Past, present, and future of hybrid plasmonic waveguides for photonics integrated circuits

Current advances in nanofabrication technology have provided researchers with unprecedented control over material properties. Sophisticated nanofabrication processes, coupled with high-performance computing facilities, have ushered in transformative changes across various branches of science and technology, among which photonics is an outstanding example. Unlike conventional electronic circuits, which lag behind in terms of both high data storage and fast delivery, photonic circuits, incorporating devices such as resonators, waveguides, and their integration with electronic components,¹ have garnered increasing attention. Owing to their larger size compared with electronic connections, optical fibers acting as optical interconnects can efficiently transmit signals with approximately three times as much data.^2–5 

Nevertheless, despite their promise of enhanced signal delivery, the miniaturization of photonic devices has proved challenging owing to the diffraction limit. A potential solution to this problem has emerged in recent years with the emergence of plasmonic waveguide (PWG)-based devices.^6–21 Operating at the metal–dielectric interface, these devices possess the capability to confine and guide light at a sub-wavelength scale. PWG-based devices thereby enable device dimensions to be shrunk to those of electronic circuits while maintaining high-speed data transmission. Beyond sub-wavelength mode confinement, PWGs offer numerous advantages, such as strong electromagnetic field enhancement, optical gradient forces, surface enhanced Raman scattering (SERS),¹⁵ nonlinear optics, and optical trapping.

Over the last two decades, a variety of PWG types have been proposed, including, among others, stripe waveguides, metallic wires, metallic nanoparticle chain waveguides, wedge waveguides, and plasmonic channels,^7–10 slot PWGs,¹¹ dielectric-loaded PWGs,^12,17,18 plasmonic semiconductor–insulator–semiconductor (SIS) waveguide systems,¹⁹ metal–insulator–metal (MIM) PWGs,²⁰ and hybrid plasmonic waveguides (HPWs).^22–26 Despite the potential benefits, a significant challenge associated with guiding surface plasmonic (SP) waveguides below the sub-wavelength diffraction limit is the high loss incurred, as observed in slot waveguides with low L_p. Similarly, MIM PWG structures exhibit tight confinement, but suffer from very high loss. In summary, PWGs have emerged as ideal candidates for maintaining tight mode confinement with a longer L_p. Numerous types of surface PWG (SPWG) have been proposed recently, and Fig. 1 provides a pictorial representation of these different SPWG types.

The surface plasmon polariton (SPP) mode (also understood as the short SP mode) is generated at the interface between dielectric and metal in a PWG. SPPs represent ideal candidates for optical confinement at the sub-wavelength scale. However, conventional dielectric waveguides with long-range SPPs suffer from high optical losses and demanding fabrication requirements. To address this, a novel hybrid optical waveguide system has been introduced, comprising a dielectric nanowire separated from a metal surface.^27,28 The nanoscale dielectric gap in this HPW system achieves mode confinement through optical coupling with the PWG, resulting in low loss and a longer L_p. The concept of the HPW²⁶ aims to overcome challenges associated with pure PWGs, such as short L_p and high loss. In an HPW, a nanoscale low-index region is placed between the metal layer and a high-refractive-index semiconductor strip. HPWs can operate across a broad range of frequencies, offering low loss and nanoscale subwavelength mode confinement compared with pure PWGs. In HPWs, mode confinement occurs in the nanoscale dielectric region, which exhibits a lower optical loss than a PWG, where all optical energy is confined in the metal, leading to higher ohmic losses. Figure 2 presents a schematic illustration of basic HPW design. Figure 2(a) shows a front view of the vertical HPW coupler on a silicon-on-insulator (SOI) platform. The vertical HPW coupler provides even and odd mode switching between the two low-index regions separated by metal. Figure 2(b) shows the nanoscale hybrid plasmonic (HP) mode confinement of odd modes in the Si₃N₄ region. This confinement is due to plasmonic and photonic modes and leads to the formation of HPP modes with low loss and longer L_p. During the past two decades, there has been tremendous development of HPWs, which have evolved into a mature technology with diverse applications. Various types of HPWs have been proposed in recent years to serve different purposes, including ultrashort coupling,²⁹ low loss with variable nonlinearity,³⁰ plasmonic and photonic mode hybridizations,³¹ compact lasers,³² electro-optic modulators (EOMs),^33–35 biosensors,^36,37 polarization control devices,^38–40 thermo-optic switches,^41,42 polarization rotation (PR),⁴³ MIM HPWs,^44–46 and photodetectors (PDs).^47–50 

