Nanoscale spectroscopy and imaging, a hybrid technique that combines a scanning probe microscope (SPM) with spectroscopy, can provide nanoscale topographical, spectral, and chemical information of a sample. In recent years, developments in nanofabrication technology have dramatically advanced the field of nanospectroscopy for applications in various fields including nanoscale materials, electronics, catalysis, and biological systems. However, challenges in nanofocusing of light for excitation and extracting weak signals of individual molecules from the background signal persist in conventional nanoscale spectroscopy including tip-enhanced Raman spectroscopy, scanning near-field microscopy (SNOM/NSOM), and photoluminescence spectroscopy. This article reviews new approaches to design plasmonic SPM probes that improve important aspects of nanospectroscopy such as nanofocusing, far-to-near-field-coupling efficiency, background suppression, and ease of fabrication. The authors survey a diverse range of novel schemes to excite propagating surface plasmon polaritons on the probe surface to attain highly enhanced nanofocused light at the apex for nanoscale spectroscopies. These schemes include grating coupler configurations on the plasmonic SPM probes, aperture and apertureless plasmonic SNOM probes, nanostructured resonators coupled with a high-quality-factor photonic cavity, interfacing of the optical fiber with plasmonic nanowires, and nanoparticle-coupled plasmonic nanowires. These innovative probes merge the field of fiber optics, plasmonics, quantum optics, and nanomaterials. The authors provide a perspective on new approaches that combine the advantages of these probes and have the potential for significant advancement in nanoscale imaging and other types of nanoscale spectroscopies including scanning quantum spin spectroscopy and scanning thermal imaging microscopy.

Advances in nanoscale spectroscopy and imaging have catalyzed rapid progress in our understanding of nanoscale materials, electronics, catalysis, and biological systems.1,2 Nanoscale spectroscopy and imaging is a hybrid technique that combines a scanning probe microscope (SPM) with spectroscopy to provide nanoscale topographical, spectral, and chemical information of a sample in two or three spatial dimensions. Tip-enhanced Raman spectroscopy (TERS) and atomic force microscopy-based infrared spectroscopy (AFM-IR) technique are chemical imaging techniques that have undergone dramatic advances in recent years.3–5 TERS and AFM-IR can be categorized as scattering-type scanning near-field optical microscopy (S-SNOM). TERS achieves chemical imaging with high spatial resolution by focusing light in a nanoscale optical mode at the tip apex and collecting enhanced Raman scattering from the SPM tip. AFM-IR provides nanoscale chemical analysis by using the AFM tip to locally detect the thermal expansion of a sample resulting from local absorption of IR radiation. Compared to AFM-IR and other chemical microscopies, including aperture SNOM (Refs. 6–9) and the Nobel Prize-winning super-resolved fluorescence microscopy,2 TERS offers the highest spatial resolution, typically ∼10 nm, without the need of molecule labeling. In ambient conditions, AFM-TERS has been used to detect single isolated molecules on gold substrates, image intermolecular reactions of self-assembled monolayers on semiconductor substrates, image carbon nanotubes with a resolution of 1.7 nm, and perform sequencing of DNA with single base precision.10–15 In ultrahigh vacuum, the resolution of TERS has been shown to be capable of astonishing angstrom level spatial resolution and has been used to image the internal vibrations of individual molecules.16–20 Nanoscale spectroscopies, which include TERS, SNOM, and photoluminescence spectroscopy, hold great promise of a wide range of applications in the characterization of different samples in material science, biosensing, optoelectronics, and catalysis.21–26 

However, there are challenges in current nanoscale spectroscopy apparatuses that hinder their widespread applications. Low excitation/collection efficiencies and high far-field background noise are the main limiting factors in conventional direct-illumination configurations. Nanoscale optical chemical imaging requires efficient delivery of light for a spatially nanoconfined excitation. For example, the central working principle of TERS is to effectively focus light from a diffraction-limited optical mode (e.g., free space mode) into a nanoscale optical mode, e.g., localized surface plasmon (LSP) mode at the SPM tip, to achieve high spatial resolution and large electromagnetic field enhancement of the optical signal.3,4,27–30 Since the light is localized at a nanoscale metal tip, TERS overcomes the diffraction limit.31 However, the mode mismatch between the diffraction-limited far-field laser excitation and the desired near-field localization at the tip results in a three-to-four order of magnitude loss in excitation efficiency, which significantly limits the light intensity achievable at the apex of conventional TERS probes.32 In addition, the micrometer-size spot that the laser beam excites on the sample surface will create a far-field background signal that significantly limits the signal-to-noise ratio (SNR) of the weak tip-enhanced Raman signal from the nanoscale tip apex.31 As for collection, typically only a few percent (∼3%) of scattered Raman signals can be collected because of the limited numerical aperture of the objective lens.33,34

Restricted optical alignment and cost-effective reproducible fabrication of the tip are other factors that make the application of nanoscale spectroscopy with the conventional configurations challenging. To enhance the weak Raman signal level, it is necessary to focus the laser beam on the tip with a spot size close to the diffraction limit (<1 μm). However, with such a tightly focused spot, a small misalignment (∼100 nm) between the microscale focused laser beam and the nanoscale-confined mode at the SPM metal tip will result in an orders-of-magnitude reduction in signal. Various TERS, SNOM, and photoluminescence spectroscopy illumination configurations, including side,35,36 bottom,27,37,38 top,39–42 and parabolic mirror,43,44 share the same difficulty.

Several articles have reviewed recent advancements of nanoscale optical microscopy mostly in conventional configurations.21–26 This article reviews new approaches that utilize various plasmonic coupling configurations with the SPM probe to address the challenges of nanofocusing, coupling efficiency, far-field background signal, and practical fabrication of the probe. In particular, we focus on probes that take advantage of propagating surface plasmon polaritons (SPPs) that converge and nanofocus at the sharp end of a metallic tip. This article also describes potential new directions.

SPPs are a versatile mode of plasmons that are not confined to a directly illuminated nanostructure but can travel some distance along a dielectric-metal interface from their point of excitation.45–47 The propagation property of SPPs leads to a new means of nanofocusing energy by the implementation of tapered metal waveguide (WG) structures. The SPP propagates along the tapered waveguide, effectively squeezing the optically excited plasmon energy into LSPs at the sharp apex of the taper (Sec. II A). The LSP is confined at the tip to a spatial dimension of a few nanometers and generates a nanoscale hotspot of high intensity electric field energy useful for nanoscale spectroscopy.48,49 This propagation has been utilized for remote-excitation TERS by spatially separating the far-field laser excitation from the location of the nanoscale excitation, which generates a far-field background-free TERS.

Section II includes a brief introduction of LSPs and SPPs that are the key elements in the nanofocusing and delivering of the light. In Sec. III, we survey a diverse range of approaches in coupling the photons to SPPs on the SPM probe apex for nanoscale Raman and photoluminescence spectroscopies. Section III A reviews various surface plasmon grating coupler configurations on the typical scanning tunneling microscope (STM) and AFM probes. Section III B discusses the revival of the traditional aperture and apertureless SNOM fiber probes with plasmonic nanofocusing in which an internal optical WG mode of optical fiber is coupled to propagating SPP mode and/or LSP gap mode. Approaches reviewed in Sec. III C include nanostructured resonators such as bowtie, monopole antenna, coupled with aperture or high-quality-factor photonic cavity (PC). Probes described in Sec. III D take advantage of the long interaction length between the optical fiber and plasmonic nanowires for efficient coupling. Incorporating newly developed nanocavity lasers in these designs would be a new direction for significant advancements. Designs described in Sec. III E also utilize the nanofocusing SPPs on plasmonic nanowires but use the scattered light from the nanoparticles to excite SPPs. This coupling scheme could be readily extended to the development of a scanning quantum probe microscope.

These innovative probes merge the field of fiber optics, plasmonics, quantum optics, and nanomaterials. New approaches that combine the advantages of these probes have the potential for significant advancement in nanoscale chemical imaging. Revolutionary new nanoscale spectroscopies, e.g., spin spectroscopy and thermal imaging, could be based on these probes.

Plasmons are discrete oscillations of free electron gas density when there is some localized perturbation of a conducting medium's electric field. They can occur in materials that possess a negative real and small positive imaginary dielectric constant. The two forms of plasmons that have come into use in nanospectroscopy are LSPs and SPPs.

LSPs occur on metal particles or structures that are smaller than the wavelength of the exciting light source.46,47,50 They are nonpropagating excitations of the conduction electrons of the particles that are resonantly coupled to the exciting field. The local charge of the nanoparticle becomes displaced and collectively oscillates with the optical source frequency. Particles composed of silver and gold have negative real dielectric constants capable of supporting plasmon resonances in the visible spectral region and can range in size from 10 to 300 nm for visible excitation wavelengths.

LSP resonances are the foundational component of conventional TERS that accounts for the focusing of far-field energy to the nanoscale. The tip of a metallic scanning probe directly excited with light effectively behaves as a nanoantenna, concentrating laser irradiation to a few-nanometer sized point of enhanced electromagnetic field energy. This point has an electric field intensity up to several orders of magnitude greater than the excitation source concentrated in an area just a few nanometers in diameter at the tip apex. This enhancing hot spot at the end of the tip enables delivery of an exciting source to a few-nanometer region to perform Raman scattering, photoluminescence, nanofluorescence, or other nanoscale spectroscopic investigations. Beside far-field excitation, LSPs could be confined through nanofocusing of SPPs (Sec. II B).

The resonant conditions and focusing capacity of nanoparticles can be modified by the shape and geometry of the nanoparticles. A smaller particle diameter leads to higher spatial confinement while the shape of the particles mediates the resonance frequency; widely used particles are nanospheres, nanorods, nanodisks, and patterned structures. Bowtie structures have been used to couple specific wavelengths of light and focus optical energy down to less than 1 nm.51–53 Particles spaced close together, typically only a few nanometers, operate in an effective “gap” mode that further increases the spatial confinement of the electric field energy. This effect has been the key to experiments demonstrating subnanometer spatial resolution TERS using a metal tip simultaneously with a metal substrate to create a gap-mode LSP between the tip and the sample.19,54,55 The highest spatial confinement and field enhancement has been observed with both substrates and tips being gold or silver.56–58 A wide array of metals and even semiconducting substrates have been used to experimentally demonstrate TERS with a reduced localized field enhancement.10,59–61 The reduced field confinement of non-noble metal substrates and tips limits the effective spatial resolution and field intensity enhancement of TERS and increases the difficulty of achieving subnanometer spatial resolution.

