Broad-band near-infrared plasmonic nanoantennas for higher harmonic generation

Light and Carrier Transportation Management in Transparent Electrode for Achieving over 30% Efficiency Perovskite/Silicon Tandem Solar Cells

Perovskite/silicon tandem solar cells have drawn widespread attention owing to their higher power conversion efficiency (PCE). However, reducing the reflection and parasitic absorption as much as possible in the transparent electrode is of considerable interest to promote the tandem device to obtain higher circuit current density (JSC). Furthermore, the carrier vertical and lateral transport capability of transparent electrodes also affects the electrical performance of solar cells. Herein, we designed and realized a stacked structure of a columnar-equiaxed zirconium-doped indium oxide (IZrO) film. The optimal stacked IZrO thin film shows carrier density and mobility of 9.4 × 1020 cm-3 and 29.7 cm2 V-1 s-1, respectively. Additionally, it also shows superior optical transmittance and lower parasitic absorption in the visible-to-near-infrared region. In addition, reflectance in the perovskite/c-Si tandem solar cell shows an obvious reduction after the application of a stacked IZrO transparent electrode because of the gradient refractive index. Finally, the stacked IZrO transparent electrode was incorporated into P-I-N-type perovskite/textured-silicon tandem solar cells, and the champion stacked IZrO-based device showed PCE of 30.12% with an active area of 1.05 cm2.

Real-space imaging of confined infrared surface plasmon polaritons on doped semiconductors covered with phase-change materials

Surface plasmon polaritons (SPPs) describe the excitation of photons coupled with free charge carriers at the interface of metals (visible) or doped semiconductors (infrared). While SPPs in the mid-infrared spectral range have been demonstrated in 2D materials such as graphene, their short propagation length combined with weak confinement in bulk materials has prevented real-space imaging of those SPPs. Here, we demonstrate real-space imaging of propagating SPPs on the doped semiconductors CdO and InAs with tunable plasma frequencies in the infrared via scattering-type scanning near-field optical microscopy. Adding a thin film of phase-change materials (PCMs) to these doped semiconductors increases the polariton confinement, leading to simplified SPP imaging and SPP resonator fabrication. We investigate optically written circular resonators of the plasmonic PCM In3SbTe2 on CdO with near-field spectroscopy and Fabry-Perot resonators of the dielectric PCM Ge3Sb2Te6 on InAs with far-field spectroscopy. Our work enables rapid prototyping of reconfigurable SPP resonators in mid-infrared.

Dendric nanostructures for SARS-CoV-2 proteins detection based on surface enhanced infrared absorption spectroscopy

Infrared spectroscopy is a non-destructive and rapid characterization tool that can distinguish different viral proteins by spectral details. However, traditional infrared spectroscopy has insufficient absorption signal intensity contrast when measuring low-concentration samples. In this work, surface enhanced infrared absorption (SEIRA) spectroscopy is proposed by deploying a novel nanostructure array as SEIRA substrates. An array of gold dendric nanostructures are designed and fabricated with a precision resonance control to achieve surface enhancement covering a broadband molecular "finger-print" region. The spectral positions of the multiple resonances accurately correspond to the characteristic absorption peaks of the SARS-CoV-2 proteins. An approach for SARS-CoV-2 protein detection based on SEIRA spectroscopy is then proposed. A low concentration detection of 40 μg/ml diluted SARS-CoV-2 nucleocapsid protein is experimentally demonstrated and the enhancement factor (EF) achieved is in good agreement with simulation results. The SEIRA methodology based on broadband resonance nanostructure design provides a systematic approach for sensitive, non-destructive and rapid protein molecular detection, which could be extended to various kind of molecular characterization and biomedical diagnostics.

Madelung Formalism for Electron Spill-Out in Nonlocal Nanoplasmonics

Current multiscale plasmonic systems pose a modeling challenge. Classical macroscopic theories fail to capture quantum effects in such systems, whereas quantum electrodynamics is impractical given the total size of the experimentally relevant systems, as the number of interactions is too large to be addressed one by one. To tackle the challenge, in this paper we propose to use the Madelung form of the hydrodynamic Drude model, in which the quantum effect electron spill-out is incorporated by describing the metal-dielectric interface using a super-Gaussian function. The results for a two-dimensional nanoplasmonic wedge are correlated to those from nonlocal full-wave numerical calculations based on a linearized hydrodynamic Drude model commonly employed in the literature, showing good qualitative agreement. Additionally, a conformal transformation perspective is provided to explain qualitatively the findings. The methodology described here may be applied to understand, both numerically and theoretically, the modular inclusions of additional quantum effects, such as electron spill-out and nonlocality, that cannot be incorporated seamlessly by using other approaches.

