Metallic Nanoantennas Research Articles

Emitting-polarization of surface plasmons coupling in metallic nanoantennas

Surface plasmons coupling of metallic nanoantennas plays an important role in light manipulation at the nanometer scale. In this work, we reveal surface plasmons coupling via emitting-polarization dependent measurements (under unpolarized excitation). By detecting the scattering at different emitting polarization angles, the surface plasmons coupling modes (resonance positions and preferential polarized orientations) can be well revealed, which can not only provide the polarization state of nanoantennas emission but also improve the peaks resolution of surface plasmon modes, especially when these modes are polarized orthogonally with each other. The emission polarization is greatly dependent on the surface plasmons coupling, and the coupled nanoantenna can work as a polarized light emitter at different resonant wavelengths. These results are of high importance for both the fundamental understanding of the interaction between light and plasmonic nanostructures, and the development of nanophotonic devices for light manipulation and sensing applications.

Relative merits of phononics vs. plasmonics: the energy balance approach

Abstract The common feature of various plasmonic schemes is their ability to confine optical fields of surface plasmon polaritons (SPPs) into subwavelength volumes and thus achieve a large enhancement of linear and nonlinear optical properties. This ability, however, is severely limited by the large ohmic loss inherent to even the best of metals. However, in the mid- and far-infrared ranges of the spectrum, there exists a viable alternative to metals – polar dielectrics and semiconductors, in which dielectric permittivity (the real part) turns negative in the Reststrahlen region. This feature engenders the so-called surface phonon polaritons, capable of confining the field in a way akin to their plasmonic analogs, the SPPs. Since the damping rate of polar phonons is substantially less than that of free electrons, it is not unreasonable to expect that phononic devices may outperform their plasmonic counterparts. Yet a more rigorous analysis of the comparative merits of phononics and plasmonics reveals a more nuanced answer, namely, that while phononic schemes do exhibit narrower resonances and can achieve a very high degree of energy concentration, most of the energy is contained in the form of lattice vibrations so that enhancement of the electric field and, hence, the Purcell factor is rather small compared to what can be achieved with metal nanoantennas. Still, the sheer narrowness of phononic resonances is expected to make phononics viable in applications where frequency selectivity is important.

Optically saturated and unsaturated collective resonances of flat metallic nanoantenna arrays

We study collective optical properties of arrays of flat gold nanoantennas, demonstrating they can support optically saturated and unsaturated plasmonic lattice modes when the incident light is polarized along their short axes. The saturated mode is nearly immune to the variation of the refractive index of the environment, while the unsaturated mode undergoes a large red shift without degradation as the refractive index increases. Our results show that when the incident light becomes polarized along the long axes of the nanoantennas, an increase of the refractive index of the superstrate leads to the formation of secondary plasmon peaks. These peaks are spectrally narrow and can detect variations of the ambient refractive index with a sensitivity of up to 620 nm/RIU (refractive index unit). The results suggest that the periodic arrays of flat metallic nanostructures can support hybridization of their multipolar plasmonic resonances with diffraction orders with distinct similarities and differences compared to those seen in cases of arrays of metallic nanorods.

Ultra-broadband enhancement of nonlinear optical processes from randomly patterned super absorbing metasurfaces

Broadband light trapping and field localization is highly desired in enhanced light-matter interaction, especially in harmonic generations. However, due to the limited resonant bandwidth, most periodic plasmonic nanostructures cannot cover both fundamental excitation wavelength and harmonic generation wavelength simultaneously. Therefore, most previously reported plasmonic nonlinear optical processes are low in conversion efficiency. Here, we report a strong enhancement of second harmonic generation based on a three-layered super absorbing metasurface structure consisting of a dielectric spacer layer sandwiched by an array of random metallic nanoantennas and a metal ground plate. Intriguingly, the strong light trapping band (e.g. >80%) was realized throughout the entire visible to near-infrared spectral regime (i.e., from 435 nm to 1100 nm), enabling plasmonically enhanced surface harmonic generation and frequency mixing across a broad range of excitation wavelengths, which cannot be achieved with narrow band periodic plasmonic structures. By introducing hybrid random antenna arrays with small metallic nanoparticles and ultra-thin nonlinear optical films (e.g. TiO2) into the nanogaps, the nonlinear optical process can be further enhanced. This broadband light-trapping metastructure shows its potential as a building block for emerging nonlinear optical meta-atoms.

Directional Emission from Dielectric Leaky-Wave Nanoantennas.

An important source of innovation in nanophotonics is the idea to scale down known radio wave technologies to the optical regime. One thoroughly investigated example of this approach are metallic nanoantennas which employ plasmonic resonances to couple localized emitters to selected far-field modes. While metals can be treated as perfect conductors in the microwave regime, their response becomes Drude-like at optical frequencies. Thus, plasmonic nanoantennas are inherently lossy. Moreover, their resonant nature requires precise control of the antenna geometry. A promising way to circumvent these problems is the use of broadband nanoantennas made from low-loss dielectric materials. Here, we report on highly directional emission from hybrid dielectric leaky-wave nanoantennas made of Hafnium dioxide nanostructures deposited on a glass substrate. Colloidal semiconductor quantum dots deposited in the nanoantenna feed gap serve as a local light source. The emission patterns of hybrid nanoantennas with different sizes are measured by Fourier imaging. We find for all antenna sizes a highly directional emission, underlining the broadband operation of our design.