Another plasmon modality, SPPs, occur at the interface of metals and dielectrics. SPPs consist of a traveling packet of electromagnetic energy bound to a surface. SPPs can propagate for long distances, up to several micrometers. Plasmons are not limited by diffraction because the electric field is evanescent perpendicular to the surface. Babadjanyan et al. theoretically studied SPP propagation along conical tapers and suggested the possibility for spatial confinement of SPP fields.49 Stockman first theoretically predicted the three-dimensional nanofocusing of SPPs into nano-LSPs and the concentration the optical radiation energy to nanoscale without major losses using a conical plasmonic taper.48 The lowest order mode of SPPs propagating toward the tip of a tapered plasmonic waveguide are slowed down and stopped when they reach to the tip apex. Such a phenomenon leads to the accumulation of optical energy and giant local fields at the tip. This nanofocusing property of SPPs is the cornerstone of the various tapered metal waveguide structures in nanoscale optical and chemical imaging that are discussed below in Sec. III. The diverse approaches use various means to effectively excite SPPs and squeeze the optically excited plasmon energy into a few nanometers at the SPM tip for the excitation of the molecules.

The excitation of SPPs occurs when the wavevector of incident photons is equal to the wavevector of the surface plasmon that can be sustained by a material. Figure 1(a) qualitatively shows a dispersion curve of surface polaritons along with the dispersion line of the light propagating in a dielectric having a refractive index nd (e.g., air). For a given frequency and a metal such as silver or gold, the wavevector of the free space photon is less than that of a surface plasmon. Therefore, the free space propagating light cannot excite the surface polariton. In order to couple propagating light with surface polaritons, a coupler is needed to shift the dispersion line of the light to match the parallel (or in-plane) wavevector component of the SPPs.62–66 

FIG. 1.

Grating-coupled plasmonic probes. (a-i) Dispersion curve of surface polaritons along with the dispersion line of the light propagating in a dielectric having a refractive index nd illustrating the wavevector-matching condition. Reproduced with permission from Heitmann, J. Phys. C Solid State Phys. 10, 397 (1977). Copyright 1977, IOP Publishing. (a-ii) Grating coupling of surface plasmons and focusing at tip apex. Adapted with permission from Ropers et al., Nano Lett. 7, 2784 (2007). Copyright 2007, American Chemical Society. (b-i) SEM image of the probe of remote-excitation TERS. (b-ii) The TERS spectra obtained via remote TERS. Adapted from Berweger et al., J. Phys. Chem. Lett. 1, 3427 (2010). Copyright 2010, American Chemical Society. (c) The plasmonic nanofocusing probe fabricated on a commercially available AFM probe. Scale bar: 5 μm, 500 nm and 50 nm for (c-ii), inset in (c-ii) and (c-iii) respectively. Adapted with permission from Zhang et al., Sci. Rep. 3, 2803 (2013). Copyright 2013, Nature Publishing Group. (d-i) A plasmonic lens illumination configuration with symmetry-breaking semiannular slits (d-iii) for TERS (d-ii). Adapted with permission from Zhang et al., Opt. Express 21, 9414 (2013). Copyright 2013, Optical Society of America. (e) A 3D p-tip with annular slit fabricated on a commercial SiO2 AFM probe with no nanoaperture at the tip apex. Adapted from Jiang et al., Nano Lett. 18, 881 (2018). Copyright 2017, American Chemical Society. (f) Patterned metallic pyramids with no aperture (f-i) and with aperture [(f-ii) and (f-iii)] for three-dimensional plasmonic nanofocusing. Adapted from Lindquist et al., Nano Lett. 10, 1369 (2010). Copyright 2010, American Chemical Society.

FIG. 1.

Grating-coupled plasmonic probes. (a-i) Dispersion curve of surface polaritons along with the dispersion line of the light propagating in a dielectric having a refractive index nd illustrating the wavevector-matching condition. Reproduced with permission from Heitmann, J. Phys. C Solid State Phys. 10, 397 (1977). Copyright 1977, IOP Publishing. (a-ii) Grating coupling of surface plasmons and focusing at tip apex. Adapted with permission from Ropers et al., Nano Lett. 7, 2784 (2007). Copyright 2007, American Chemical Society. (b-i) SEM image of the probe of remote-excitation TERS. (b-ii) The TERS spectra obtained via remote TERS. Adapted from Berweger et al., J. Phys. Chem. Lett. 1, 3427 (2010). Copyright 2010, American Chemical Society. (c) The plasmonic nanofocusing probe fabricated on a commercially available AFM probe. Scale bar: 5 μm, 500 nm and 50 nm for (c-ii), inset in (c-ii) and (c-iii) respectively. Adapted with permission from Zhang et al., Sci. Rep. 3, 2803 (2013). Copyright 2013, Nature Publishing Group. (d-i) A plasmonic lens illumination configuration with symmetry-breaking semiannular slits (d-iii) for TERS (d-ii). Adapted with permission from Zhang et al., Opt. Express 21, 9414 (2013). Copyright 2013, Optical Society of America. (e) A 3D p-tip with annular slit fabricated on a commercial SiO2 AFM probe with no nanoaperture at the tip apex. Adapted from Jiang et al., Nano Lett. 18, 881 (2018). Copyright 2017, American Chemical Society. (f) Patterned metallic pyramids with no aperture (f-i) and with aperture [(f-ii) and (f-iii)] for three-dimensional plasmonic nanofocusing. Adapted from Lindquist et al., Nano Lett. 10, 1369 (2010). Copyright 2010, American Chemical Society.

Close modal

This section reviews the various plasmonic SPM probe designs utilizing different coupling schemes to excite the SPPs for nanofocusing at the tip apex. Grating couplers with metalized AFM probes are most commonly used since grating structures on the metal surface can directly couple the impinging free space radiation to SPPs (Sec. III A). The transverse wavevectors of the beam diffracted by the grating can be made to match that of SPP's by the grating width and period. The plasmonic lens design improves upon the conventional grating coupler by extending the coupling region to three dimensions. When a high index light-guiding medium such as glass is available, the Kretschmann configuration can be used (Sec. III B), as is the case with probes built on an optical fiber platform. In this configuration, evanescent waves excited through the attenuated total internal reflection possess wave vectors close to the SPPs on the metal-dielectric interface, and thus, energy can tunnel from evanescent modes to SPP modes. Direct interfacing of nanoresonators and the photonic cavity can resonantly excite the plasmonic modes of the nanoprobes and thus can be implemented in both optical fiber and AFM platforms (Sec. III C). For metallic nanowire probes attached to optical fibers, evanescent wave coupling of parallel waveguides is utilized, where the evanescent tail of the dielectric waveguide excites the SPP mode on the metallic nanowire placed in close vicinity (Sec. III D). Furthermore, the structures composed of a single scattering particle or slit have been shown to couple incident light to SPPs by scattering a wide range of wavevector components of the incident photon. This phenomenon allows the design of metal nanowire probes incorporated into AFM and STM for remote TERS (Sec. III E).

Implementation of these techniques on SPM probes has different impacts on nanofocusing, coupling efficiency, far-field background signal suppression, and practical fabrication of the probe. The fraction of the optical energy carried by the input free space radiation that is converted to SPP modes on the metal waveguide is mainly determined by momentum matching, mode overlap, and coupling length. One or more of these parameters can be optimized depending on the coupling scheme. In a grating coupling scheme and planar Kretschmann configuration, while momentum mismatch can be controlled by grating periods and angle of incidence, there is no degree of freedom in coupling length and mode overlap. The three-dimensional (3D) plasmonic lens design improves on the conventional grating coupler by extending the coupling length. On the other hand, in internal excitation through a tapered fiber, all three parameters can be optimized by varying the metal coating thickness, radial vector mode excitation, and taper angle. Similarly, in photonic-to-plasmonic nanowire coupling, momentum matching and mode overlap are achieved due to the high index of the photonic fiber and close proximity of the wires; in addition, the length of the coupling region can also be adjusted. However, the trade-off of the control over coupling efficiency lies in the fabrication complexity and the ease of integration into existing setups. Once excited, the nanofocusing performance of the coupled SPP is further limited by the ohmic loss of the metal and radiative scattering that largely depend on the quality of the metal surface. Innovative fabrication techniques have been developed to not only improve the quality of the probes but also to lower the fabrication cost. In the following discussion, we will report on promising plasmonic nanofocusing probes and assess their different merits such as coupling efficiency, effective nanofocusing, far-field background signal suppression, and ease of fabrication.

The most attractive merits of grating-coupled plasmonic probes are the flexibility in the design, the ease of incorporating the grating structure on a readily available SPM probes, and the high coupling efficiency.

In a grating coupler, the edge of the grating slits can be thought of as a line of SPP point sources. When the light is incident on the grating structure, the in-plane wavevector of the diffracted light increases with a magnitude depending on the diffraction order in the direction along the film. This change shifts the light dispersion to couple with the SPP dispersion as shown in Fig. 1(a-i).62–66 The grating coupler can excite multiple surface polaritons even at the normal incidence. The fabrication of the grating structure on the shaft of the SPM enables a far-field background-free TERS scheme [Fig. 1(b)].67,68 Recently, the development of a 3D plasmonic lens (a circular or elliptical grating) and the incorporation of the plasmonic lens on the SPM probe showed the potential for high efficiency delivering of nanoscale (down to ∼10 nm) confined light.69,70

1. Remote-excitation TERS

To realize 3D nanofocusing of SPPs on a probe, Raschke’s67,71 and Lienau's72–74 groups experimentally fabricated a grating coupler configuration on the probe for remote excitation [Fig. 1(a-ii)].75 One-dimensional gratings are written onto the tip shaft by focused ion beam (FIB) sputtering about 10 μm away from the apex. The resonant SPPs are generated by illumination of the grating with a broadband femtosecond laser and travel to the tip apex. The SPPs are adiabatically transformed into LSPs leading to 3D nanofocusing with a giant concentration of energy on the nanoscale as predicted by Stockman.48 

Raschke's group experimentally demonstrated the TERS application of these grating-assisted probes [Fig. 1(b)].67,71 This remote TERS scheme allows for background-free nanospectroscopy since the far-field illumination of the grating coupler for the excitation of SPPs is spatially separated (∼10 μm away) from the nanoscale near-field tip apex for excitation of the molecule. High nanofocusing efficiency is achieved in this scheme. At 632 nm, 2% of the light incident on the grating is nanofocused into the tip apex region with a propagation-induced loss of 98%. A TERS signal equivalent to conventional (direct-illumination) TERS was obtained. At 785 nm, 9% of the light incident on the grating is nanofocused into the tip apex region. This high nanofocusing efficiency allows for the extension of TERS into the near-IR (λ = 800 nm).