Self-Constructed Multiple Plasmonic Hotspots on an Individual Fractal to Amplify Broadband Hot Electron Generation

Plasmonic nanoparticles are ideal candidates for hot-electron-assisted applications, but their narrow resonance region and limited hotspot number hindered the energy utilization of broadband solar energy. Inspired by tree branches, we designed and chemically synthesized silver fractals, which enable self-constructed hotspots and multiple plasmonic resonances, extending the broadband generation of hot electrons for better matching with the solar radiation spectrum. We directly revealed the plasmonic origin, the spatial distribution, and the decay dynamics of hot electrons on the single-particle level by using ab initio simulation, dark-field spectroscopy, pump-probe measurements, and electron energy loss spectroscopy. Our results show that fractals with acute tips and narrow gaps can support broadband resonances (400-1100 nm) and a large number of randomly distributed hotspots, which can provide unpolarized enhanced near field and promote hot electron generation. As a proof-of-concept, hot-electron-triggered dimerization of p-nitropthiophenol and hydrogen production are investigated under various irradiations, and the promoted hot electron generation on fractals was confirmed with significantly improved efficiency.

Growth of Au Hollow Stars and Harmonic Excitation Energy Transfer

Optical excitation, subsequent energy transfer, and emission are fundamental to many physical problems. Optical antennas are ideal candidates for manipulating these processes. We extend energy transfer to second- and third-harmonic (SH and TH) fields through the collaborative susceptibility χ⁽ⁿ⁾ (n = 1, 2, 3) resonances of nonlinear optical antennas. Hollow gold stars, with a broadband response covering the fundamental, SH, and TH frequencies, are synthesized as nonlinear antennas. Harmonic resonance energy transfer through a χ⁽³⁾ → χ⁽¹⁾ collaboration is revealed. A χ⁽³⁾ → χ⁽²⁾ collaboration is uncovered, with largely enhanced SH radiation demonstrated by exciting the three resonances at the fundamental, SH, and TH frequencies. A theoretical model of the effective nonlinear susceptibilities is proposed to calculate the efficiencies of the two nonlinear energy transfer processes.

Magnetic Plasmon-Enhanced Second-Harmonic Generation on Colloidal Gold Nanocups

The magnetic plasmons of three-dimensional nanostructures have unique optical responses and special significance for optical nanoresonators and nanoantennas. In this study, we have successfully synthesized colloidal Au and AuAg nanocups with a well-controlled asymmetric geometry, tunable opening sizes, and normalized depths ( h/ b, where h is depth and b is the height of the templating PbS nanooctahedrons), variable magnetic plasmon resonance, and largely enhanced second-harmonic generation (SHG). The most-efficient SHG of the bare Au nanocups is experimentally observed when the normalized depth h/ b is adjusted to ∼0.78-0.79. We find that the average magnetic field enhancement is maximized at h/ b = ∼0.65 and reveal that the maximal SHG can be attributed to the joint action of the optimized magnetic plasmon resonance and the "lightning-rod effect" of the Au nanocups. Furthermore, we demonstrate for the first time that the AuAg heteronanocups prepared by overgrowth of Ag on the Au nanocups can synergize the magnetic and electric plasmon resonances for nonlinear enhancement. By the tailoring of the dual resonances at the fundamental excitation and second-harmonic wavelengths, the far-field SHG intensity of the AuAg nanocups is enhanced 21.8-fold compared to that of the bare Au nanocups. These findings provide a strategy for the design of nonlinear optical nanoantennas based on magnetic plasmon resonances and can lead to diverse applications ranging from nanophotonics to biological spectroscopy.

Exciton-enhanced infrared spectroscopy with organometallic perovskite nanoplatelets

Organometallic perovskite nanoplatelets enhance molecular infrared absorption as their excitons can be excited by infrared photons and amplify molecular polarization.

Ultrashort pulse synthesis for energy concentration control in nanostructures

A waveform synthesis technique is introduced and applied to the femtosecond pulse excitation of plasmonic nanoantennas for temporal and spatial energy concentration control. The waveform synthesis process is based on phase and polarization shaping and an understanding of the electromagnetic response of the nanostructure. Linear and radial nano-dipole arrays are analyzed before the log-periodic toothed nanoantenna is investigated as a nanostructure capable of combining the benefits of the nano-dipole arrays. The consistent superiority of the log-periodic toothed nanoantenna is established by comparing its electromagnetic response to that of the radial nano-dipole array using a variety of synthesized excitation waveforms.

Understanding quantum emitters in plasmonic nanocavities with conformal transformation: Purcell enhancement and forces

Nanogaps supporting cavity plasmonic modes with unprecedented small mode volume are attractive platforms for tailoring the properties of light-matter interactions at the nanoscale and revealing new physics. Hitherto, there is a concerning lack of analytical solutions to divide the complex interactions into their different underlying mechanisms to gain a better understanding that can foster enhanced designs. Bowtie apertures are viewed as an effective and appealing nanocavity and are studied here within the analytical frame of conformal transformation. We show how the non-radiative Purcell enhancement of a quantum emitter within the bowtie nanocavity depends strongly not only on the geometry of the nanocavity, but also on the position and orientation of the emitter. For a 20 nm diameter (∅ 20 nm) bowtie nanocavity, we report a change of up to two orders of magnitude in the maximum non-radiative Purcell enhancement and a shift in its peak wavelength from green to infra-red. The changes are tracked down to the overlap between the emitter field and the gap plasmon mode field distribution. This analysis also enables us to understand the self-induced trapping potential of a colloidal quantum dot inside the nanocavity. Since transformations can be cascaded, the technique introduced in this work can also be applied to a wide range of nanocavities found in the literature.

