The nonlinear optical effect of second harmonic generation can be very strong when originating from nanoplasmonic structures, due to enhancement of the surrounding material's intrinsic non-linear optical properties or due to its occurrence as a result of the plasmonic structure. However, manufacturing of large-scale three dimensional nanoplasmonic structures is still a challenge. Here, we demonstrate the two-photon luminescence and second-harmonic generation in a Bi2O3-Ag eutectic-based metamaterial exhibiting a hierarchic structure of nano- and micro-sized silver precipitates. The investigations employed a microscope system combined with polarimetric analysis. It appears that the second-harmonic-generation arises from the silver plasmonic structure rather than from the nonlinear effects of the bismuth oxide matrix. Both quadrupolar and dipolar modes of polarization are observed.
Nonlinear effects such as the second-harmonic generation (SHG) can be very profound when originating from nanoplasmonic structures which enhance the nonlinear optical properties of the matrix or give rise to significant nonlinear optical effects by themselves.1 Thus, there is a strong interest in investigation and development of nanoplasmonic materials. In standard nonlinear materials, the SHG effect arises from the noncentrosymmetry of a material. In the case of nanoplasmonic materials, even small centrosymmetric nanoparticles can provide a nonlinear response, due to symmetry breaking on the nanoparticle surface and electromagnetic field enhancement. In the case of bigger plasmonic particles, it can also occur due to retardation effects.2,3 With plasmonic structures, the nonlinear effects can be enhanced by plasmonic resonances in the incident or/and generated SH beams.2 The enhancement has been shown both when propagating plasmon polaritons could be generated4 and for localized surface plasmon resonances (LSPRs) in finite nanoplasmonic elements.5 Plasmonic structures can be used to enhance the nonlinear properties of dielectric materials.6 However, the SHG from the plasmonic structure itself is often as high as from the same plasmonic structure combined with a nonlinear material.7 While the phase matching, necessary in nonlinear crystals, is absent in plasmonic structures due to size effects, it is compensated by mode-matching.8 Metallodielectric materials which possess the plasmonic properties are intensively studied due to their potential applications in various fields such as photonics, photovoltaics, medicine, and others. However, the fabrication methods used to obtain large-scale three-dimensional structures are still a challenge.
Eutectic self-organization is a promising method for manufacturing volumetric highly crystalline nano- and microplasmonic materials9,10 and metamaterials.11–13 It enables the combination of two crystalline phases in one composite for a wide range of materials, e.g., nonlinear materials and metals, often in a special self-organized micro/nanostructure. It also provides possibilities for post-growth engineering of the obtained materials. When necessary, they can be easily doped with optically active elements.9,14,15 However, neither plasmon-enhanced nonlinear optical processes, nor their manifestation has yet been demonstrated in bulk metallodielectric structures obtained by eutectic solidification. Here we demonstrate the two-photon luminescence (TPL) and second-harmonic generation in a volumetric eutectic-based Bi2O3-Ag nanoplasmonic metamaterial.
Bismuth oxide, Bi2O3, is characterized by a high refractive index16 which can be accompanied by large third-order optical nonlinearities, so it has an excellent potential for applications in optical communication devices. For example, glasses which contain Bi2O3 can exhibit 15–60 times larger third-order non-linear optical susceptibilities χ(3) than that of pure silica glass,17 which makes them potentially useful for all-optical signal processing.18 Zhou et al.19,20 demonstrated a high third-order nonlinearities in thin films of Ag nanoparticles embedded in Bi2O3 matrices manufactured by co-sputtering of Ag and Bi2O3 in a multi-target magnetron sputtering system. The highest value (χ(3) = 4.13 × 10−10 esu)21 obtained at 35.7% concentration of silver was assessed to be two orders of magnitude higher than that reported in the literature for pure bismuth oxide. According to Zhou et al.22 these results suggest that Ag:Bi2O3 composite films could be applied in ultrafast all-optical switches.
In this work, the Bi2O3–Ag eutectic composite was grown in the Institute of Electronic Materials Technology (ITME) in Warsaw from pure starting materials of bismuth oxide powder (Alfa Aesar, 99.99% purity) and silver (Alfa Aesar, 99.95% purity). The materials were mixed with isopropanol in an alumina mortar with a composition of 84.6 mol. % Bi2O3 and 15.4 mol. % Ag, leading to 7.8 vol. % of Ag in the composite. Materials were obtained by the micro-pulling down method under a N2 atmosphere.23 This method has been previously applied for the growth of single crystals,24,25 eutectic materials,11,26 and nanoplasmonic materials based on direct doping of glass matrices with plasmonic nanoparticles.27 Additionally, after growth, the samples were annealed in an air atmosphere at 600 °C for 10 h.