These HPW-based devices have not only found applications in optical communication but also expanded into domains such as sensing,^51–54 nonlinear photonics,^55–57 and deep learning.^58–60 The incorporation of HPW components into photonics helps to address the limitations of PWG, particularly excessive Ohmic losses, and provides functional standards for photonic integrated circuits (PICs). Despite the advances that have been achieved, however, there remains a need for further simplification of hybrid device structures to achieve high integration density in integrated HPW circuits.^61–66 Previous HPW-based devices have been complex, often consisting of multiple components. Integrating HPWs into PICs is a challenging task because of the complexity of multilayer fabrication. Various material configurations and systems have been explored to overcome these challenges, enabling feasible multilayer fabrication of HPW ICs. Materials such as silicon,^67,68 silicon nitride, gallium arsenide, indium phosphide, aluminum nitride, silicon carbide, and hydrogen silsesquioxane (HSQ) have been utilized, each offering unique properties for high-resolution electron-beam lithography and compatibility with diverse materials and fabrication processes.^69–86 

HPWs offer the dual advantages of high integration density and compatibility with CMOS technology, thanks to their matched refractive index and adaptability to existing SOI designs. Advances in nanoscale measurement and fabrication techniques have facilitated the integration of HPWs with PICs, combining the low propagation loss of HPWs with plasmonic confinement and providing an intermediate interface for coupling between plasmonic and photonic waveguides.^87–93 The synergy between PICs and HPWs has been experimentally examined, and tight confinement and low propagation losses in HPWs have been demonstrated.^94–96 Although experiments involving nonlinear processes based on PIC-compatible photonic and plasmonic platforms have been somewhat limited,^97–102 the potential for the development of future structures lies in their integration into more complex circuits featuring multiple modular and functional elements.^103–105 This aligns with the trajectory of development of traditional PICs, aiming for completely chip-based structures, the fabrication of which is feasible with normal manufacturing processes for photonic components and that allow streamlined integration with conventional technologies. The technological trajectory of HPW-based PICs extends beyond applications in communication and ventures into various other domains.

Among the various types of PWG that have been discussed above, HPWs are superior owing to their wide range of applications and advantages compared with other types of PWG. Table I presents a performance comparison of various type of PWG and illustrates the technological advantages of HPWs in comparison with pure PWGs.

Table I covers the various types of PWG with basic modal propagation properties. Among these various types, HPWs stand out as the most superior, possessing the lowest modal propagation loss while maintaining the highest L_p with nanoscale confinement. Exploitation of their properties could enable the development of various types of PICs, ranging from passive to active devices. On-chip integration of PICs and HPWs opens up new areas of application, such as sensing, quantum emission, machine learning, and carbon reduction and other environmental applications.

This review underscores the importance of HPWs in conjunction with PICs for next-generation technologies. It presents a comprehensive overview of past and current research efforts focused on HPWs and their sensing applications, spanning a range of devices from passive to active. The remainder of the paper is organized as follows. Section II delves into the utilization of passive PICs based on HPWs, including resonators, grating couplers, splitters, demultiplexers, and polarization management devices. Section III presents a thorough examination of active PICs based on HPWs. Section IV explores the hybrid integration of HPWs into various PICs, showcasing their applications across different domains. The paper concludes with Sec. V, providing insights into the prospects of these technologies in the future applications.

The synergy between HPWs and PICs has attracted much research attention owing to its relevance to a wide range of passive PICs, operating at wavelengths from visible to near-infrared. HPWs also have the advantage of providing a material platform compatible with complementary metal oxide semiconductor (CMOS) technology. This section presents a comprehensive discussion of passive HPW-based PICs with applications in domains as diverse as biology, communication, quantum information processing, sensing, computing, and energy. Considering properties including polarization, wavelength, and mode, the presentation encompasses passive HPW devices including interferometers, rotators, polarization beam splitters, resonators, couplers, and mode division conversion devices. Beginning with devices such as wavelength filters, refractive index sensors, and temperature sensors, the discussion emphasizes the pivotal role of HPW-based wavelength filters in wavelength division multiplexing (WDM) systems. As demonstrated by their use in structures such as resonators, gratings, interferometers, and arrayed waveguide gratings (AWGs), these filters can serve as single-channel drop filters or demultiplexing all-wavelength channels simultaneously. A crucial criterion for such applications is the ability to achieve a flat-top transmission band for filtering or blocking a desired spectral component.