Multiresonant Composite Optical Nanoantennas by Out-of-plane Plasmonic Engineering

Optical nanoantennas can concentrate light and enhance light-matter interactions in subwavelength domain, which is useful for photodetection, light emission, optical biosensing, and spectroscopy. However, conventional optical nanoantennas operating at a single wavelength band are not suitable for multiband applications. Here, we propose and exploit an out-of-plane plasmonic engineering strategy to design and create composite optical nanoantennas that can support multiple nanolocalized modes at different resonant wavelengths. These multiresonant composite nanoantennas are composed of vertically stacked building blocks of metal-insulator-metal loop nanoantennas. Studies of multiresonant composite nanoantennas demonstrate that the number of supported modes depends on the number of vertically stacked building blocks and the resonant wavelengths of individual modes are tunable by controlling the out-of-plane geometries of their building blocks. In addition, numerical studies show that the resonant wavelengths of individual modes in composite nanoantennas can deviate from the optical response of building blocks due to hybridization of magnetic modes in neighboring building blocks. Using Au nanohole arrays as deposition masks to fabricate arrays of multilayered composite nanoantennas, we experimentally demonstrate their multiresonant optical properties in good agreement with theory predictions. These studies show that out-of-plane engineered multiresonant composite nanoantennas can provide new opportunities for fundamental nanophotonics research and practical applications involving optical multiband operations, such as multiphoton process, broadband solar energy conversion, and wavelength-multiplexed optical system.

Gradient metasurfaces: a review of fundamentals and applications

In the wake of intense research on metamaterials the two-dimensional analogue, known as metasurfaces, has attracted progressively increasing attention in recent years due to the ease of fabrication and smaller insertion losses, while enabling an unprecedented control over spatial distributions of transmitted and reflected optical fields. Metasurfaces represent optically thin planar arrays of resonant subwavelength elements that can be arranged in a strictly or quasi periodic fashion, or even in an aperiodic manner, depending on targeted optical wavefronts to be molded with their help. This paper reviews a broad subclass of metasurfaces, viz. gradient metasurfaces, which are devised to exhibit spatially varying optical responses resulting in spatially varying amplitudes, phases and polarizations of scattered fields. Starting with introducing the concept of gradient metasurfaces, we present classification of different metasurfaces from the viewpoint of their responses, differentiating electrical-dipole, geometric, reflective and Huygens' metasurfaces. The fundamental building blocks essential for the realization of metasurfaces are then discussed in order to elucidate the underlying physics of various physical realizations of both plasmonic and purely dielectric metasurfaces. We then overview the main applications of gradient metasurfaces, including waveplates, flat lenses, spiral phase plates, broadband absorbers, color printing, holograms, polarimeters and surface wave couplers. The review is terminated with a short section on recently developed nonlinear metasurfaces, followed by the outlook presenting our view on possible future developments and perspectives for future applications.

Indium phosphide metasurface with enhanced nonlinear absorption

We solve the nonlinear Maxwell equations in an InP-based dielectric metamaterial, considering both two-photon absorption and photo-induced free-carrier absorption. We obtain the intensity-dependent reflection, absorption, and effective permittivity and permeability of the metamaterial. Our results show that nonlinear absorption dampens both the electric and magnetic Mie resonance, although the magnetic resonance is more affected because it occurs at longer wavelengths where the free-carrier absorption cross section is larger. Owing to field concentration in the metamaterial at resonance, the threshold intensity for nonlinear absorption is reduced by a factor of about 30 compared to a homogeneous layer of the same thickness. Our results have implications on the use of dielectric metamaterials for nonlinear applications such as higher harmonic generation, optical limiting, and ultrafast modulation.

Self-Similarity of Plasmon Edge Modes on Koch Fractal Antennas

We investigate the plasmonic behavior of Koch snowflake fractal geometries and their possible application as broadband optical antennas. Lithographically defined planar silver Koch fractal antennas were fabricated and characterized with high spatial and spectral resolution using electron energy loss spectroscopy. The experimental data are supported by numerical calculations carried out with a surface integral equation method. Multiple surface plasmon edge modes supported by the fractal structures have been imaged and analyzed. Furthermore, by isolating and reproducing self-similar features in long silver strip antennas, the edge modes present in the Koch snowflake fractals are identified. We demonstrate that the fractal response can be obtained by the sum of basic self-similar segments called characteristic edge units. Interestingly, the plasmon edge modes follow a fractal-scaling rule that depends on these self-similar segments formed in the structure after a fractal iteration. As the size of a fractal structure is reduced, coupling of the modes in the characteristic edge units becomes relevant, and the symmetry of the fractal affects the formation of hybrid modes. This analysis can be utilized not only to understand the edge modes in other planar structures but also in the design and fabrication of fractal structures for nanophotonic applications.