Raschke's group further demonstrated femtosecond near-field imaging with the same configuration.68 The nonlinear plasmonic nanofocused four-wave mixing signal was highly localized in a nanoscopic volume at the tip apex due to the asymptotic compression of the SPPs associated with the nanofocusing process. This nonlinear nanoprobe was used to image the few-femtosecond coherent dynamics of plasmonic hotspots on a nanostructured gold surface with a spatial resolution of a few tens of nanometers. Ultrafast excitation and nonlinear excitation of molecules have been introduced in conventional TERS.76–79 The phase matching requires colinear illumination of two or three beams, tight focusing of the laser spot, and spatial alignment with the tip apex. This grating-assisted plasmonic probe relaxes these strict requirements and could be a highly sensitive nanoprobe for ultrafast near-field microscopy and spectroscopy.

With the same grating coupling principle, Zhang et al. fabricated the grating pattern on the shaft of a metallic coated AFM probe to excite and focus propagating SPPs [Fig. 1(c)].80 The scheme has been used for near-field imaging of surface plasmon cavity modes. This design should extend the remote TERS application to AFM-based nanoscale spectroscopy setups. The main advantage of this design is that they are simple to adopt since they are compatible with commercial nanoscale spectroscopes.

2. Tip-enhanced Raman spectroscopy assisted by plasmonic lens

The use of a plasmonic lens with a circular or elliptical grating is another approach to break the diffraction limit. This setup can focus the light to nanoscale (∼100 nm), which is not as tight as the use of a plasmonic metal tip apex (∼10 nm). Symmetric circular and elliptical slit grating structures on the metal film act as a plasmonic lens that focuses radially polarized light by exciting the propagating SPP waves.81 The surface plasmons interfere and concentrate the electromagnetic field at the focal points to a subwavelength light spot. The sharp edge of a slit milled via FIB through a metallic film is commonly fabricated to couple light into SPPs.

Zhang et al. used a metallic plasmonic lens instead of a conventional optical objective in the illumination configuration in TERS to enhance the Raman scattering at the subwavelength focal point [Fig. 1(d)].69 In this study, instead of using radially polarized light, a plasmonic lens with symmetry-breaking semiannular slits corrugated on a gold film was designed to generate concentrated subwavelength light spots with a strong longitudinal electric field [Fig. 1(d-iii)]. The symmetry breaking generates a π-phase shift in the propagating SPPs leading to constructive interference in the center of the plasmonic lens. This asymmetric photoluminescence (PL) realizes a strong longitudinal electric field illumination for TERS, i.e., in the z-component of the electric field (EZ), with a linearly polarized incident beam. Experimental TERS results [Fig. 1(d-ii)] with carbon nanotubes at the focus point with a silver tip shows that the Raman scatting signal is significantly enhanced. However, the enhancement due to the plasmonic lens is not separated from the TERS enhancement from the plasmonic tip in this study.

3. Plasmonic SNOM probes based on a plasmonic lens

The adaption of a 2D plasmonic lens to a 3D conic probe has been proposed and fabricated for near-field imaging with high efficiency delivering of nanoscale (down to ∼10 nm) confined light. These probes could potentially further improve the TERS sensitivity. Zhang et al. fabricated a circular plasmonic lens on the metal coating of a conical SNOM probe surrounding a nanoaperture at the tip apex.70 Similar to the plasmonic lens on the planar surface, the conic plasmonic lens can focus light energy into a subdiffraction-limited intense spot. In this study, while the transmission (36 times) is significantly improved compared to a setup without the 3D grating, the spatial resolution is limited by the size of the aperture (∼100 nm).

Jiang et al. designed a plasmonic lens tip (p-tip) on a commercial SiO2 AFM probe with no nanoaperture at the tip apex [Fig. 1(e)].82 Subwavelength circular gratings were fabricated to couple the internal radial polarized illumination to SPPs that led to an ultrastrong, superfocused spot via adiabatic nanofocusing. This p-tip supports both a radial symmetric SPP excitation and a Fabry–Perot resonance. The performance of the p-tip was optimized by deliberately tailoring the locations of these annular slits to satisfy the phase matching condition between the phase delay of the SPP wave propagation on a curved surface and the phase delay of the excitation light propagation in media (i.e., SiO2 in this case). The SPP wave propagates and constructively interferes at the tip apex, when the phase difference satisfies the condition of Fabry–Perot resonance, leading to the superfocusing mode with a spot size of 8 nm.70,83 Near-field optical experiments were performed with this p-tip on a standard Fischer's pattern. This design rendered an optical resolution of 10 nm, a topographic resolution of 10 nm, a throughput of 3.28% that is approximately four orders higher than a commercial SNOM probe, and an outstanding SNR of up to 18.2 (nearly free of background). Another S-SNOM probe design combined a sharp metal tip with two half-circular gratings on its base, which shows the potential of optimizing the nanofocusing by fine tuning of the plasmonic lens and the plasmonic probe.84 

These plasmonic lens probes can be fabricated more economically by use of a reusable template stripping method.85–87 Pyramidal probes with various periodic patterns with nanoaperture or without nanoaperture have been fabricated [Fig. 1(f)].88,89 A 103-fold increase of the transmitted optical intensity was observed after patterning with elliptical grooves without an aperture on the pyramid apex.88 Surface-enhanced Raman spectroscopy (SERS) and second harmonic generation have been demonstrated using probes without nanoaperture.89 

It is expected that the combination of the plasmonic lens/grating structure with the other plasmonic tips (e.g., campanile probe discussed in Sec. III B) could further improve the enhancement of electric field at the tip apex and, therefore, improve the near-field sensitivity.

This section overviews probes and structures that are internally excited and create SPPs on the external or internal surfaces of a coated SNOM dielectric waveguide. The techniques listed in this section have the benefit of inherent coupling of free space optical energy to plasmonic modes without the requirement of alignment of focusing objectives to tip apexes, gratings, or any other form of plasmonic coupling structure. The exciting optical energy is remotely coupled to common dielectric waveguiding structures, such as optical fibers, which can have plasmonic probes directly fabricated on their ends. The probes discussed below consist of apertureless conical tapered probes based on dielectric waveguides, metal–insulator–metal (MIM) gap mode tips, and multilayered dielectric gap structures.

This inherent optical alignment to the nanofocusing device further reduces the background signal by avoiding direct illumination of the sample with a diffraction-limited focused beam. This offers the possibility of exciting only the target sample with the nanofocused energy, leading to exceptionally high SNRs.

1. Conical apertureless plasmonic SNOM probes

Apertureless plasmonic SNOM probes are an advancement of the aperture scanning near-field optical microscopy (aSNOM), a widely employed technique to deliver background-free nanoscale energy to a sample. aSNOM probes typically consist of a tapered optical fiber coated with ∼100 nm of metal, and a subwavelength aperture is etched in the metal film on the facet of the tapered end.90,91 This probe can focus optical energy to a sub-50 nm diameter area by effectively squeezing light through an aperture at the end of a tapered dielectric fiber. However, aSNOM suffers from very low throughput, typically 10−4 of the input optical power, and, because it utilizes a nanoscale aperture, it cannot confine its excitation volume to the single nanometer regime.92 Similar to grating and plasmonic lens coupled probes, plasmonic SNOM probes take advantage of nanofocusing of SPPs. An internal optical WG mode can be coupled to an external SPP on the metal surface, which then propagates to the tip apex, resulting in nanofocusing of the delivered optical energy to the single nanometer regime [Fig. 2(a-i)].

FIG. 2.

Plasmonic internally excited SNOM techniques. (a-i) Internally excited apertureless probe coupling schematic. (a-ii) Simulation result of internal WG mode exciting nanofocused SPP modes of a 50 nm Au film on a tapered optical fiber. Adapted with permission from Chen and Zhan, Opt. Express 15, 4106 (2007). Copyright 2007, Optical Society of America. (a-iii) Optical image of SPP emission from a coated fiber tip overlaid on an SEM image of a conical tip. Adapted with permission from Tugchin et al., ACS Photonics 2, 1468 (2015). Copyright 2015, American Chemical Society. (a-iv) Raman spectra collected from tip with internally excited radial WG mode (Inset: experiment schematic). Adapted with permission from Liu et al., Nanophotonics 8, 921 (2019). Copyright 2019, De Gruyter. (b-i) SEM image of two photon 3D printed asymmetric conical tip structures and (b-ii) simulation of internal linear polarization WG excitation nanofocused to tip apex, and (b-iii) Raman collected from symmetric plasmonic conical structure (bottom spectrum) vs. asymmetric structure (top spectrum) (Inset: Nile blue molecule and Raman experiment scheme). Adapted with permission from Sun et al., ACS Photonics 5, 4872 (2018). Copyright 2018, American Chemical Society. (c) Design of a helical nanofocusing structure and simulation of the structure with the internal radial WG mode focused to the tip apex. Adapted with permission from Kuang et al., IEEE Photonics J. 6, 1 (2014). Copyright 2014, IEEE. (d-i) Schematic of WG-SPP coupling of internally excited metal–insulator–metal probe. (d-ii) SEM image of a campanile pyramid tip with two Au coated facets and two bare facets. Adapted from Tuniz and Schmidt, Nanophotonics 7, 1279 (2018). Copyright 2018, De Gruyter. (d-iii) and (d-iv) Simulation of internally excited linear WG modes focused to gap at tip apex. Adapted from Bao et al., Opt. Express 21, 8166 (2013). Copyright 2013, Optical Society of America. (d-v) Photoluminescence map of monolayer MoS2 excitonic emission collected with a scanning campanile tip near-field microscope and a scanning confocal microscope. Scale bar: 1 μm. Reproduced with permission from Bao et al., Nat. Commun. 6, 7993 (2015). Copyright 2015, Springer Nature. (e-i) Schematic of multilayer conductor-dielectric-gap-substrate conical probe with normalized electric field intensity vs layer thickness inset, (e-ii) simulation of probe with radially polarized light launched into silica layer with (right) and without (left) lower-index dielectric layer, and (e-iii) curve of enhancement of electric field at tip apex with (solid) and without (dashed) a lower-index dielectric layer. Adapted from Li et al., Opt. Eng. 58, 077101 (2019). Copyright 2019, SPIE.