Subsequent discussions consider recent advances in filters, featuring the typical structures illustrated in Fig. 3. As a versatile component within PICs, optical resonators have attracted considerable attention owing to their wide range of applications, for example, as light sources,¹⁰⁶ optical filters,¹⁰⁷ modulators/switches,¹⁰⁸ and optical sensors,¹⁰⁹ as well as to their contributions to nonlinear optics.¹¹⁰ The use of HPW-based devices enables exceptionally precise ring bending, holding great promise for the development of ultracompact resonators. Notably, the HPW resonators demonstrated to date exhibit subwavelength or even sub-micrometer bending radii, coupled with a Q factor ranging from 10² to 10³. The anticipated high Purcell factor, arising from the ultrasmall volume, proves advantageous for applications such as lasing, single-photon sources, or light–matter interactions. An experimental study of a composite HPW¹¹¹ has demonstrated promising characteristics as depicted in Fig. 3(a), with a reduced mode area of 0.002 mm², a low propagation loss of 0.03 dB/mm, a full width at half maximum of 4.8 nm, and a free spectral range of 44.8 nm.

Circular HPWs (CHPWs) have been reported to exhibit a quality factor of 320 and a high extinction ratio (ER) of 29 dB. Furthermore, CHPW ring resonators with a 2.5 mm radius achieved a normalized Purcell factor of 6507 in experimental trials.¹¹¹ In another study,¹¹² a micro-ring resonator based on HPWs demonstrated superior sensitivity, reaching 333.3 nm/refractive index unit. In comparison, a slot waveguide-based micro-ring resonator outperformed in terms of both Q-factor (593.6) and figure of merit (75.2). Another noteworthy design is the Si based hybrid dielectric-loaded PWG,¹¹³ which yielded an L_p of 0.35 mm with a tight mode confinement of 0.029 μm² with a wavelength span of 1490–1625 nm. This waveguide exhibited a Q factor of ∼1900 for a moderate magnitude of 1.25 μm². A sub-micrometer donut resonator featuring a pure dielectric access waveguide¹¹⁴ demonstrated the potential to provide long-distance optical interconnects without requiring an additional mode converter to integrate HP circuits with pure dielectric waveguides. These HPW-based ring resonators are crucial elements for passive photonic devices, serving as integral components for various applications, including filters, switches, and modulators, within integrated photonics. Mach–Zehnder interferometer (MZI) structures have also been implemented using HPWs to achieve broader bandwidth, improve sensing capabilities, and create wavelength-selective devices. Figure 3(b) shows an ultra-wideband directional coupler designed with HPWs as a 2 × 2 MZI switch. This HPW-based MZI switch not only provides a broad bandwidth, but also rapid switching times, low power consumption, and a small overall size.¹¹⁵ Such MZI-based devices also find applications in sensing. For instance, an on-chip HPW-based MZI designed for temperature and chemical concentration sensing has been proposed.¹¹⁶ This MZI configuration exhibits an outstanding temperature sensitivity of 243.9 pm/°C and a liquid concentration sensitivity of 437.3 nm/RIU, with total maximum loss and ER of 2.56 dB and >25 dB, respectively. A compatible SOI platform with HPW-based MZI for liquid refractive index sensors has also been proposed,¹¹⁷ in which MZI features a 20 μm-long hollow HPW in the sensing arm, achieving a sensitivity of about 160 nm/RIU, with an ER of >40 dB. A highly sensitive sensor for refractive index and pressure sensing using an MZI based on double slot HPWs has been proposed.¹¹⁸ It demonstrates a very high sensitivity in air as well as water, with values of 40 000 and 80 000 nm/RIU, respectively, as well as an 800 nm/MPa sensitivity for the pressure sensor. Bragg grating-based devices, known for their wavelength-dependent properties, have also been studied extensively.¹¹⁹ 

Figure 3(c) shows a schematic of Bragg reflectors and high-Q micro cavities based on Si HPWs.¹²⁰ These HP microcavities achieve a Q factor of 1235 at λ = 1550 nm, with low loss compared with conventional MIM or IMI plasmonic Bragg grating reflectors. The grating structure is mostly intended to reflect only the fundamental even mode, providing wavelength filtering with an ER of ∼16.7 dB. These devices exhibit ultra-high mode confinement and reduced propagation loss, making them promising for telecommunication systems and future PICs.¹²¹ A mode-selective grating reflector based on an HPW has been proposed¹²² to cater to the demands of wavelength-selective add-drop multiplexing systems. To address the increasing bandwidth requirements of future massively parallel chip multiprocessors, a compact and broadband HPW-based multiplexer and demultiplexer were proposed a decade ago.¹²³ This design, shown in Fig. 3(d), has been extended to an array waveguide grating for a mode multiplexing system, enhancing capacity for transverse electric (TE) polarization. More recently, a compact low-crosstalk mode demultiplexer using a triple plasmonic dielectric waveguide-based directional coupler has been proposed.¹²⁴ It features a reduced coupling length of 7.5 μm, an ultra-low mode crosstalk of −30 dB, an insertion loss (IL) of 0.32 dB at λ = 1550 nm, and an ER of >15 dB over the entire C-band. These developments showcase the versatility and potential applications of HPW-based structures in various photonic devices.