Tunable optical switching in the near-infrared spectral regime by employing plasmonic nanoantennas containing phase change materials

We propose the design of switchable plasmonic nanoantennas (SPNs) that can be employed for optical switching in the near-infrared regime. The proposed SPNs consist of nanoantenna structures made up of a plasmonic metal (gold) such that these nanoantennas are filled with a switchable material (vanadium dioxide). We compare the results of these SPNs with inverted SPN structures that consist of gold nanoantenna structures surrounded by a layer of vanadium dioxide (VO₂) on their outer surface. These nanoantennas demonstrate switching of electric-field intensity enhancement (EFIE) between two states (On and Off states), which can be induced thermally, optically or electrically. The On and Off states of the nanoantennas correspond to the metallic and semiconductor states, respectively of the VO₂ film inside or around the nanoantennas, as the VO₂ film exhibits phase transition from its semiconductor state to the metallic state upon application of thermal, optical, or electrical energy. We employ finite-difference time-domain (FDTD) simulations to demonstrate switching in the EFIE for four different SPN geometries - nanorod-dipole, bowtie, planar trapezoidal toothed log-periodic, and rod-disk - and compare their near-field distributions for the On and Off states of the SPNs. We also demonstrate that the resonance wavelength of the EFIE spectra gets substantially modified when these SPNs switch between the two states.

Surface-Enhanced Infrared Spectroscopy Using Resonant Nanoantennas

Infrared spectroscopy is a powerful tool widely used in research and industry for a label-free and unambiguous identification of molecular species. Inconveniently, its application to spectroscopic analysis of minute amounts of materials, for example, in sensing applications, is hampered by the low infrared absorption cross-sections. Surface-enhanced infrared spectroscopy using resonant metal nanoantennas, or short "resonant SEIRA", overcomes this limitation. Resonantly excited, such metal nanostructures feature collective oscillations of electrons (plasmons), providing huge electromagnetic fields on the nanometer scale. Infrared vibrations of molecules located in these fields are enhanced by orders of magnitude enabling a spectroscopic characterization with unprecedented sensitivity. In this Review, we introduce the concept of resonant SEIRA and discuss the underlying physics, particularly, the resonant coupling between molecular and antenna excitations as well as the spatial extent of the enhancement and its scaling with frequency. On the basis of these fundamentals, different routes to maximize the SEIRA enhancement are reviewed including the choice of nanostructures geometries, arrangements, and materials. Furthermore, first applications such as the detection of proteins, the monitoring of dynamic processes, and hyperspectral infrared chemical imaging are discussed, demonstrating the sensitivity and broad applicability of resonant SEIRA.

Controlling second-harmonic generation at the nanoscale with monolithic AlGaAs-on-AlOx antennas

We review recent achievements in the field of nanoscale nonlinear AlGaAs photonics based on all-dielectric optical antennas. After discussing the motivation and main technological challenges for the development of an AlGaAs monolithic platform for χ ⁽²⁾ nonlinear nanophotonics, we present numerical and experimental investigations of the second-order nonlinear response and physical reasons for high efficiency of second-order nonlinear interactions in the AlGaAs nano-antennas. In particular, we emphasize the role of the dipolar resonances at the fundamental frequency and the multipolar resonances at the second harmonic wavelength. We also discuss second-harmonic generation directionality and show possible strategies to engineer the radiation pattern of nonlinear antennas.

Broadband localized electric field enhancement produced by a single-element plasmonic nanoantenna

We propose a novel design of a broadband plasmonic nanoantenna, investigate it numerically using finite-difference time-domain methods, and explain its performance using the analysis of charge distribution in addition to a multipole expansion.

Exploration of organic–inorganic hybrid perovskites for surface-enhanced infrared spectroscopy of small molecules

Organic–inorganic hybrid perovskites allow the infrared absorption of small molecules to be efficiently enhanced.

Anticrossing double Fano resonances generated in metallic/dielectric hybrid nanostructures using nonradiative anapole modes for enhanced nonlinear optical effects

Third-harmonic generation with metallic or dielectric nanoparticles often suffer from, respectively, small modal volumes and weak near-field enhancements. This study propose and demonstrate that a metallic/dielectric hybrid nanostructure composed of a silver double rectangular nanoring and a silicon square nanoplate can be used to overcome these obstacles for enhanced third-harmonic generation. It is shown that the nonradiative anapole mode of the Si plate can be used as a localized source to excite the dark subradiant octupole mode of the Ag ring, and the mode hybridization leads to the formation of an antibonding and a bonding subradiant collective mode, thereby forming anticrossing double Fano resonances. With the strong coupling between individual particles and the effectively suppressed radiative losses of the Fano resonances, several strong hot spots are generated around the Ag ring due to the excitation of the octupole mode, and electromagnetic fields within the Si plate are also strongly amplified, making it possible to confine more incident energy inside the dielectric nanoparticle. Calculation results reveal that the confined energy inside the Si plate and the Ag ring for the hybrid structures can be about, respectively, more than three times and four orders stronger than that of the corresponding isolated nanoparticles, which makes the designed hybrid nanostructure a promising platform for enhanced third-harmonic generation.