In the Bi2O3-Ag composite, the Ag phase is rather homogeneously distributed as a net of triangular micron-scale precipitates interconnected with Ag nanosheets with lengths of a few/tens micrometers, Fig. 1. The triangular precipitates are elongated in the growth direction and their size as well as the thickness of the nanosheets mostly depend on the pulling rate. For high pulling rates, the thickness of the nanosheets is in the range of tens of nanometers. After annealing in an air atmosphere, silver also diffuses into the Bi2O3 phase, and Ag and Bi nanoparticles are formed.9 This material exhibits localized surface plasmon resonance at ∼595 nm due to the presence of silver nanoparticles, with the localized surface plasmon resonance (LSPR) wavelength controlled by annealing conditions.10,12 The enhancement of Er3+ luminescence in the Bi2O3-Ag composite demonstrated in our laboratory was the proof of plasmon-enhanced linear optical properties in a eutectic-based material.9 The eutectic composition enables us in the case of this material to distribute silver relatively homogeneously in the bulk of the sample. Thus after annealing, it enables us to distribute homogeneously silver nanoparticles in the bulk material. This is rather a rear case. It is rather difficult to find other methods which would enable to obtain metallic nanoparticles inside a crystalline material.
Here, we investigated the nonlinear optical effects observed in this material using a home-built two-photon microscope system combined with polarimetric analysis. The measurements were performed at Wroclaw University of Science and Technology. Excitation from a Ti:Sapphire laser (Quantronix Ti-Light, 100 fs, repetition rate 80 MHz) with 20 mW incident power was applied with an incident wavelength λ = 830 nm. We applied the Nikon Plan Fluor 40×/0.75 objective to excite the sample. The emission was collected in an epi configuration, after the filtering with dichroic mirror and appropriate short-pass filters in order to block the laser light. The emission is basically confined in three-dimensions to a small volume, where the intensity of the incident radiation is sufficiently high.
In the case of both as-grown and annealed Bi2O3-Ag materials, two-photon excited emission was collected with horizontal polarization of the incident beam. The two-photon intensity maps (Figs. 2(b) and 3(b)) clearly correspond to the microstructure of the observed samples (Figs. 2(a) and 3(a)).
The emission spectra were recorded at selected points of interest, which were then followed by polarimetric analysis. Polarization analysis of the signal at three points in each sample, highlighted by arrows in (Figs. 2(a) and 3(a)), shows a high emission intensity. Emission collected from the as-grown Bi2O3-Ag sample exhibits a broad band centered at 560 nm, (Figs. 2(c) and 2(d)), which is more pronounced for bigger silver precipitates. We identify this as two-photon luminescence (TPL).
The as-grown Bi2O3-Ag composite also contains forms of silver other than metallic. The spectra of two-photon excited luminescence observed for the as-grown Bi2O3-Ag samples (Figs. 2(c)–2(e)) are consistent with that expected for silver oxide, which has an energy bandgap of about 552 nm (2.25 eV);28 and silver nanoclusters, which can exhibit high intensity luminescence.29
The post-annealing of Bi2O3-Ag leads to an oxidation/reduction interplay between the silver and bismuth in bismuth compounds; it also leads to the formation of Ag and Bi nanoparticles.10 Annealed Bi2O3-Ag samples exhibit localized surface plasmon resonance due to the presence of silver nanoparticles due to spontaneous reduction of Ag2O to Ag metal above 195 °C, according to the reaction: 2Ag2O = 4Ag + O2.30 Also the presence of bismuth ions from the Bi2O3 matrix can help in the reduction of silver. When bismuth is in a lower oxidation state like Bi2+, Ag+ ions can be reduced to metallic silver, Ag0, by the following reaction Bi2+/Bi+ + Ag+ → Bi3+/Bi2+ + Ag0.31
In the annealed Bi2O3-Ag composite, where mostly metallic silver should be present, in some of the selected places, two-photon excited luminescence is still observed, (Fig. 3(c)). This may prove that, even in the annealed samples, silver may still exist in forms other than metallic. As demonstrated previously, luminescence in the visible region can also result from oxidized silver clusters on silver nanoparticles, nano-sized silver particles exposed to silver ions,32 or Agx nanoclusters.28 Ag clusters can be formed by IR femtosecond pulses according to the following reaction Ag0 + Ag+ → Ag2+.33,34
Figs. 3(d)–3(e) show the emission spectra of an Ag nanosheet and a triangular inclusion in the annealed Bi2O3-Ag sample, respectively. Here the spectra are dominated by a strong emission at around 415 nm, which is obviously SHG from the 830 nm excitation laser beam. The shapes of polarization patterns in these two cases are also markedly different from the previous ones, and are consistent with a SHG process occurring in quadrupolar symmetry, (Figs. 3(d)–3(e)).35,36 Red and blue plots correspond to the X and Y component of the emitted SHG, respectively, and they are plotted as a function of the incident beam polarization angle.