FIG. 2.

Plasmonic internally excited SNOM techniques. (a-i) Internally excited apertureless probe coupling schematic. (a-ii) Simulation result of internal WG mode exciting nanofocused SPP modes of a 50 nm Au film on a tapered optical fiber. Adapted with permission from Chen and Zhan, Opt. Express 15, 4106 (2007). Copyright 2007, Optical Society of America. (a-iii) Optical image of SPP emission from a coated fiber tip overlaid on an SEM image of a conical tip. Adapted with permission from Tugchin et al., ACS Photonics 2, 1468 (2015). Copyright 2015, American Chemical Society. (a-iv) Raman spectra collected from tip with internally excited radial WG mode (Inset: experiment schematic). Adapted with permission from Liu et al., Nanophotonics 8, 921 (2019). Copyright 2019, De Gruyter. (b-i) SEM image of two photon 3D printed asymmetric conical tip structures and (b-ii) simulation of internal linear polarization WG excitation nanofocused to tip apex, and (b-iii) Raman collected from symmetric plasmonic conical structure (bottom spectrum) vs. asymmetric structure (top spectrum) (Inset: Nile blue molecule and Raman experiment scheme). Adapted with permission from Sun et al., ACS Photonics 5, 4872 (2018). Copyright 2018, American Chemical Society. (c) Design of a helical nanofocusing structure and simulation of the structure with the internal radial WG mode focused to the tip apex. Adapted with permission from Kuang et al., IEEE Photonics J. 6, 1 (2014). Copyright 2014, IEEE. (d-i) Schematic of WG-SPP coupling of internally excited metal–insulator–metal probe. (d-ii) SEM image of a campanile pyramid tip with two Au coated facets and two bare facets. Adapted from Tuniz and Schmidt, Nanophotonics 7, 1279 (2018). Copyright 2018, De Gruyter. (d-iii) and (d-iv) Simulation of internally excited linear WG modes focused to gap at tip apex. Adapted from Bao et al., Opt. Express 21, 8166 (2013). Copyright 2013, Optical Society of America. (d-v) Photoluminescence map of monolayer MoS2 excitonic emission collected with a scanning campanile tip near-field microscope and a scanning confocal microscope. Scale bar: 1 μm. Reproduced with permission from Bao et al., Nat. Commun. 6, 7993 (2015). Copyright 2015, Springer Nature. (e-i) Schematic of multilayer conductor-dielectric-gap-substrate conical probe with normalized electric field intensity vs layer thickness inset, (e-ii) simulation of probe with radially polarized light launched into silica layer with (right) and without (left) lower-index dielectric layer, and (e-iii) curve of enhancement of electric field at tip apex with (solid) and without (dashed) a lower-index dielectric layer. Adapted from Li et al., Opt. Eng. 58, 077101 (2019). Copyright 2019, SPIE.

Close modal

Excitation of propagating plasmons on metal films cannot be achieved by direct illumination due to momentum mismatch between the WG and SPP modes.93 The introduction of a dielectric layer or prism can be used to achieve matching between the in-plane component of the excitation wavevector with the SPP wavevector. For a metal-coated prism coupler (or Kretschmann coupler), the effective index of the in-plane wavevector of free space light becomes kx = npk0sin θ. Here, np is the refractive index of the prism, k0 = ω/c is the propagating wavevector in vacuum for light, and the light is incident on the metallic thin film at angle θ. For a specific incidence angle, the planar wavevector of the exciting light will match the SPP wavevector.94,95 The taper of the optical fiber behaves similarly to the Kretschmann coupler. The decreasing mode volume leads to a decreasing effective index of the supported optical WG mode while the effective index of the SPP increases. At some radius, the WG and SPP momentums match and SPPs will be excited by WG modes. The SPP then propagates down the outer metal skin of the fiber to the tip while higher-order modes decay radiatively or are absorbed by the film. As the fiber radius is reduced, the SPP wavelength shortens, resulting in the nanofocusing effect.

The coupling of radial WG modes to highly localized SPP modes has been simulated and experimentally demonstrated.96–99 The nanofocusing of tapered conical tips is dependent on the symmetry of the plasmonic waveguiding structure and the exciting WG mode. Radially polarized light effectively excites SPPs and constructively generates significant field intensity at the symmetric conical tip apex. However, perfectly symmetrical metal-coated conical tapers are prone to destructive interference of their SPP modes when excited by linearly polarized light due to propagating plasmons generated on opposite sides of the cone being of opposite phase.96,97 Chen and Zhan simulated the coupling of radial WG modes to SPP modes on the metal film of a conical tip [Fig. 2(a-ii)].100 Their result shows the generation of SPPs at a fiber radius of 600 nm and nanofocusing of radially symmetric SPPs to the tip resulting in a strong electric field at the apex. Tugchin et al. experimentally demonstrated coupling of a radially polarized WG mode in a step index optical fiber with a tapered end coated with a 130 nm Au layer [Fig. 2(a-iii)].93 The achieved SPP focusing was visualized by an emission at the tip apex of the coated fiber.

The optical energy focused to the tip apex is sufficient to perform spectroscopic measurements. Liu et al. utilized a silver-coated conical fiber tip to detect low concentrations of malachite green adsorbed onto the fiber tip, which is internally excited by a radially WG mode [Fig. 2(a-iv)].101 They estimate that 17% of the input optical energy is delivered to the nanofocused spot at the tip apex. In their configuration, the Raman signal was collected by an objective positioned along the fiber axis with its focus set at the tip apex. Furthermore, the adsorbed analyte Raman signal exhibited 15 times greater intensity when internally excited with a radially polarized source rather than linear polarization.

The requirement of a radially polarized optical source is potentially cost prohibitive, and the technique requires complex optical components to generate the required polarization. In the work by Sun et al., asymmetric cone structures are designed to break the taper radial symmetry and allow the SPPs to constructively interfere.102 Their optimized asymmetric designs lead to an average field enhancement of 500 times the input intensity [Fig. 2(b-ii)]. These structures can be grown in arrays by two photon polymerization followed by 60 nm of silver coating. The asymmetric cone structures show a Raman signal 1.8 times stronger when illuminated internally than when illuminated directly [Fig. 2(b-iii)]. The symmetric cones show no Raman signal when internally or directly illuminated [Figure 2(b-iii)]. Their result indicates that internally excited nanofocused SPPs are a highly efficient way to deliver energy to the nanoscale for spectroscopic analysis.

Coupling efficiency of WG to SPP modes has been further improved by tuning the excitation polarization to specifically designed waveguides. Kuang et al. utilized a combination of structure engineering and a radial polarized source to further improve the excitation efficiency of the nanofocused hotspot through use of a metal-coated helical nanocone [Fig. 2(c-i)].103 The helical nanocone design allows radial SPPs to be excited at the base of the of the helical structure and propagate along the boundary of the helix to the tip. Simulations of this configuration show that the electric field component in the transverse and longitudinal directions can be tuned by careful choice of geometry parameters to achieve better than 70% energy conversion to the longitudinal component of electric field at the tip apex with potential enhancement factors of 104, substantially higher than typical conical or asymmetric structures [Fig. 2(c-ii)].

Apertureless external plasmon adiabatic focusing structures have the potential to vastly improve nanoscale spectroscopy by delivering a substantial amount of the input electric field energy to a few-nanometer excitation spots with virtually no external background. In addition, these plasmonic fiber probes could potentially collect the Raman signal from the nanoscale emission. Tugchin et al. experimentally demonstrated the collection of SPPs generated at the tip apex by direct illumination; 0.01% of the total focused beam energy was converted to SPPs and then into WG modes in the fiber.93 Their relatively low SPP generation efficiency was attributed to mismatched polarization components to the ideal SPP generation in their direct-illumination configuration. In a similar plasmonic collection scheme, Chen et al. demonstrated 70% total collection of a single emitter source by the generation of SPPs on nanowires coupled to dielectric fibers.104 

Yet, no published work has demonstrated the spectroscopic collection capabilities of plasmonic SNOM probe tips. Plasmonic SNOM probes demonstrate sufficient WG-SPP nanofocusing efficiency for near-field spectroscopic measurements, and further improvements to SPP-WG should yield a powerful single-probe-excitation-collection near-field spectroscopy system. The demonstration of nanoscale imaging and spectroscopy using fiber-based, apertureless conical probes is a promising direction for the advancement of the nanospectroscopy field.

2. Metal–insulator–metal probe

A similar modality utilizing SPP modes to focus energy to the nanoscale with internally excited WG modes involves MIM structures with a planar geometry and nanoscale gap (∼10 nm) at the tip apex, commonly known as campanile probes. These gap mode tips utilize an internal WG to excite a traveling SPP on the internal surface of the metal coating that propagates to the gap [Fig. 2(d-i)]. While similar in appearance to aperture NSOM probes, the campanile probes utilize tapered planar geometry with metal films coated on two opposing bare facets on a silica pyramid [Fig. 2(d-ii)].105 MIM waveguides are known to support guided symmetric SPP modes without any cutoff frequency, leading to a broader range of optical modes getting converted to SPP modes.105,106 MIMs with optimized gap size and taper angle have been shown to achieve better than 80% energy conversion from the WG to SPP mode.107,108 It has been reported that the combination of high SPP conversion efficiency with the gap structure at the probe apex leads to a further enhancement of the nanofocused light. This type of device capitalizes on similar principles employed by gap-mode TERS, which has demonstrated astounding subnanometer resolved Raman images.11,54,109 However, the gap structure limits the spatial resolution of the campanile structure to approximately the gap width (∼10–40 nm).105,106

Unlike the internally excited adiabatic focusing tips discussed in this section, campanile probes have demonstrated spectral acquisition in the backward SPP to WG collection mode. Bao et al. imaged the photoluminescence emission of monolayer molybdenum disulfide edge sites and grain boundaries using campanile-style probes. A comparison between confocal and spectral scanning probe images [Fig. 2(d-v)] of MoS2 excitonic emission peaks demonstrates the subwavelength resolving powers of the SPP-WG collection probe, which provides ∼60 nm of spatial resolution revealing a substantial nanoscale optoelectronic disorder.8 

Although several modalities of WG to SPP focusing have demonstrated high conversion of waveguided excitation to nanoscale volumes, they are yet to achieve 100% coupling of optical energy to nanofocused SPPs. In one such work to improve coupling efficiencies, Li et al. proposed a hybrid plasmonic probe consisting of a gold layer, two different low index dielectric layers, and a semiconductor substrate [Fig. 2(e-i)].110 The low index and silica layers behave as a traditional waveguide while confining the WG field energy close to the outer metallic layer. This decreased interaction volume increases the WG conversion to SPP. Simulation results [Fig. 2(e-ii)] show electric field energies at the apex of a tip with the low index layer (right) and without (left) when radially polarized light is launched into the silica layer. The electric field intensity delivered to the tip apex is nearly four times higher with the included lower-index guiding layer. The enhancement is dependent on the thickness of the low index and silica layers (0–100 nm) and reaches a maximum enhancement of over ∼109.