Plasmonic Circuit Theory for Multiresonant Light Funneling to a Single Spatial Hot Spot

We present a theoretical framework, based on plasmonic circuit models, for generating a multiresonant field intensity enhancement spectrum at a single "hot spot" in a plasmonic device. We introduce a circuit model, consisting of an array of coupled LC resonators, that directs current asymmetrically in the array, and we show that this circuit can funnel energy efficiently from each resonance to a single element. We implement the circuit model in a plasmonic nanostructure consisting of a series of metal bars of differing length, with nearest neighbor metal bars strongly coupled electromagnetically through air gaps. The resulting nanostructure resonantly traps different wavelengths of incident light in separate gap regions, yet it funnels the energy of different resonances to a common location, which is consistent with our circuit model. Our work is important for a number of applications of plasmonic nanoantennas in spectroscopy, such as in single-molecule fluorescence spectroscopy or Raman spectroscopy.

The Interplay of Symmetry and Scattering Phase in Second Harmonic Generation from Gold Nanoantennas

Nonlinear phenomena are central to modern photonics but, being inherently weak, typically require gradual accumulation over several millimeters. For example, second harmonic generation (SHG) is typically achieved in thick transparent nonlinear crystals by phase-matching energy exchange between light at initial, ω, and final, 2ω, frequencies. Recently, metamaterials imbued with artificial nonlinearity from their constituent nanoantennas have generated excitement by opening the possibility of wavelength-scale nonlinear optics. However, the selection rules of SHG typically prevent dipole emission from simple nanoantennas, which has led to much discussion concerning the best geometries, for example, those breaking centro-symmetry or incorporating resonances at multiple harmonics. In this work, we explore the use of both nanoantenna symmetry and multiple harmonics to control the strength, polarization and radiation pattern of SHG from a variety of antenna configurations incorporating simple resonant elements tuned to light at both ω and 2ω. We use a microscopic description of the scattering strength and phases of these constituent particles, determined by their relative positions, to accurately predict the SHG radiation observed in our experiments. We find that the 2ω particles radiate dipolar SHG by near-field coupling to the ω particle, which radiates SHG as a quadrupole. Consequently, strong linearly polarized dipolar SHG is only possible for noncentro-symmetric antennas that also minimize interference between their dipolar and quadrupolar responses. Metamaterials with such intra-antenna phase and polarization control could enable compact nonlinear photonic nanotechnologies.

Broadband gold nanoantennas arrays with transverse dimension effects

Broadband resonance in gold paired-rods nanoantennas and paired-strips gratings is investigated when the nanostructure's transverse (non-polarization) dimension is changed from paired-rods to paired-strips. Increasing the transverse dimension blue shifts the resonance wavelength and widens its bandwidth due to cancellation of the magnetic field between nanoantennas. A derived resistor-inductor-capacitor (RLC) equivalent circuit model verifies the nanostructures' resonance when elongating the transverse dimensions. Paired-strips gratings have a bandwidth 2.04 times that of paired-rods nanoantennas.

Toward Surface Plasmon-Enhanced Optical Parametric Amplification (SPOPA) with Engineered Nanoparticles: A Nanoscale Tunable Infrared Source

Active optical processes such as amplification and stimulated emission promise to play just as important a role in nanoscale optics as they have in mainstream modern optics. The ability of metallic nanostructures to enhance optical nonlinearities at the nanoscale has been shown for a number of nonlinear and active processes; however, one important process yet to be seen is optical parametric amplification. Here, we report the demonstration of surface plasmon-enhanced difference frequency generation by integration of a nonlinear optical medium, BaTiO3, in nanocrystalline form within a plasmonic nanocavity. These nanoengineered composite structures support resonances at pump, signal, and idler frequencies, providing large enhancements of the confined fields and efficient coupling of the wavelength-converted idler radiation to the far-field. This nanocomplex works as a nanoscale tunable infrared light source and paves the way for the design and fabrication of a surface plasmon-enhanced optical parametric amplifier.