Until recently, SHG in eutectic composites had only been demonstrated in organic compounds.37–39 It has now been shown that eutectic materials can exhibit SHG signals many times higher than that of pure noncentrosymmetric component materials due to the eutectic microstructure enabling quasi phase matching (QPM).40
In the case of the Bi2O3-Ag composite, the SHG could originate from the nonlinear response of the plasmonic nano/microstructure itself, or its combination with nonlinearity of the Bi2O3 matrix. In our material, Bi2O3 grows in two of five possible polymorphs:41 mainly as α-Bi2O3 (space group P21/c) and traces of γ-Bi2O3 (I23).9 α-Bi2O3 is centrosymmetric, while γ-Bi2O3 exhibits non-centrosymmetry; thus this could be a potential source of SHG. However, we have not observed SHG in the as-grown samples, so the presence of γ-Bi2O3 only does not sufficiently explain its origin. In the annealed Bi2O3-Ag eutectic-based nanoplasmonic composite, TPL can be enhanced by the LSPR (λmax ∼ 595 nm) which overlaps with the luminescence peak (λmax ∼ 560 nm). However, neither SHG (λmax ∼ 415 nm) nor the excitation wavelength (λmax ∼ 830 nm) overlap with the LSPR peak here.
Symmetry breaking, which could provide the proper environment for SHG, is possible by, e.g., creation of a spatially modulated static electric field distribution,34 with local space charge separation.42 Such effects can be generated by femtosecond laser irradiation, a type of light source used in our experiment. However, optical poling processes are time-dependent (i.e., one should observe the growth of the SHG signal in time) and lead to the formation of a second-order susceptibility that has a dipolar character, not quadrupolar, as clearly seen in our studies, (Figs. 3(d)–3(e)).
Metal nanostructures are known to exhibit the second-order nonlinear effects due to the presence of surface normal and surface tangential components of the second order susceptibility.43 In the case of purely centrosymmetrically shaped nanoparticles, i.e., spherical, the effect is cancelled out in the far field and has to be caused by another symmetry breaking coming from the nanoparticle shape or field variation.2 In a real material, the particles are not perfectly shaped and uniform, which is why strong SHG is observed.2 The annealed Bi2O3-Ag eutectic-based nanoplasmonic composite exhibits a hierarchical structure with micrometer-size Ag precipitates with triangular cross-sections and micrometer-long Ag sheets as well as Ag spherical nanoparticles. Thus, the SHG can originate from both the local field-distortion at the metal-matrix interface and the retardation effects, i.e., the spatial variation of the electromagnetic field over a wavelength, which cannot be neglected when the particles are comparable in size with the wavelength. It could also originate from the noncentrosymmetric shapes of the bigger silver elements. The enhancement of SHG in metallic structures with triangular cross-section,44 as well as in other non-centrosymmetric structures, has been already shown.45
where θ is the incident light polarization angle, A, B, C are related to dipolar response and D and E are related to retardation effects giving contributions to the quadrupolar response.
Depending on the investigated place on the sample, we observe a different polarization behavior, i.e., for SHG originating from a silver sheet, both X and Y components have a quadrupolar character (Fig. 3(d)). On the other hand, for triangular structure X components, the radiation field retains a dominant quadrupolar character, whereas the Y component is distorted with a stronger contribution of a dipole, (Fig. 3(c)). The values of the parameters A to E obtained are in good agreement with those reported by Nappa et al.47 for big nanoparticles. A quantitative assessment of dipolar and quadrupolar contributions can be made using the weighting parameter ξV.46,47 A quantitative assessment of dipolar and quadrupolar contributions can be made using the weighting parameter ξ.46,47 It is evaluated from the parameters in Eq. (1) as follows:
The parameter ξ takes values from 0 to 1. It equals 0 and 1 for the pure dipolar and the pure quadrupolar case, respectively. In the case of our results, Y components of Figs. 3(d) and 3(e) contain 80% and 34% of the quadrupolar mode, respectively. The distortion in dipolar response is related to a grain-size effect47—the larger the grain, the less homogenous is the optical field over their volume, so additional terms must be included in the scattering model. However, in order to draw conclusions about the SHG polarization-dependence, we would need detailed information about the shapes of the considered Ag structures.
In summary, the polarization-dependent nonlinear optical (NLO) properties of a Bi2O3-Ag eutectic-based metamaterial have been investigated. The as-grown samples only exhibit a two-photon luminescence, while a strong two-photon luminescence and SHG are present in the annealed samples exhibiting localized surface plasmon resonance. The observed SHG is not likely to be a bulk phenomenon; rather, it is likely to be due to surface effects occurring at the interface between the Ag nanoparticles formed after annealing and the Bi2O3 matrix. Thus, the SHG is initiated here due to the local distortion at the metal interface and the shape of the silver precipitates, as well as retardation effects. We used the scattering model proposed by Nappa et al. and specified a contribution of dipolar and quadrupolar modes in our SHG signal. This gives us the basis for further investigation of the mechanism of multiphoton excitation and emission in Bi2O3-Ag metamaterial.
The authors thank the Maestro Project No. 2011/02/A/ST5/00471 and the Harmonia Project No. 2013/10/M/ST5/00650 from the National Science Centre, and the U.S. Air Force Office of Scientific Research under Grant No. FA9550-14-1-0061 for support of this work.