Internally excited propagating SPP probes utilizing adiabatic plasmon focusing, MIM mode confinement, and multiple-layered structures offer substantial advantages over conventional direct illumination. The reduced far-field background signal, inherent coupling of optical energy to tip structures without the need for external alignment, and flexible structural and excitation polarization parameters allow for a wide range of possibilities for substantially improved nanoscale spectral imaging. The collection capabilities of internal WG to SPP probes have not yet been fully demonstrated; but given the high coupling efficiency that can be achieved in the WG to SPP generation, it is likely that strides will be made for simultaneous signal collection using SPP to WG coupling. Further advancements could be aided by the integration of elements such as gratinglike coupling structures, resonant structures, or further tuning of dielectric and metallic layers.

Both the signal intensity and the spatial resolution of the aperture SNOM tips can be improved by incorporating nanostructured resonators into the fiber tip such as a bowtie nanoantenna,111 monopole antenna,112 and tapered metallic tip.113 As discussed earlier, the guided mode of the fiber decays along the length of the aSNOM probe, and only the evanescent field remains at the tapered end. A nanoantenna probe whose dimensions are smaller or comparable to the free space excitation wavelength can be driven to its resonances by the electric field parallel to its axis. Since the electric field at the aperture has longitudinal components parallel to the axis of the fiber, these evanescent field components couple to the plasmonic resonant modes of the nanoantenna, judiciously placed at the aperture opening. The spectral range of the antenna resonances depends on the shape and size of the antenna. In fact, the spectral bandwidth of such probes can be tuned by coupling multiple plasmonic structures.114 Near-field probes that incorporate one or more nanoantennae on the tapered fiber benefit from the advantages of both plasmonic and photonic worlds. The concept of a photonic-plasmonic hybrid nanofocusing probe is not limited to the optical fiber platform. For straightforward integration with the AFM platform, a coupled cavity-antenna system has also been explored. In this system, the field inside the high-quality-factor photonic cavity excites the plasmonic resonant modes of the nanoantenna. Similar to the fiber-based probe, the cavity-based probe provides low background noise with nanoscale spatial resolution.

1. Aperture-coupled nanoantenna on fiber

Taminiau et al. showed that the spatial resolution of an aperture NSOM probe can be greatly improved by incorporating a nanoantenna on the apertures [Fig. 3(a-i)].112 At the edge of the aperture, the electric field has components parallel to the axis of the probe, which can excite the resonant modes in the nanoantenna, resulting in the nanofocused spot shown in Fig. 3(a-ii). By single molecule luminescence imaging, the authors showed that the antenna efficiency is dependent on the antenna length and that the antenna contribution to the field at resonance is about four times larger than the contribution from the aperture [Fig. 3(a-iii)]. The main advantage of the inclusion of the nanoantenna is the tighter field confinement that leads to a better spatial resolution of 25 nm FWHM, compared to the ∼80 nm FWHM of aperture probe.112 

FIG. 3.

Coupled nanoresonator probes. (a-i) SEM image of the aNSOM probe enhanced with nanoantenna. (a-ii) Simulated electric field vector distribution on a cross section of the antenna structure. (a-iii) Experimental verification of dependence of antenna efficiency on antenna length. Adapted with permission from Taminiau et al., Nano Lett. 7, 28 (2007). Copyright 2007, American Chemical Society. (b-i) Hybrid nanoaperture-antenna probe for dual color, nanoscale fluorescence imaging. Scale bar: 300 nm (b-ii) Confocal fluorescence image of a mixture of DiD and DiI molecules with 560 and 633 nm excitation lasers. (b-iii) Near-field fluorescence image obtained with the hybrid probe of the same sample using the same excitation lasers. Scale bar: 200 nm. Adapted with permission from Mivelle et al., Nano Lett. 14, 4895 (2014). Copyright 2014, American Chemical Society. (c-i) SEM image of the pyramid and C-shaped aperture structure fabricated by template stripping. (c-ii) and (c-iii) Microscope image of transmitted light from the C-shaped aperture pyramid with and without bright-field illumination. Adapted with permission from Lindquist et al., Sci. Rep. 3, 1857 (2013). Copyright 2013, Springer Nature. (d-i) Hybrid photonic-plasmonic cavity consisting of bowtie plasmonic antenna embedded in a 1D silicon photonic cavity. (d-ii) SEM image of the cavity. (d-iii) Transmission spectra of the hybrid cavity showing a Q-factor of 800. Adapted with permission from Conteduca et al., APL Photonics 2, 086101 (2017). Copyright 2017, AIP Publishing LLC. (e-i) Photonic-plasmonic nanofocusing probe in an AFM-based tip-enhanced Raman spectroscopy setup. (e-ii) Tip-enhanced Raman intensity map across the edge of a silicon nanocrystal grating. (e-iii) Intensity of the c-Si raman peak at 520 cm−1 (black dots with error bars, right vertical axis) and corresponding AFM line scan red squares (red squares without error bars, left vertical axis). Adapted with permission from De Angelis et al., Nat. Nanotechnol. 5, 67 (2010). Copyright 2010, Springer Nature. (f) Virtual nanofocusing probe with hyperbolic metamaterial (HMM) and plasmonic cavity lens. (f-i) Schematic of the device consisting concentric annular grating, HMM layers, and Ag-photoresist-Ag plasmonic cavity. (f-ii) and (f-iii) SEM images of the photoresist after exposure by the focusing probe at the spacer distance of 0 and 80 nm, respectively. Adapted with permission from Liu et al., Mater. Horiz. 4, 290 (2017). Copyright 2017, Royal Society of Chemistry.

FIG. 3.

Coupled nanoresonator probes. (a-i) SEM image of the aNSOM probe enhanced with nanoantenna. (a-ii) Simulated electric field vector distribution on a cross section of the antenna structure. (a-iii) Experimental verification of dependence of antenna efficiency on antenna length. Adapted with permission from Taminiau et al., Nano Lett. 7, 28 (2007). Copyright 2007, American Chemical Society. (b-i) Hybrid nanoaperture-antenna probe for dual color, nanoscale fluorescence imaging. Scale bar: 300 nm (b-ii) Confocal fluorescence image of a mixture of DiD and DiI molecules with 560 and 633 nm excitation lasers. (b-iii) Near-field fluorescence image obtained with the hybrid probe of the same sample using the same excitation lasers. Scale bar: 200 nm. Adapted with permission from Mivelle et al., Nano Lett. 14, 4895 (2014). Copyright 2014, American Chemical Society. (c-i) SEM image of the pyramid and C-shaped aperture structure fabricated by template stripping. (c-ii) and (c-iii) Microscope image of transmitted light from the C-shaped aperture pyramid with and without bright-field illumination. Adapted with permission from Lindquist et al., Sci. Rep. 3, 1857 (2013). Copyright 2013, Springer Nature. (d-i) Hybrid photonic-plasmonic cavity consisting of bowtie plasmonic antenna embedded in a 1D silicon photonic cavity. (d-ii) SEM image of the cavity. (d-iii) Transmission spectra of the hybrid cavity showing a Q-factor of 800. Adapted with permission from Conteduca et al., APL Photonics 2, 086101 (2017). Copyright 2017, AIP Publishing LLC. (e-i) Photonic-plasmonic nanofocusing probe in an AFM-based tip-enhanced Raman spectroscopy setup. (e-ii) Tip-enhanced Raman intensity map across the edge of a silicon nanocrystal grating. (e-iii) Intensity of the c-Si raman peak at 520 cm−1 (black dots with error bars, right vertical axis) and corresponding AFM line scan red squares (red squares without error bars, left vertical axis). Adapted with permission from De Angelis et al., Nat. Nanotechnol. 5, 67 (2010). Copyright 2010, Springer Nature. (f) Virtual nanofocusing probe with hyperbolic metamaterial (HMM) and plasmonic cavity lens. (f-i) Schematic of the device consisting concentric annular grating, HMM layers, and Ag-photoresist-Ag plasmonic cavity. (f-ii) and (f-iii) SEM images of the photoresist after exposure by the focusing probe at the spacer distance of 0 and 80 nm, respectively. Adapted with permission from Liu et al., Mater. Horiz. 4, 290 (2017). Copyright 2017, Royal Society of Chemistry.

Close modal

The above configuration still has some drawbacks, especially since the narrow spectral bandwidth of the antenna resonance limits the device to single excitation wavelengths. To overcome this, Mivelle et al. replaced the conventional circular aperture with a bowtie nanoaperture (BNA).114 At the edge of the BNA gap, a monopole antenna was planted [Fig. 3(b-i)]. Under incident polarization transverse to the gap of BNA, the local electric field is highly confined to the nanoscale gap region, and localized plasmons can be efficiently coupled to the nanoantenna whose apex diameter determines the spatial field confinement. Coupling between the BNA and nanoantenna loosens the dependence on the antenna length and broadens the spectral linewidth of the resonance of the hybrid mode. Mivelle et al. took advantage of this fact and showed that simultaneous excitation and imaging of two different colored dyes could be done with an optical resolution of ∼20 nm [Figs. 3(b-ii) and 3(b-iii)]. They also demonstrated that spectrally separated molecules can be resolved within 2.1 nm spatial resolution. Their work opens the door to new applications of near-field probes in biological imaging.

Since the spatial resolution is determined by the radius of curvature of the antenna, it is important to be able to fabricate “sharp” tips with large throughput and reproducibility. Conventional tip sharpening involves direct FIB milling, which introduces rough surfaces and rounding of the tip. Lindquist et al. reported the fabrication of sharp pyramidal tips using template stripping of silver deposited on a silicon preform whose tip was formed by anisotropic etching (<10 nm radius). The C-shaped aperture is formed by milling from the backside without contaminating the surface of the tip [Fig. 3(c-i)].85 The fabrication procedures are similar to that of the pyramidal plasmonic lens discussed earlier in Sec. III A. The authors demonstrated nanofocusing of their C-aperture-tip device indirectly using transmission measurements [Figs. 3(c-ii) and 3(c-iii)]. Incorporating such novel fabrication methods into aperture-antenna hybrid devices have a great potential in improving the spatial resolution of near-field probes.