Polarization-Independent Multiple Fano Resonances in Plasmonic Nonamers for Multimode-Matching Enhanced Multiband Second-Harmonic Generation

Plasmonic oligomers composed of metallic nanoparticles are one class of the most promising platforms for generating Fano resonances with unprecedented optical properties for enhancing various linear and nonlinear optical processes. For efficient generation of second-harmonic emissions at multiple wavelength bands, it is critical to design a plasmonic oligomer concurrently having multiple Fano resonances spectrally matching the fundamental excitation wavelengths and multiple plasmon resonance modes coinciding with the harmonic wavelengths. Thus far, the realization of such a plasmonic oligomer remains a challenge. This study demonstrates both theoretically and experimentally that a plasmonic nonamer consisting of a gold nanocross surrounded by eight nanorods simultaneously sustains multiple polarization-independent Fano resonances in the near-infrared region and several higher-order plasmon resonances in the visible spectrum. Due to coherent amplification of the nonlinear excitation sources by the Fano resonances and efficient scattering-enhanced outcoupling by the higher-order modes, the second-harmonic emission of the nonamer is significantly increased at multiple spectral bands, and their spectral positions and radiation patterns can be flexibly manipulated by easily tuning the length of the surrounding nanorods in the nonamer. These results provide us with important implications for realizing ultrafast multichannel nonlinear optoelectronic devices.

Optical Second Harmonic Generation in Plasmonic Nanostructures: From Fundamental Principles to Advanced Applications

Plasmonics has emerged as an important research field in nanoscience and nanotechnology. Recently, significant attention has been devoted to the observation and the understanding of nonlinear optical processes in plasmonic nanostructures, giving rise to the new research field called nonlinear plasmonics. This review provides a comprehensive insight into the physical mechanisms of one of these nonlinear optical processes, namely, second harmonic generation (SHG), with an emphasis on the main differences with the linear response of plasmonic nanostructures. The main applications, ranging from the nonlinear optical characterization of nanostructure shapes to the optimization of laser beams at the nanoscale, are summarized and discussed. Future directions and developments, made possible by the unique combination of SHG surface sensitivity and field enhancements associated with surface plasmon resonances, are also addressed.

Fractal nanoparticle plasmonics: the Cayley tree

There has been strong, ongoing interest over the past decade in developing strategies to design and engineer materials with tailored optical properties. Fractal-like nanoparticles and films have long been known to possess a remarkably broad-band optical response and are potential nanoscale components for realizing spectrum-spanning optical effects. Here we examine the role of self-similarity in a fractal geometry for the design of plasmon line shapes. By computing and fabricating simple Cayley tree nanostructures of increasing fractal order N, we are able to identify the principle behind how the multimodal plasmon spectrum of this system develops as the fractal order is increased. With increasing N, the fractal structure acquires an increasing number of modes with certain degeneracies: these modes correspond to plasmon oscillations on the different length scales inside a fractal. As a result, fractals with large N exhibit broad, multipeaked spectra from plasmons with large degeneracy numbers. The Cayley tree serves as an example of a more general, fractal-based route for the design of structures and media with highly complex optical line shapes.

Incident angle-tuned, broadband, ultrahigh-sensitivity plasmonic antennas prepared from nanoparticles on imprinted mirrors

We have used a direct imprint-in-metal method that is cheap and rapid to prepare incident angle-tuned, broadband, ultrahigh-sensitivity plasmonic antennas from nanoparticles (NPs) and imprinted metal mirrors. By changing the angle of incidence, the nanoparticle-imprinted mirror antennas (NIMAs) exhibited broadband electromagnetic enhancement from the visible to the near-infrared (NIR) regime, making them suitable for use as surface-enhanced Raman scattering (SERS)-active substrates. Unlike other SERS-active substrates that feature various structures with different periods or morphologies, the NIMAs achieved broadband electromagnetic enhancement from single configurations. The enhancement of the electric field intensity in the NIMAs originated from coupling between the localized surface plasmon resonance of the NPs and the periodic structure-excited surface plasmon resonance (SPR) of the imprinted mirror. Moreover, the coupling wavelengths could be modulated because the SPR wavelength was readily tuned by changing the angle of the incident light. Herein, we demonstrate that such NIMAs are robust substrates for visible and NIR surface-enhanced resonance Raman scattering under multiple laser lines (532, 633, and 785 nm) of excitation. In addition, we have found that NIMAs are ultrasensitive SERS-active substrates that can detect analytes (e.g., rhodamine 6G) at concentrations as low as 10(-15) M.

Plasmonic spiderweb nanoantenna surface for broadband hotspot generation

In this study, we demonstrate a general framework for obtaining a plasmonic nanoantenna surface with a broadband polarization-independent response. The plasmonic spiderweb nanoantenna surface is composed of unit cells, which form multiple resonance paths due to patterning of the metallic conductor such that electrons can find multiple ways to oscillate between the poles of the conductor. The tailoring of the conductor paths and shapes of the unit cells' patterns results in a broadband spectral response. At various resonance frequencies, the electrons oscillate along different paths between the poles of the antenna, generating broadband hot spots around those poles.