Polarization division multiplexing (PDM) plays a crucial role in manipulating optical signals for the chip-scale PICs that handle the increasing demands for transmission in optical communication systems. Essential components in this context include PRs, polarization beam splitters (PBSs), and polarization splitter–rotators (PSRs). This review will first present results on PBSs based on HPWs for PICs, followed by a discussion of PSRs, which combines the main functions of PRs and PBSs. PBSs are crucial elements that separate transverse magnetic (TM) and TE modes, enabling the independent processing of these two polarization modes and effectively doubling the traffic bandwidth. In the realm of optical HPWs, one of the key components is the PBS, which splits the HP mode into different ports and couples it to TE and TM modes to implement polarization splitting functions.¹²⁵ Much research effort has been dedicated to the development of different methods and structures for implementation of PBSs, including investigations of mode evolution and multimode interference¹²⁶ and various structural designs such as MZIs,¹¹⁶ photonic crystals,¹²⁷ gratings,^128,129 and directional couplers.¹³⁰ A comprehensive examination of the work that has been done on PBSs reveals that directional coupling stands out as the simplest and most flexible design.

Figure 4 presents a variety of PBSs and polarization beam reflectors (PBRs) employing HPW devices. These components play a crucial role in manipulating polarized light within PICs, demonstrating their versatility and potential for enhancing optical communication systems. The assessment of PBSs involves multiple criteria, such as device dimensions, IL, polarization ER (PER), operational bandwidth, structural complexity, and acceptable fabrication tolerances. The pursuit of minimal PBS dimensions while maintaining acceptable device performance is essential in order to construct coherent receivers and is also of crucial importance for the development of the next generation of ultra-dense PICs. Figure 4(a) depicts an ultracompact broadband HPW-based PBS with a significantly reduced length on the sub-micrometer scale of 920 nm.¹³¹ This design offers high PERs of 19 and ∼18 dB for TE and TM modes, respectively, and low ILs of ∼0.6 and ∼0.3 dB, respectively, over a wide bandwidth of about 400 nm (i.e., λ = 1300–1700 nm). Additionally, as shown in Fig. 4(b), this PBS operates in the mid-infrared region,¹³² where it achieves an ER of ∼19.78 dB and an IL of nearly 1.64 dB for the TE mode, and an ER of 7.78 dB and an IL of 2.64 dB for the TM mode while operating at λ = 3.5 μm. Figure 4(c) demonstrates the high performance of a compact PBS on an x-cut lithium niobate on insulator (LNOI) platform, covering the entire C- and L-bands with at least 200 nm bandwidth and excellent ER.¹³³ In addition to directional coupling-based PBSs, an alternative design for achieving high ER features a symmetric three-core HP splitter in the communication wavelength range.¹³⁴ This design, shown in Fig. 4(d), demonstrates high extinction ratios (>38 dB and >40 dB) for the output ports, with minimal losses (<0.1 dB and <0.5 dB). A compact and broadband TE pass and TM stop polarizer based on a HP grating on an x-cut LNOI platform has been shown to achieve an ER over 20 dB for λ = 1470–1700 nm, with an IL below 2.3 dB in the C-band.¹³⁵ 

PSRs combine polarization rotation and splitting simultaneously to realize dense polarization diversity circuits. An asymmetrical cross shape is required to break the waveguide symmetry in PSR-type waveguides.

There are two primary types of PSRs: based on mode evolution and mode coupling, respectively. In the case of mode-evolution-based PSRs, one approach involves transforming the TM₀ mode into the TE₁ mode within a mode evolution waveguide. Subsequently, a mode converter is employed to convert the TE₁ mode back to the TE₀ mode. This transformation between the TM₀ and TE₁ modes requires a mode hybridization region within a high-index-contrast optical waveguide featuring an asymmetrical cross-section. Around a decade ago, an ultracompact HP polarization rotator, designed for integrated Si photonics circuits and operating at approximately λ = 1.55 μm, was experimentally demonstrated.¹³⁶ This design achieved rotation of TM polarization in a Si waveguide to TE polarization, exhibiting a low IL of 1.5 dB and a PER exceeding 13.5 dB.