2. Cavity-coupled nanoantenna

A variation of the photonic-plasmonic hybrid nanofocusing probe is a coupled cavity-antenna system. High-quality-factor photonic cavities have been widely utilized in sensing,115 microlasers,116 nonlinear processes,117 etc. But their mode confinement is diffraction-limited. On the other hand, plasmonic nanoantennae have high mode confinement but small quality factor. The hybridization of phonic cavities and plasmonic antenna results in a coupled device with remarkable quality factor to mode volume ratios.118,119 Conteduca et al. reported a photonic-plasmonic hybrid cavity that consists of a one-dimensional photonic cavity coupled to a bowtie plasmonic nanoantenna [Figs. 3(d-i) and 3(d-ii)].120 The hybrid cavity exhibits high transmission (20%) and possesses a high Q/V of 106 (λ/n)−3 [Fig. 3(d-iii)]. The principle of the cavity-antenna hybrid system can also be applied in the context of the near-field probe where the far-field radiation is coupled to the plasmonic nanofocusing tip with the assistance of a photonic cavity.121 De Angelis et al. demonstrated the first realization of a photonic-plasmonic nanofocusing probe in an AFM-based tip-enhanced Raman spectroscopy setup.122 In their device, a PC whose resonance is matched with the excitation laser wavelength was milled on an AFM cantilever made of Si3N4 [Fig. 3(e-i)]. At the center of the PC, a sharp metal tip was deposited as the plasmonic probe. The excitation laser beam was focused onto the PC. The coupling of PC mode to surface plasmon polariton on the tip led to nanofocusing that provided background-free illumination at the tip apex. Using this device, the authors were able to obtain the Raman image of silicon nanocrystal gratings with 10 nm spatial resolution [Figs. 3(e-ii) and 3(e-iii)]. Since cavity-antenna hybrid probes can be directly implemented into mature nanopositioning and mapping technology such as AFM, they have great potential to be widely applied in nanoscale material characterization.

For samples that have large topological variations or strongly interact with the probe, near-field imaging with a scanning probe can be challenging. For those samples, it is desirable to develop a virtual probe that relaxes the working distance between the imaging device and the sample while sacrificing lateral resolution. Such virtual probes can be achieved by means of a plasmonic cavity lens that provides high spatial frequency light sources, giving a resolution up to ∼λ/6.123,124 Liu et al. designed a concentric metasurface and plasmonic cavity lens consisting of silver-insulator-silver layers as shown in Fig. 3(f-i).124 In contrast to the plasmonic lens discussed in Sec. III A 2, the metal–insulator–metal plasmonic cavity in this case increases the ratio of transverse to longitudinal electric field components of the incident circularly polarized Bessel beam and focuses it to a subdiffraction spot of 70 nm at a working distance of 80 nm [Figs. 3(f-ii) and 3(f-iii)]. The tip-free nanofocusing of this setup has great potential in nonintrusive super-resolution imaging, especially in biomedical applications. In addition, the virtual probe can be integrated with physical plasmonic probes similar to the 3D plasmonic lens probe and could potentially lead to improved coupling efficiency and thus better sensitivity of TERS and other near-field microscopes.

In Secs. III AIII C, we have discussed how background suppression and spatial resolution of the near-field probes have improved by the development of hybrid systems where microscale photonic components are coupled to nanoscale plasmonic antennae. The major challenge of such hybrid devices is to eliminate the coupling losses from the photonic to plasmonic modes. Coupling efficiency can be improved by tailoring the overlapping length over which the two modes exchange energy.125–127 When the two nanowires are brought to contact, the evanescent fields interact with each other and mode coupling occurs. As shown in Fig. 4(a), the eigenmode guided in the input wire has an evanescent tail extending from the wire surface.128 When it reaches the overlap region, energy exchange occurs between the two wires, and light is coupled to the output wire after the overlap region. The coupling efficiency depends on the overlapping length of the two wires. As calculated by Huang et al., loss to the radiative modes at the coupling region of two silica wire is minimal at the optimal overlap length [Fig. 4(a)].128 In the grating coupling schemes where the impinging beam and the gratings have a very small interacting area, the overlap length cannot be engineered, and thus it is difficult to improve the coupling between the free space radiation and surface plasmon modes supported by the gratings.

FIG. 4.

Nanofocusing with nanowire couplers. (a) Evanescent coupling between two parallel silica nanowires showing the effect of overlapping lengths. Adapted with permission from Huang et al., Appl. Opt. 46, 1429 (2007). Copyright 2007, Optical Society of America. (b) Coupling between silica nanofibers, ZnO nanowires, and Ag nanowires. Adapted with permission from Guo et al., Nano Lett. 9, 4515 (2009). Copyright 2009, American Chemical Society. (c) SPP mode coupling from a fiber radial vector (TM01) mode. (c-i) Schematic of TM01-to-SPP mode coupling. Reproduced from Tuniz and Schmidt, Nanophotonics 7, 1279 (2018). Copyright 2018, Author(s), licensed under a Creative Commons Attribution License (CC BY-NC-ND 4.0). (c-ii) Microscope image of light scattered to the side from the nanotip for radially polarized input. (c-iii) SEM image of the nanoprobe end face showing a nanotip with 10 nm apex diameter (scale bar: 1 μm). Adapted with permission from Tuniz et al., Nano Lett. 17, 631 (2017). Copyright 2017, American Chemical Society. (d-i) Schematic of the mode overlap coupling of tapered fiber and nanowire. (d-ii) Near-field Raman spectra of R6G molecules obtained using the device in (d-i) as both excitation and collection probes. Adapted with permission from Kim et al., Nat. Photonics 13, 636 (2019). Copyright 2019, Springer Nature. (e) Schematic and SEM image of the plasmonic laser consisting of a CdS semiconductor nanowire, nanoscale MgF2 spacer layer, and silver substrate. Adapted with permission from Oulton et al., Nature 461, 629 (2009). Copyright 2009, Springer Nature. (f) Schematic illustration of a hybrid photon-plasmon NW laser consisting of an Ag NW and an ultralong CdSe NW coupled into an X shape. The dashed line indicates the coupled hybrid lasing cavity. Adapted with permission from Wu et al., Nano Lett. 13, 5654 (2013). Copyright 2013, American Chemical Society.

FIG. 4.

Nanofocusing with nanowire couplers. (a) Evanescent coupling between two parallel silica nanowires showing the effect of overlapping lengths. Adapted with permission from Huang et al., Appl. Opt. 46, 1429 (2007). Copyright 2007, Optical Society of America. (b) Coupling between silica nanofibers, ZnO nanowires, and Ag nanowires. Adapted with permission from Guo et al., Nano Lett. 9, 4515 (2009). Copyright 2009, American Chemical Society. (c) SPP mode coupling from a fiber radial vector (TM01) mode. (c-i) Schematic of TM01-to-SPP mode coupling. Reproduced from Tuniz and Schmidt, Nanophotonics 7, 1279 (2018). Copyright 2018, Author(s), licensed under a Creative Commons Attribution License (CC BY-NC-ND 4.0). (c-ii) Microscope image of light scattered to the side from the nanotip for radially polarized input. (c-iii) SEM image of the nanoprobe end face showing a nanotip with 10 nm apex diameter (scale bar: 1 μm). Adapted with permission from Tuniz et al., Nano Lett. 17, 631 (2017). Copyright 2017, American Chemical Society. (d-i) Schematic of the mode overlap coupling of tapered fiber and nanowire. (d-ii) Near-field Raman spectra of R6G molecules obtained using the device in (d-i) as both excitation and collection probes. Adapted with permission from Kim et al., Nat. Photonics 13, 636 (2019). Copyright 2019, Springer Nature. (e) Schematic and SEM image of the plasmonic laser consisting of a CdS semiconductor nanowire, nanoscale MgF2 spacer layer, and silver substrate. Adapted with permission from Oulton et al., Nature 461, 629 (2009). Copyright 2009, Springer Nature. (f) Schematic illustration of a hybrid photon-plasmon NW laser consisting of an Ag NW and an ultralong CdSe NW coupled into an X shape. The dashed line indicates the coupled hybrid lasing cavity. Adapted with permission from Wu et al., Nano Lett. 13, 5654 (2013). Copyright 2013, American Chemical Society.

Close modal

1. Optical fiber to nanowire coupling

One way to implement the scheme of parallel wire coupling is by using photonic-to-plasmonic nanowire couplers. Efficient light coupling can be achieved via the near-field interaction between metallic and dielectric nanowire waveguides that are brought into physical proximity with each other.126 Guo et al. demonstrated that a coupling efficiency of up to 80% can be achieved between zinc oxide photonic nanowires and silver plasmonic nanowires [Fig. 4(b)].129 Such hybridized couplers are a potential nanoscale light source for photonic circuits since they have lower loss than plasmonic wires and higher mode confinement than photonic wires.

An alternative fiber-plasmonic coupling scheme that makes use of higher-order radial vector modes in the fiber is proposed by Tuniz et al.130 In this scheme, the plasmonic nanowire is embedded at the center of the fiber core, rather than attached to the tapered end, providing mechanical stability to the device [Fig. 4(c-i)]. The radially polarized TM01 mode of the fiber is coupled directly to the radial plasmons on the center gold wire, which is then nanofocused at the tip apex upon exiting the fiber [Fig. 4(c-ii)]. The simple fabrication process makes use of a specialty fiber that bears a hollow nanochannel at the center of its core, which is filled with gold by pressure-assisted melting that leads to a sharp gold tip with an apex diameter around 10 nm protruding from the core after cleaving the fiber [Fig. 4(c-iii)]. Since the fiber TM01 mode to plasmon mode coupling efficiency is only weakly dependent on the beat length, the nanowire embedded inside the fiber can have a wide range of lengths, further relaxing the fabrication requirements. The collection efficiency of the device is estimated from simulation results to be around 2% at the 650 nm wavelength. Despite the promising performance of the device, its direct application in spectroscopic imaging has yet to be demonstrated. The application of a fiber-plasmonic-wire coupled probe in near-field imaging and sensing is promising in a large part due to the performance in the reverse mode, coupling light back from the plasmonic nanowire to the photonic fiber.