Fano resonances in the nonlinear optical response of coupled plasmonic nanostructures

The coupling between metallic nanostructures is a common and easy way to control the optical properties of plasmonic systems. Even though the coupling between plasmonic oscillators has been widely studied in the linear regime, its influence on the nonlinear optical response of metallic nanostructures has been sparsely considered. Using a surface integral equation method, we investigate the second order nonlinear optical response of plasmonic metamolecules supporting Fano resonances revealing that the typical lineshape of Fano resonances is also clearly observable in the nonlinear regime. The physical mechanisms leading to nonlinear Fano resonances are revealed by the coupled oscillator model and the symmetry subgroup decomposition. It is found that the origin of the nonlinear scattered wave, i. e. the active plasmonic oscillator, can be selectively chosen. Furthermore, interferences between nonlinear emissions are clearly observed in specific configurations. The results presented in this article pave the way for the design of efficient nonlinear plasmonic metamolecules with controlled nonlinear radiation.

Third Harmonic Mechanism in Complex Plasmonic Fano Structures

We perform third harmonic spectroscopy of dolmen-type nanostructures, which exhibit plasmonic Fano resonances in the near-infrared. Strong third harmonic emission is predominantly radiated close to the low energy peak of the Fano resonance. Furthermore, we find that the third harmonic polarization of the subradiant mode interferes destructively and diminishes the nonlinear signal in the far-field. By comparing the experimental third harmonic spectra with finite element simulations and an anharmonic oscillator model, we find strong indications that the source of the third harmonic is the optical nonlinearity of the bare gold enhanced by the resonant plasmonic polarization.

Third-harmonic-upconversion enhancement from a single semiconductor nanoparticle coupled to a plasmonic antenna

The ability to convert low-energy quanta into a quantum of higher energy is of great interest for a variety of applications, including bioimaging, drug delivery and photovoltaics. Although high conversion efficiencies can be achieved using macroscopic nonlinear crystals, upconverting light at the nanometre scale remains challenging because the subwavelength scale of materials prevents the exploitation of phase-matching processes. Light-plasmon interactions that occur in nanostructured noble metals have offered alternative opportunities for nonlinear upconversion of infrared light, but conversion efficiency rates remain extremely low due to the weak penetration of the exciting fields into the metal. Here, we show that third-harmonic generation from an individual semiconductor indium tin oxide nanoparticle is significantly enhanced when coupled within a plasmonic gold dimer. The plasmonic dimer acts as a receiving optical antenna, confining the incident far-field radiation into a near field localized at its gap; the indium tin oxide nanoparticle located at the plasmonic dimer gap acts as a localized nonlinear transmitter upconverting three incident photons at frequency ω into a photon at frequency 3ω. This hybrid nanodevice provides third-harmonic-generation enhancements of up to 10(6)-fold compared with an isolated indium tin oxide nanoparticle, with an effective third-order susceptibility up to 3.5 × 10(3) nm V(-2) and conversion efficiency of 0.0007%. We also show that the upconverted third-harmonic emission can be exploited to probe the near-field intensity at the plasmonic dimer gap.

Photonic-plasmonic-coupled nanoantennas for polarization-controlled multispectral nanofocusing

We report on the design and experimental demonstration of array-enhanced nanoantennas for polarization-controlled multispectral nanofocusing in the near-IR spectral range. We design plasmonic double bow-tie nanoantennas-coupled to multiple-periodic nanoparticle arrays to harvest radiation of designed wavelengths from a large spatial area and to focus it into a targeted nanoscale region. Near-field calculations were performed on a gold nanoantenna array using three-dimensional finite difference time domain simulations. Cross-shaped optical nanoantennas were fabricated on glass substrates using electron beam lithography. The optical characterization of the fabricated nanoantennas was performed using second harmonic excitation spectroscopy that demonstrates multiwavelength photonic coupling in good agreement with the antenna modeling. The nanoantenna structures introduced in this Letter provide the ability to focus optical energy into deep subwavelength areas and to address multiple spectral regions with polarization control. Such attributes are highly desirable in optical biosensing, enhanced Raman scattering, and for nonlinear plasmonic applications.

Tunable and broadband plasmonic absorption via dispersible nanoantennas with sub-10 nm gaps

Plasmonic nanoparticles have traditionally been associated with relatively narrow absorption profiles. But, for many of the most exciting potential applications for these particles, such as solar energy applications, broadband absorption is desirable. By utilizing on-wire lithography, nanostructures which absorb light through the visible and near-IR portions of the electromagnetic spectrum can be synthesized.

Augmenting second harmonic generation using Fano resonances in plasmonic systems

Significant augmentation of second harmonic generation using Fano resonances in plasmonic heptamers made of silver is theoretically and experimentally demonstrated. The geometry is engineered to simultaneously produce a Fano resonance at the fundamental wavelength, resulting in a strong localization of the fundamental field close to the system, and a higher order scattering peak at the second harmonic wavelength. These results illustrate the versatility of Fano resonant structures to engineer specific optical responses both in the linear and nonlinear regimes thus paving the way for future investigations on the role of dark modes in nonlinear and quantum optics.