Although the designs described above do not primarily involve coupling to the hybrid modes, they are based on a combination of mode coupling and mode evolution. To overcome this limitation, a polarization rotation and coupling (PRC) scheme based upon mode-evolution was introduced.¹³⁷ This scheme rotates the TE mode adiabatically and couples the rotated mode into the HP mode. Mode-evolution-based PRs with a coupling scheme have achieved a high coupling efficiency (CE) of 92%, a broad spectral range of over 64% covering more than 500 nm, and near-zero reflection. This innovative approach enhances the performance of PSRs by concentrating on efficient coupling to hybrid modes and expanding the operational bandwidth.

Grating couplers (GCs) play a crucial role in PICs, serving as a key component through which signals enter and exit a circuit. Their efficiency and compactness are of paramount importance for the overall performance of the circuit¹³⁸ GCs based on a Si waveguide and SOI platform and covering the mid-IR region have recently been proposed.^139,140 GCs offer the advantage of being placeable anywhere on a chip, enabling signals to enter through vertical coupling. While plasmonic GCs are less common than fully dielectric photonic GCs owing to their higher losses, the combination of inverse design with plasmonics holds promise for creating small GCs with lower losses than conventional designs of plasmonic gratings. An on-chip GCs designed for Si HPWs has demonstrated a theoretical CE of 79% at λ = 1550 nm. This coupler achieves a 3 dB bandwidth at about 73 nm between fiber and a 100 nm wide Si HPW incorporating a bottom metal layer.¹⁴¹ Another notable development is a compressed vertical coupler designed for dense PICs based on an HPW. This coupler features a wavelength-scale footprint of 1.07 × 0.62 μm². Finite element simulations indicate a maximum CE of −3.4 dB across the bands C-, L-, and U-bands for telecommunications, as well as a 230 nm wider 3-dB bandwidth. Notably, this CE surpasses by more than 9 dB those of recently reported dense plasmonic couplers.¹⁴² Furthermore, a plasmonic GC, incorporating both apodized and periodic gratings, has demonstrated experimentally a 1-dB bandwidth of 115 nm with a CE of 2.9 dB.¹⁴³ A nanophotonic vertical coupler has been proposed with an ultrashort coupling length of 0.461 μm. This coupler is based on an HPW featuring a lateral subwavelength grating. Its design is aimed at improving the efficiency and performance of GCs in the context of dense PICs.¹⁴⁴ 

This section discusses the development of HPWs for PICs. The review considers mainly HPW-based active devices for high-speed modulation,^145–150 switching,^151–153 and photodetection.^154–158 Optical modulators based on HPWs offer several advantages with regard to speed, modulation depth, device voltage, temperature sensitivity, compactness, optical loss, polarization dependence, and wavelength range. An example of such a device is the HPW-based optical phase modulator proposed in Ref. 145, where, a π phase shift in a 13 μm-long PWG has been achieved. This optical modulator boasts a modulation bandwidth of up to 100 GHz, with a low power consumption of about ∼9 fJ/bit and a CE of 91%. Figure 5 illustrates various types of HPW-based modulator. A notable development is the ultra-compact Si/In₂O₃ HPW modulator presented in Fig. 5(a). This utilizes an asymmetric DC, consisting of a Si photonic waveguide and Si/In₂O₃ HPW.¹⁴⁶ With its 3.5 μm-long asymmetric DC, the fabricated waveguide modulator demonstrates high performance, exhibiting a modulation bandwidth of >40 GHz.