Kim et al. fully utilized the optical fiber to nanowire hybrid coupling scheme in their lens-free, fiber-in-fiber-out, TERS microscope.34 In contrast to a conventional TERS microscope where the gap plasmon mode between the metal tip and substrate is excited by the free space illumination, in their work, the nanoscale light source is attained by the adiabatic focusing of the propagating SPPs, coupled onto the silver nanowire via tapered optical fiber as shown in Fig. 4(d-i). They demonstrated the ability of photonic-plasmonic nanowire couplers to both deliver the light and collect the signal from a nanoscale hotspot by obtaining Raman signal from R6G molecules with low background noise [Fig. 4(d-ii)]. They were also able to map the chemical fingerprint of single-walled carbon nanotubes with 1 nm spatial resolution. This study has established a new platform for integrating low-cost metal nanowires onto optical fibers as an SPM probe for nanofocusing.

The low-cost probes based on chemically synthesized silver nanowires have also been integrated into AFM-based TERS setups. The defining feature of a nanowire-on-AFM-tip device is that instead of parallel-wire coupling, nanoparticles or nanostructures are introduced on the wire as a nanoscale plasmonic light source. Due to the unique nature of the coupling scheme, we will leave the detailed discussion about those probes for Sec. III E.

2. Nanowire laser

So far, all the nanofocusing probes that have been discussed use an external light source, implying the inherent loss of light energy going from micro- to nanoconfinement. Moving toward a truly compact, low loss optical near-field system, nanocavity lasers consisting of chemically synthesized metal nanowires discussed in Sec. III D 1 have a huge potential to be the integrated light sources. Nanometer-scale hybrid plasmonic lasers have been demonstrated by Oulton et al., who made use of gap plasmons between a semiconductor (CdS) nanowire and a metal (silver) substrate as shown in Fig. 4(e).131 By comparing the pure photonic and hybrid plasmonic modes, the authors showed that subdiffraction confined gap plasmons leads to amplified spontaneous emission of 52 nm diameter CdS nanowire and then to full laser oscillation at a pump fluence of 76 mW cm−2. In this hybridized scheme, the emission comes from the gap mode where the photonic and plasmonic modes overlap spatially, and outcoupling of light to a truly plasmonic mode is yet needed for adiabatic plasmon focusing. Thus, it will be challenging to implement this configuration as the excitation light source in a scanning probe microscope.

Wu et al. solved this problem by photonic to plasmonic coupling along the length of the wires.132 The authors devised an X-shaped coupler consisting of a CdSe nanowire and a silver nanowire. The spontaneously emitted light from the CdSe photonic nanowire is guided along its length and couples to the plasmon modes on the 100 nm diameter silver plasmonic nanowire in the waist region of the X-shaped structure as shown in Fig. 4(f). The excitation arm of the CdSe wire and the front end of the silver nanowire forms the cavity for nanolasing with the lasing threshold of 12 kW cm−2. By spatially separating the photonic and plasmonic ends of the nanolasing cavity, their work provided a foundation needed to integrate nanolaser sources in nanofocusing probes.

For the probes with parallel wire couplers discussed in Sec. III D, the coupling efficiency has been dramatically improved. However, the excitation from the photonic mode of the fiber will still contribute to the background signal. Targeted on background-free remote excitation, probes with nanoparticle-coupled plasmons on the chemically synthesized nanowire have been developed to eliminate the background but still take advantage of the nanofocusing of the propagating SPPs on the plasmonic nanowire. To enable efficient coupling, optical signal injection into plasmonic waveguides can be conducted by illuminating directly onto a symmetry-broken point, such as nanowire defects, kinks, wire intersections, or a nearby scatter [Fig. 5(a-i)].133–139 Incident light at these points can be scattered into components with a wide range of wave vectors, a part of which can match with SPPs on the nanowire and enable the excitation. When exciting SPPs via a nanoparticle, plasmons on the nanowire and nanoparticles mix and hybridize, yielding new “hybridized” plasmon states that couple the LSPs to SPPs [Fig. 5(a-ii)].137 The excited SPPs propagate and emit from both ends of the nanowire [Fig. 5(a-iii)].139 

FIG. 5.

Nanoparticle-coupled plasmonic probes. (a-i) Schematic of the excitation of SPPs on NW wire by illuminating directly onto a nanoparticle. (a-ii) Finite difference time domain calculation image of the coupling between the LSPs on nanoparticles and the SPPS on the wire. Adapted with permission from Hao and Nordlander, Appl. Phys. Lett. 89, 103101 (2006). Copyright 2006, AIP Publishing LLC. (a-iii) Optical image of the emission from both ends of the Ag nanowire. Adapted with permission from Knight et al., Nano Lett. 7, 2346 (2007). Copyright 2007, American Chemical Society. (b-i) Schematic diagram of remote-excitation TERS using gold nanoparticle-attached silver nanowire STM tip. (b-ii) SEM image of the probe. (b-iii) TERS spectra obtained via remote STM-TERS. Spectra from top to bottom: tip retracted; direct illumination by p-polarized light; remote illumination by s-polarized light. Adapted with permission from Fujita et al., Jpn. J. Appl. Phys. 55, 08NB03 (2016). Copyright 2016, Japan Society of Applied Physics. (c-i)–(c-iii) SEM images of the silver nanoparticle-attached silver nanowire probe for remote-excitation AFM-TERS. (c-iv) AFM image, TERS spectrum, and (c-v) TERS map cross section of a carbon nanotube obtained via remote TERS. Adapted with permission from Ma et al., Nano Lett. 19, 100 (2019). Copyright 2019, American Chemical Society. (d-i) SEM image, (d-ii) geometry, and (d-iii) a PL spectrum of a single InAsP quantum dot in an InP-tapered nanowire. Adapted with permission from Reimer et al., Nat. Commun. 3, 737 (2012). Copyright 2012, Springer Nature. (e-i) Scheme and (e-ii) design of a single-photon nonlinear device: well-aligned emitters (e.g., DBT molecules) intensely interact with the SPPs in a short plasmonic gold stripe waveguide (golden color). Adapted with permission from Kewes et al., Sci. Rep. 6, 28877 (2016). Copyright 2016, Springer Nature. (f) Schematic of the scanning nanospin ensemble microscope. The probe consists of a nanodiamond containing a small ensemble of electronic spins (symbolized by the red arrows) grafted onto the tip of an AFM. Adapted from Tetienne et al., Nano Lett. 16, 326 (2016). Copyright 2016, American Chemical Society.

FIG. 5.

Nanoparticle-coupled plasmonic probes. (a-i) Schematic of the excitation of SPPs on NW wire by illuminating directly onto a nanoparticle. (a-ii) Finite difference time domain calculation image of the coupling between the LSPs on nanoparticles and the SPPS on the wire. Adapted with permission from Hao and Nordlander, Appl. Phys. Lett. 89, 103101 (2006). Copyright 2006, AIP Publishing LLC. (a-iii) Optical image of the emission from both ends of the Ag nanowire. Adapted with permission from Knight et al., Nano Lett. 7, 2346 (2007). Copyright 2007, American Chemical Society. (b-i) Schematic diagram of remote-excitation TERS using gold nanoparticle-attached silver nanowire STM tip. (b-ii) SEM image of the probe. (b-iii) TERS spectra obtained via remote STM-TERS. Spectra from top to bottom: tip retracted; direct illumination by p-polarized light; remote illumination by s-polarized light. Adapted with permission from Fujita et al., Jpn. J. Appl. Phys. 55, 08NB03 (2016). Copyright 2016, Japan Society of Applied Physics. (c-i)–(c-iii) SEM images of the silver nanoparticle-attached silver nanowire probe for remote-excitation AFM-TERS. (c-iv) AFM image, TERS spectrum, and (c-v) TERS map cross section of a carbon nanotube obtained via remote TERS. Adapted with permission from Ma et al., Nano Lett. 19, 100 (2019). Copyright 2019, American Chemical Society. (d-i) SEM image, (d-ii) geometry, and (d-iii) a PL spectrum of a single InAsP quantum dot in an InP-tapered nanowire. Adapted with permission from Reimer et al., Nat. Commun. 3, 737 (2012). Copyright 2012, Springer Nature. (e-i) Scheme and (e-ii) design of a single-photon nonlinear device: well-aligned emitters (e.g., DBT molecules) intensely interact with the SPPs in a short plasmonic gold stripe waveguide (golden color). Adapted with permission from Kewes et al., Sci. Rep. 6, 28877 (2016). Copyright 2016, Springer Nature. (f) Schematic of the scanning nanospin ensemble microscope. The probe consists of a nanodiamond containing a small ensemble of electronic spins (symbolized by the red arrows) grafted onto the tip of an AFM. Adapted from Tetienne et al., Nano Lett. 16, 326 (2016). Copyright 2016, American Chemical Society.

Close modal

1. Nanoparticle-coupled remote TERS

To incorporate nanowire plasmonic waveguides into the application of SPM, You et al. reported a method to attach silver nanowires on a tungsten (W) tip (referred to as silver nanowire tip) and performed SERS using the tip.140 Fujita et al. adopted the method of You et al. and demonstrated atomic-resolution STM imaging and TERS spectroscopy using a chemically synthesized silver nanowire.141 Later, in order to remove the background signal from direct excitation, Fujita et al. developed a remote-excitation TERS probe utilizing gold nanoparticles attached to the silver nanowire probe to couple visible light into SPPs on the nanowire [Fig. 5(b)].142 Similar to other SPPs on plasmonic waveguides, adiabatic nanofocusing of SPPs to LSPs at the tip apex is used for nanoscale TERS measurements. Lower background TERS spectra were demonstrated on a benzenethiol-modified Au (111) substrate when collected with laser light focused onto a nanoparticle than with direct excitation of the nanowire apex. Similar to the grating-assisted probes, the far-field background Raman signal in the conventional TERS scheme was removed via remote excitation. However, the reported TERS signal for remote excitation was 10–15 times weaker than that of direct excitation.