Ultrasensitive optical shape characterization of gold nanoantennas using second harmonic generation

Second harmonic generation from plasmonic nanoantennas is investigated numerically using a surface integral formulation for the calculation of both the fundamental and the second harmonic electric field. The comparison between a realistic and an idealized gold nanoantenna shows that second harmonic generation is extremely sensitive to asymmetry in the nanostructure shape even in cases where the linear response is barely modified. Interestingly, minute geometry asymmetry and surface roughness are clearly revealed by far-field analysis, demonstrating that second harmonic generation is a promising tool for the sensitive optical characterization of plasmonic nanostructures. Furthermore, defects located where the linear field is strong (e.g., in the antenna gap) do not necessarily have the strongest impact on the second harmonic signal.

Ultrasensitive broadband probing of molecular vibrational modes with multifrequency optical antennas

Optical antennas represent an enabling technology for enhancing the detection of molecular vibrational signatures at low concentrations and probing the chemical composition of a sample in order to identify target molecules. However, efficiently detecting different vibrational modes to determine the presence (or the absence) of a molecular species requires a multispectral interrogation in a window of several micrometers, as many molecules present informative fingerprint spectra in the mid-infrared between 2.5 and 10 μm. As most nanoantennas exhibit a narrow-band response because of their dipolar nature, they are not suitable for such applications. Here, we propose the use of multifrequency optical antennas designed for operating with a bandwidth of several octaves. We demonstrate that surface-enhanced infrared absorption gains in the order of 10(5) can be easily obtained in a spectral window of 3 μm with attomolar concentrations of molecules, providing new opportunities for ultrasensitive broadband detection of molecular species via vibrational spectroscopy techniques.

Cascaded four-wave mixing in tapered plasmonic nanoantenna

We theoretically study the cascaded four-wave mixing (FWM) in broadband tapered plasmonic nanoantennas. In comparison with nonlinear bulk medium they demonstrate a 300-fold increase in nonlinear frequency conversion detected in the main lobe of the nanoantenna far-field pattern. This is achieved by tuning the elements of the nanoantenna to resonance frequencies involved in the FWM interaction. Our findings have a potentially broad application in ultrafast nonlinear spectroscopy, sensing, on-chip optical frequency conversion, nonlinear optical metamaterials, and photon sources.

Multiresonant broadband optical antennas as efficient tunable nanosources of second harmonic light

We report the experimental realization of efficient tunable nanosources of second harmonic light with individual multiresonant log-periodic optical antennas. By designing the nanoantenna with a bandwidth of several octaves, simultaneous enhancement of fundamental and harmonic fields is observed over a broad range of frequencies, leading to a high second harmonic conversion efficiency, together with an effective second order susceptibility within the range of values provided by widespread inorganic crystals. Moreover, the geometrical configuration of the nanoantenna makes the generated second harmonic signal independent from the polarization of the fundamental excitation. These results open new possibilities for the development of efficient integrated nonlinear nanodevices with high frequency tunability.

Transformation-optics description of plasmonic nanostructures containing blunt edges/corners: from symmetric to asymmetric edge rounding

The sharpness of corners/edges can have a large effect on the optical responses of metallic nanostructures. Here we deploy the theory of transformation optics to analytically investigate a variety of blunt plasmonic structures, including overlapping nanowire dimers and crescent-shaped nanocylinders. These systems are shown to support several discrete optical modes, whose energy and line width can be controlled by tuning the nanoparticle geometry. In particular, the necessary conditions are highlighted respectively for the broadband light absorption effect and the invisibility dips that appear in the radiative spectrum. More detailed discussions are provided especially with respect to the structures with asymmetric edge rounding. These structures can support additional subradiant modes, whose interference with the neighboring dipolar modes results in a rapid change of the scattering cross-section, similar to the phenomenon observed in plasmonic Fano resonances. Finite element numerical calculations are also performed to validate the analytical predictions. The physical insights into blunt nanostructures presented in this work may be of great interest for the design of broadband light-harvesting devices, invisible and noninvasive biosensors, and slowing-light devices.

Quantitative modeling of the third harmonic emission spectrum of plasmonic nanoantennas

Plasmonic dimer nanoantennas are characterized by a strong enhancement of the optical field, leading to large nonlinear effects. The third harmonic emission spectrum thus depends strongly on the antenna shape and size as well as on its gap size. Despite the complex shape of the nanostructure, we find that for a large range of different geometries the nonlinear spectral properties are fully determined by the linear response of the antenna. We find excellent agreement between the measured spectra and predictions from a simple nonlinear oscillator model. We extract the oscillator parameters from the linear spectrum and use the amplitude of the nonlinear perturbation only as scaling parameter of the third harmonic spectra. Deviations from the model only occur for gap sizes below 20 nm, indicating that only for these small distances the antenna hot spot contributes noticeable to the third harmonic generation. Because of its simplicity and intuitiveness, our model allows for the rational design of efficient plasmonic nonlinear light sources and is thus crucial for the design of future plasmonic devices that give substantial enhancement of nonlinear processes such as higher harmonics generation as well as difference frequency mixing for plasmonically enhanced terahertz generation.