Graphene has emerged as a promising material for the creation of highly efficient and broadband electro-optic devices, particularly when combined with HPWs. Graphene is employed for its favorable properties, including high carrier mobility and broad absorption capabilities, as well as its high responsivity in Si-based slot waveguide photodetectors (PDs).^147,148 Figure 5(b) shows a PSW with a Si/hBN/graphene/dielectric/graphene/hBN/Si stack.¹⁴⁹ On the application of an electric field and by adjusting the carrier density in graphene, the real part of the permittivity can be manipulated to become zero and even negative. This modulation induces a plasmonic mode that exhibits high losses in the middle waveguide, effectively attenuating the optical signal. This modulator achieves impressive performance metrics, including a high ER of 11.01 dB/μm and a wide 3 dB modulation bandwidth of 72.2 GHz, with a low power consumption of 19.36 fJ at λ = 1.55 μm. Figure 5(c) shows a tunable graphene-based HP modulator that provides nanoscale light confinement with low loss along with a notable modulation depth of nearly 0.3 dB/μm.¹⁵⁰ Described in detail in Ref. 151, this modulator features low transmission loss and high speed, and serves as an electro-absorption modulator within an HPW at about 1.55 μm. Modulation of absorption is achieved by adjusting the in-plane permittivity of anisotropic graphene, covering a substantial operating bandwidth of 0.4 THz. The modulator has a modulation length of 8.5 μm and a compact footprint of 1.6 μm². To facilitate efficient waveguide coupling, a tapered Si coupler is employed, resulting in an 80% CE. The HP modulator demonstrates a 3 dB ER for an 8.5 μm-long device. Its compact size, with a power consumption of 17 fJ/bit and a modulator footprint of 0.18 μm², makes it highly efficient while having a smaller footprint. Graphene-based HPWs not only excel in nanoscale confinement with low loss, but also demonstrate excellent performance in broadband mid-IR SPP propagation and modulation, as explained in Ref. 152.

Key requisites for PDs include low power consumption, compatibility with Si technology, wide bandwidth, high efficiency, and low optical loss. Si-based PDs have become popular owing to their distinctive advantages of Si material, high cost-effectiveness, ease of fabrication, high performance, and compatibility with CMOS technology. PDs made of pure Si have been extensively developed, demonstrating outstanding performance at shorter wavelengths (<1.1 μm), where Si possesses a high absorption coefficient.¹⁵³ However, at longer wavelengths λ > 1.1 μm, such as mid-IR or near-IR, Si becomes transparent, with nearly zero absorption. In such instances, the incorporation of other materials with high absorption coefficients at λ > 1.1 μm becomes necessary.^154,155 The operational wavelength range of semiconductor PDs is generally constrained by the bandgap of the semiconductor material. For instance, Ge/Si PDs or III-V/Si are not suitable for the mid-IR range, which is of significant interest for applications like optical sensing. Referred to as the “fingerprint region,” the mid-IR range 2–7 μm is characterized by distinct absorption spectra of many molecules, making it crucial for certain applications. Figure 6 presents various types of PDs based on HPWs that have been designed to overcome the limitations associated with the photodetection range beyond 1.55 μm. An ultra-compact on-chip photothermal power monitor utilizing Si HPWs is featured in Fig. 6(a).¹⁵⁶ The small footprint of this design enhances its response speed and thermal efficiency. At a bias voltage of 2 V, an exceptionally high responsivity of 17.7 mV/mW has been measured, with a power dynamic range of 35 dB and a rise time of ∼3.1 μs. This power monitor exhibits a flat response covering the range λ = 1.3–7.8 μm, with potential extension beyond 8 μm through the use of alternative platform materials such as Ge-on-Si or SiN. Continuing the present focus on high-responsivity and high-speed Si graphene waveguide PDs for the region beyond 1.55 μm, Fig. 6(b) illustrates the utilization of an HPW with an ultrathin wide Si ridge.¹⁵⁷ This PD, operating at 2 μm, demonstrates a measured 3 dB bandwidth exceeding 20 GHz, with a responsivity of ∼70 mA/W at V_b = −0.3 V for P_in = 0.28 mW. The ultrafast photodetection capability has been verified with measurement of the frequency responses at λ =1.55 μm, revealing a 3 dB bandwidth exceeding 40 GHz. Simultaneously, the measured responsivity has been found to be about 0.4 A/W at V_b = −0.3 V for P_in = 0.16 mW. Figure 6(c) shows an avalanche PD based on an HPW fabricated from a two-dimensional material, namely, Si/black phosphorus.¹⁵⁸ This device exhibits high responsivity and high-speed operation at λ = 1.55/1.95 μm. The measured 3 dB bandwidth is found to be as high as 1.05 GHz, with a responsivity of 66 A/W at an optical power of 2.9 μW in the wavelength band of about 1.55 μm, while the responsivity is as high as 125 A/W at an optical power of 1.4 μW in the 1.95 μm wavelength band. Figure 6(d) shows a compact PD based on a graphene-loaded asymmetric HPW.¹⁵⁹ With this design, enhanced HPW absorption of up to 32.1% is achieved for a length of 5 μm. An HPW-based PD consisting of a high-index-material cap placed on top of a waveguide-fed configuration is described in Ref. 160. This device demonstrates a remarkable near 100-fold enhancement compared with previous similar PDs, exhibiting an approximate R value of 35 A/W.