Ma et al. significantly improved the remote-excitation TERS signal by carefully perfecting the morphology of the nanoparticles and the nanowire for efficient coupling and low SPP propagation loss [Fig. 5(c)].143 Colloidal silver nanocubes [Fig. 5(c-ii)] and silver nanowires [Fig. 5(c-iii)] with an ultrasharp conical tip and smooth surface were chemically synthesized. The pentagonal cross section of the silver nanowires allowed the silver nanocubes to sit with one face in complete parallel with one of its sidewalls to couple visible light into SPPs on the latter [Fig. 5(c-ii)]. The crystalline silver nanowire has smooth surfaces that minimize the SPP propagation loss to ∼0.4 dB/μm. Using ultrasharp conical tips sharpened with oxidative dissolution led to the nanoscale compression of the SPP mode at the tip apex to allow high spatial resolution TERS imaging. Ma et al. demonstrated high TERS contrast (up to 100) and fine spatial resolution [10 nm, Fig. 5(c-v)] using single-walled CNTs.143 Here, TERS contrast (Iengaged/Iretracted) is a direct measure of the signal increase by the tip.28 This reported TERS contrast is at least one order of magnitude better than most reported TERS signals. The simulation indicates that the coupling efficiency varies between 2% and 4% when the incident light is focused on the nanowire–nanocube junction.

2. Single-photon emitter in plasmonic antenna

Interestingly, the challenges involved in nanoscale chemical imaging have similarities to those in quantum optics. Single-photon emitters (SPEs) embedded in photonic nanostructures play a central role in a range of proposed quantum computing schemes for quantum-information encoding and processing.144,145 Highly efficient single-photon sources need to be implemented in a scalable quantum architecture with an efficient quantum interface between light (photons) and matter (quantum emitters). Light emitted by single emitters need to be efficiently coupled to and collected out of the dielectric and metallic nanostructures including waveguides and nanocavities.

Embedding stable single-photon emitters (such as a color center in diamond or a semiconductor quantum dot) in tapered photonic waveguides is one of the most promising methods to achieve near-unity light-extraction efficiency, which is necessary for a truly deterministic single-photon source.144,145 Reimer et al. fabricated such bright single-photon sources by positioning an InAs quantum dot SPE embedded in a tapered GaAs photonic nanowire [Fig. 5(d)].146 The light-extraction efficiency was increased by optimizing the photonic waveguide diameter to maximize the amount of light emitted from the quantum dot into the fundamental mode of the waveguide and by tapering the waveguide tip to minimize the total internal reflection at the semiconductor-air interface.

Alternatively, the quantum emitter in a plasmonic waveguide can efficiently couple single photons directly to a propagating optical mode over a wide bandwidth for immediate applications. Kewes et al. fabricated a single-photon device with single emitters integrated in plasmonic-dielectric waveguide structures [Fig. 5(e)].147 An efficient V-type photon-to-plasmon transducer transfers photons in a dielectric Si3N4 waveguide into a short SPP-waveguide with tight confinement. There, well-aligned emitters [organic dibenzoterrylene (DBT) molecules], positioned next to the plasmonic waveguide, intensely interact with the SPPs in the near field of the plasmonic waveguide. After the interaction, transmitted SPPs are converted into guided photons and eventually coupled into free space again.

As one possible future direction, these single-photon sources could utilize the various coupling schemes discussed in Secs. III AIII D to couple the emission to the fundamental mode of the waveguide and then into propagating SPPs that focus into nano-LSPs. This approach could potentially further enhance the single-photon light-extraction efficiency. The integration of these waveguide single-photon sources with nanoscale scanning probe spectroscopy is a highly promising direction. A probe integrated with a quantum emitter could be used as the single-photon source to test the quantum qubits via strong light-matter interactions.

3. Scanning quantum probe microscopy

Integrating single-photon emitters, e.g., a nitrogen-vacancy (NV) center in diamond, into a scanning probe microscope could also provide nanoscale quantum sensor capability, such as the magnetic field, electric field, pressure, or temperature in addition to detailed topography of the sample.148,149 Quantum sensors based on the spin resonance spectroscopy of solid-state spins provide tremendous opportunities in a wide range of fields from basic physics and chemistry to biomedical imaging. The NV center in nanodiamonds is a photostable point defect in a diamond lattice. The electronic ground state of the NV center is a spin triplet that can be manipulated by magnetic field, electric field, and/or optical approaches. Tetienne et al. demonstrated a scanning nanospin ensemble microscope by attaching a nanodiamond hosting multiple nanospin NV centers on an AFM probe.150 This scanning quantum probe microscopy imaged the sample's local magnetic fields by detecting the spin resonance of the NV centers. The use of multiple NV centers improved the acquisition time from tens of hours to an hour. The group also demonstrated nanoscale thermal imaging of a photoheated gold nanoparticle through spin resonance spectroscopy of the quantum probe.

Integration of an NV probe into an AFM can also be achieved by grafting a nanodiamond onto the tip of the AFM,151–154 or by fabricating an all-diamond AFM tip hosting an NV center.155,156 These approaches can be incorporated with the various plasmonic probes discussed in Secs. III AIII D to enhance the weak optical signal. The quantum probe spectroscopy can be used as a quantum sensor to characterize the magnetic field, electric field, pressure, or temperature in nanoscale.

Nanoscale spectroscopy and imaging require efficient delivery of light for a spatially nanoconfined excitation. We have reviewed and accessed various types of nanofocusing probes that convert optical energy between far-field radiation and localized surface plasmon fields at the probe apex for nanoscale Raman and photoluminescence imaging. In particular, we focused on probes that take advantage of propagation SPPs that converge and focus at the sharp end of a metallic tip. These types of probes have better field enhancement than directly illuminated tips since the SPP excitation is spatially separated from the tip-sample interaction and the background scattering can be greatly reduced. We categorized the probes based on the coupling scheme that is employed to excite SPP on the metal tip, commenting on their delivery and extraction performances as well as the complexity of the fabrication process.

The grating couplers are widely employed on readily available SPM probes. Gratings milled on the shaft of a metalized AFM probe remotely excite TERS signals for improved signal-to-noise ratios. Various carefully designed gratings on conical and pyramidal structures can serve as plasmonic lenses, which could further improve the sensitivity of the near background-free nanoimaging and can be fabricated via a more economically friendly template stripping method. On the other hand, both the signal intensity and the spatial resolution of the aperture NSOM tips have been improved by exciting surface plasmon polaritons, gap plasmons, or plasmonic nanoantenna resonances. Metal-coated conical fiber tips and commonly known campanile probes can be used to couple an internal optical WG mode of the optical fiber to a propagating SPP mode and/or LSP gap mode. Hybrid devices that incorporate multiple nanoresonators onto the tapered fiber improve the spatial resolution and bandwidth of the near-field fluorescence imaging. Hybridized photonic-cavity-plasmonic-antenna systems have also been utilized in TERS imaging to improve the signal-to-noise ratio. We expect that the next logical step is the combination of plasmonic probes such as the campanile probe with a grating-coupled plasmonic lens, which could further improve the enhancement of the electric field at the tip apex and, therefore, improve the TERS sensitivity.

Fiber-plasmonic coupled devices, where a metal nanowire is either embedded inside the fiber core or attached next to it, allow for the control of photon-plasmon coupling length and improve the coupling efficiency. We expect to see chemically synthesized metallic nanowires coupled to optical fiber probes emerging as a new and promising platform for low-cost nanospectroscopy. Moreover, we think that the integration of a fiber-nanowire platform and hybrid nanocavity lasers consisting of nanowires will lead to truly compact nanofocusing devices. The chemically synthesized nanowires have not only been integrated with optical fibers but also found their way into AFM-TERS setups. Similarly, nanoparticles are used as point scatterers for mode matching to the SPPs. A similar idea can be extended to the coupling of quantum light to scanning probes; for example, a single-photon emitter such as diamond NV centers attached to the AFM probe has been used as a scanning nanospin ensemble microscope. We speculate that these scanning quantum probes could benefit from various surface plasmon coupling schemes to couple to single-photon emitters, which will further enhance the light-extraction efficiency.

The scanning probe spectroscopy has seen great progress due to the new and improved nanofocusing probes at the convergence of fiber optics, plasmonics, quantum optics, and nanomaterials. These probes have provided a platform for novel approaches to the advancement of nanoscale chemical imaging. Furthermore, they have the potential to inspire the development of revolutionary spectroscopic techniques such as nanoscale spin spectroscopy and nanoscale thermal imaging.

This work was partially supported by the National Science Foundation (NSF) under Grant No. CHE-1905043 and the Office of Vice Provost for Research of Baylor University. The Donors of the American Chemical Society Petroleum Research Fund are acknowledged for partial support of this research.

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Khant Minn received his B.S. degree in physics in 2012 from Georgetown College, Kentucky. He taught high school physics and mathematics at Hebrew Academy in Miami Beach, Florida, before joining Baylor University's Ph.D. Physics program in 2015. His research interests are nanophotonics, plasmonics, optical imaging, and spectroscopy. He is passionate about educational outreach activities and is a founding member of the Baylor University's SPIE student chapter.

Blake Birmingham completed his B.S. degree in physics at the Baylor University in Waco, Texas, in 2014. He is currently completing a Ph.D. in physics at the Baylor University. He has experience in ultrahigh vacuum scanning tunneling microscopy, Raman microspectroscopy, and nanoscale plasmonic engineering. He is motivated by the invention of innovative technologies and the transition of laboratory techniques to industrial processes and techniques.

Zhenrong Zhang is an Associate Professor of Physics at the Baylor University. She received her B.S. and M.S. in physics from the Lanzhou University, China, and her Ph.D. in physics from the Institute of Physics, Chinese Academy of Science. After a postdoctoral position with Professor Erminald Bertel in University of Innsbruck, Austria (2002–2004), she moved to a joint postdoctoral position in Pacific Northwest National Laboratory (PNNL) with Zdenek Dohnalek and the University of Texas at Austin with J. Mike White (2004–2009). During her time at PNNL, she was awarded the M. T. Thomas award by the Department of Energy's Environmental Molecular Sciences Laboratory for her accomplishments in developing a molecular-level imaging technique for characterizing catalytic reaction events on catalyst surfaces using a scanning tunneling microscope. She joined the Baylor University in 2010 as an assistant professor of physics and was promoted to associate professor in 2016.

Zhang's research focus as a faculty member has been on understanding reaction mechanisms by imaging surface reaction processes at the atomic level using surface science techniques that operate in ultrahigh vacuum and at low temperature. These research works motivated her recent research interest that focuses on developing nanoscale chemical imaging techniques for investigation of the structure-functionality of the materials in realistic conditions. The hybrid technique of a scanning probe microscope and spectroscopy offers nanoscale topographical information together with chemical and optical information of the sample. She is the author of over 50 publications and holds one U.S. patent.

Although Zhang was ranked top 1% in high school, she never imagined herself to be a professor, especially in physics. She majored in physics in college by accident. But she found that she actually enjoyed it. Her advice to her 16-year-old self would be: “Don't be afraid to try something that is known to be difficult. You might surprise yourself.”