Recent advances in photonic and plasmonic devices have led to the proposal of on-chip optical switches and optical modulators based on phase-change materials.^161–163 These devices are pivotal for applications including optical memories, optical computing, signal processing, and in-memory computation. In a novel design presented in Ref. 164, a plasmonic switch based on phase-change materials has been introduced for use in an integrated Si photonics platform. This switch incorporates a slot-based PWG with phase-change material serving as an active material coating on the surface of the slot waveguide. It demonstrates excellent performance, with features such as a high bandwidth >400 nm, a large ER > 28 dB, a low power consumption, and a compact footprint. The insulator–metal phase transition of vanadium dioxide, a correlated electron material, has also been harnessed for electrically controlled, low-voltage, and compact plasmonic switches. These micrometer-scale devices operate near λ = 1550 nm, achieving switching bandwidths exceeding 100 nm and requiring only 400 mV for ER in excess of 20 dB.¹⁶⁵ Optical switching achieved through the resistive switching effect via lateral coupling between an input nanophotonic waveguide and an output waveguide at λ = 1550 nm has been demonstrated in Ref. 166 and has provided a high optical ER of 27 dB for a 20 μm-long device. In a recent development, an HP photonic device designed for on-chip all-optical switching and reading of ferrimagnetic bits with perpendicular magnetic anisotropy in a racetrack spintronic memory has been proposed.¹⁶⁷ This hybrid device bridges the gap between nanoscale spintronics and integrated photonics, addressing challenges such as weak magneto-optics, nonlinear absorption in waveguides, and size mismatches. The creation of such a device holds promise for achieving ultrafast as well as energy-efficient sophisticated on-chip applications.

HPWs have a vital role in the design and functionality of on-chip photonic devices. The fundamental structure of an on-chip photonic device involves a waveguide, and an HPW is constructed as a hybrid structure with a dielectric core that is sandwiched between a metallic layer-based plasmonic structure and another dielectric material. This HP architecture combines the favorable respective characteristics of traditional photonics and plasmonics, allowing for strong field confinement, high optical intensities, and the potential for significant nonlinear effects. The use of HPWs is particularly advantageous in lab-on-a-chip applications, where subwavelength light confinement and nanoscale waveguiding are essential. HPWs strike a balance between achieving strong field confinement and minimizing propagation losses. While simple HPWs have been proposed for this task, the ongoing challenge is to integrate them into more complex circuits with a number of elements that are functional in nature, similarly to conventional PICs. In an ideal scenario, HPWs would be seamlessly integrated into completely chip-based structures, accessible using ordinary photonic techniques and facilitating further integration with conventional technologies. Thus, HPWs have the potential to be pivotal in the development of nonlinear optical devices, on-chip photonic signal processing, and quantum information processing. Here, we discuss the various type of on-chip PIC-based HPWs for different applications. Recently, several on-chip HP applications have been proposed and explored, showcasing the versatility and potential of HPWs in various domains. We shall examine some of these applications here.

• HP-based light steering concentrator. A numerical simulation study of a proposed on-chip HP light steering concentrator demonstrated nearly 96% CE.¹⁶⁸ This type of device could be crucial for efficiently concentrating light on a chip, enhancing the performance of photonic circuits.

• Modular nonlinear HPW circuit. An on-chip HP integrated circuit (HPIC) operating at λ = 1.32 μm has been designed, fabricated, and characterized. It consists of two different in-series plasmonic-based elements on an SOI waveguide, namely, a focuser and a mode converter.¹⁶⁹ Such circuits could prove valuable for spectroscopy, on-chip quantum photonics, terahertz sources, and sensing.

• On-chip polarization control. An ultracompact on-chip waveplate utilizing an asymmetric HPW has been proposed and experimentally demonstrated.¹⁷⁰ It achieves a high polarization conversion efficiency from TE to TM, reaching as high as 99.2%. This could be crucial in applications where precise polarization control is needed.

• On-chip HP nano-antenna. Numerical simulations of the proposed on-chip V-shaped nano-antenna shown in Fig. 7(a) indicate that it has high gain and wide bandwidth. Such a nano-antenna could receive and transmit optical signals at telecommunication wavelengths with high realized gains.¹⁷¹ 

• On-chip quantum emitter. An on-chip HPW-based quantum emitter (QE) for plasmonic single-photon sources has been proposed.¹⁷² As shown in Fig. 7(b), light from the QE is coupled to the on-chip HPW functioning as a PR. This is crucial for quantum information processing applications.

• On-chip Raman detection and optical trapping. This has been demonstrated using an HP nanofocusing waveguide.¹⁷³  Figure 7(c) shows a proposed SERS tweezer combining SERS detection and optical trapping in an on-chip photonic waveguide. This could find applications in optical manipulation and sensing.

• Compact integrated biosensors. HPWs have been utilized for biosensing applications, an example being the compact integrated biosensor with high sensitivity shown in Fig. 7(d).¹⁷⁴ This offers a sensitivity of 270 nm/RIU with a compact footprint.

These applications highlight the broad spectrum of possibilities that on-chip HP devices can offer, ranging from quantum optics and communication to biosensing and optical manipulation. The integration of HPWs into on-chip photonic circuits opens up new avenues for compact and efficient devices in various fields.

This review has showcased the great versatility and wide range of applications, both potential and already realized, of HPW-based devices. Here, we present a summary of these diverse applications and the future prospects for HPW-based PICs. HPWs play a pivotal role in guiding light for communication purposes, in enhancing data transmission and bandwidth, and in enabling precise control and manipulation of optical beams, thereby contributing to advances in beam steering and focusing. Furthermore, the wireless optical communication that such devices offer provides a potential way to achieve high-speed, short-range wireless communication between chips. In sensing technology, HPW-based devices provide improved sensitivity and performance. In optical computing, HPW-based devices contribute to the development of compact and efficient photonic devices. The filtering, bending, and splitting capabilities of HPWs can be exploited in devices to enhance the functionality of photonic circuits. The efficient optical signal emission in HPW-based devices can be beneficial for applications in light sources and displays. HPW modulators based on microwave photonics have the potential to extend the range of applications toward the sub-terahertz range, crucial for applications such as 5G wireless and the Internet of Things (IoT). HPW-based Mach–Zehnder modulators (MZMs) are expected to meet the strict requirements of high-performance analog applications, again including 5G wireless and the IoT, as well as antenna remoting. The advances associated with HPW-based PICs hold the promise to revolutionize various areas of technology, including wireless chip-to-chip communication, machine learning, artificial intelligence, medical science, biosensors, and carbon reduction. Densely integrated circuits reflecting the compact nature of HPWs allow the creation of highly dense PICs, paving the way to further device miniaturization and increased efficiency. The demonstration of MZMs with speeds of up to 500 GHz highlights the potential of HPWs to meet the demands of emerging technologies and applications.¹⁷⁵ As research and development continue, HPW-based devices are poised to play a vital role in shaping the future of photonics and integrated circuits.

The future applications and developments in HPW technology are indeed promising and diverse. In the context of renewable energy and carbon reduction, HPWs are being explored for artificial photosynthesis and have demonstrated their potential for efficient sunlight harvesting, hydrogen generation, and carbon dioxide reduction.¹⁷⁶ Nano-antennas based on HPWs offer advantages in biosensing applications, providing enhanced sensitivity for tasks such as fluorescence detection and label-free vibrational spectroscopy.¹⁷⁷ Machine learning-based beam steering in HP nano-antenna arrays has prospects for applications including optical communications, ranging, biological sensing, light detection, imaging, and special beam generation.¹⁷⁸ CMOS-compatible HP ring resonator-based biochemistry sensors, designed for compatibility with CMOS technology, offer an ultra-compact and highly sensitive approach to biochemical sensing.¹⁷⁹ The development of wireless chip-to-chip communication is key to future applications of HPWs, overcoming the limitations of wired connections and offering ultrahigh capacity and energy efficiency.¹⁸⁰ Integration with the IoT, medical devices, and artificial intelligence, as well as combinations of HPWs with various platforms and materials, hold the potential to address challenges in a number of areas. The development of HPW-based devices can contribute to solving environmental problems, possibly through innovations in energy harvesting and reduction of carbon emissions. As the diversity of these applications demonstrates, HPW technology is positioned to have a vital role in shaping the future of various industries and technologies, contributing to advances in sustainability, healthcare, communication, and more. Continued research and development in this field will likely unlock even more possibilities for addressing complex challenges in our rapidly evolving world.

This work has not been supported by any funding agencies.

The authors have no conflicts to disclose.

Data sharing is not applicable to this article as no new data were created or analyzed in this study.