Quantum description of the optical response of charged monolayer–thick metallic patch nanoantennas

The optical response of small charged metallic nanodisks of one atomic monolayer thickness is analyzed under the excitation by an incident plane wave and by a localized pointlike dipole. Using the time-dependent density functional theory (TDDFT) and classical electrodynamical calculations we identify the bright and dark plasmon modes and study their evolution under external charging of the nanostructure. For neutral nanodisks, despite their monolayer thickness, the in-plane optical response, as obtained from TDDFT, is in agreement with classical electromagnetic results. The optical response for an incident wave polarized perpendicular to the nanostructure cannot be retrieved classically as it reflects a discrete energy structure of electronic levels. This latter situation appears most sensitive to external charging while the energy of the in-plane plasmon with dipolar character is nearly charge independent.

Single Asymmetric Plasmonic Antenna as a Directional Coupler to a Dielectric Waveguide

Directional in-coupling of light into a single-mode SixNy waveguide is demonstrated using a single subwavelength asymmetrically shaped metallic nanoantenna. In simulation as well as experiments, we show that at its quadrupolar resonance the V-shaped antenna can couple near-infrared light in the waveguide with a directivity reaching 25 dB. As an ultracompact directional coupler in the near-infrared and visible spectral range, the investigated system constitutes an important building block for integrated nanophotonic circuits and sensors.

Antenna-Enhanced Nonlinear Infrared Spectroscopy in Reflection Geometry

We propose and demonstrate antenna-enhanced nonlinear infrared (IR) spectroscopy in reflection geometry. Our approach uses resonant metal nanoantennas to enhance near-fields and to amplify the interaction between molecular vibrations and IR light. We successfully obtain amplified nonlinear vibrational signals in backscattering light of nanoantennas using IR pump energy of 10 nJ, with local signal enhancement of more than 7 orders of magnitude. This ultrasensitive and reflection-type method is useful for characterizing the structure and dynamics of minute volumes of molecules, monolayer materials, and biomolecules in aqueous environments, with additional benefit of surface sensitivity.

Polarization contrast scattering spectroscopy of individual metal nanoantennas

Metal nanoantennas give rise to strongly localized and enhanced electric fields. The enhancement is largest at the plasmon resonance, which, therefore, needs to be characterized. Here, we use polarization contrast microscopy for imaging and spectroscopy of single metal nanoantennas. The method relies on the strong suppression of incident linearly polarized light with a cross-polarized analyzer in the detection path and exploits the distinct polarization dependence of the plasmonic response of the antennas. The technique enables an easy-to-use background-free measurement of plasmon resonances of single metal nanoantennas in the visible and infrared spectral range.

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.

Plasmonic photoluminescence for recovering native chemical information from surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) spectroscopy has attracted tremendous interests as a highly sensitive label-free tool. The local field produced by the excitation of localized surface plasmon resonances (LSPRs) dominates the overall enhancement of SERS. Such an electromagnetic enhancement is unfortunately accompanied by a strong modification in the relative intensity of the original Raman spectra, which highly distorts spectral features providing chemical information. Here we propose a robust method to retrieve the fingerprint of intrinsic chemical information from the SERS spectra. The method is established based on the finding that the SERS background originates from the LSPR-modulated photoluminescence, which contains the local field information shared also by SERS. We validate this concept of retrieval of intrinsic fingerprint information in well controlled single metallic nanoantennas of varying aspect ratios. We further demonstrate its unambiguity and generality in more complicated systems of tip-enhanced Raman spectroscopy (TERS) and SERS of silver nanoaggregates.

Visible light focusing flat lenses based on hybrid dielectric-metal metasurface reflector-arrays

Conventional metasurface reflector-arrays based on metallic resonant nanoantenna to control the wavefront of light for focusing always suffer from strong ohmic loss at optical frequencies. Here, we overcome this challenge by constructing a non-resonant, hybrid dielectric-metal configuration consisting of TiO2 nanofins associated with an Ag reflector substrate that provides a broadband response and high polarization conversion efficiency in the visible range. A reflective flat lens based on this configuration shows an excellent focusing performance with the spot size close to the diffraction limit. Furthermore, by employing the superimposed phase distribution design to manipulate the wavefront of the reflected light, various functionalities, such as multifocal and achromatic focusing, are demonstrated for the flat lenses. Such a reflective flat lens will find various applications in visible light imaging and sensing systems.

Efficient Third Harmonic Generation from Metal-Dielectric Hybrid Nanoantennas.

High refractive index dielectric nanoantennas are expected to become key elements for nonlinear nano-optics applications due to their large nonlinearities, low energy losses, and ability to produce high electric field enhancements in relatively large nanoscale volumes. In this work, we show that the nonlinear response from a high-index dielectric nanoantenna can be significantly improved by adding a metallic component to build a metal-dielectric hybrid nanostructure. We demonstrate that the plasmonic resonance of a Au nanoring can boost the anapole mode supported by a Si nanodisk, strongly enhancing the electric field inside the large third-order susceptibility dielectric. As a result, a high third harmonic conversion efficiency, which reaches 0.007% at a third harmonic wavelength of 440 nm, is obtained. In addition, by suitably modifying geometrical parameters of the hybrid nanoantenna, we tune the enhanced third harmonic emission throughout the optical regime. Coupling metallic and dielectric nanoantennas to expand the potential of subwavelength structures opens new paths for efficient nonlinear optical effects in the visible range on the nanoscale.

Tunable Nanoantennas for Surface Enhanced Infrared Absorption Spectroscopy by Colloidal Lithography and Post-Fabrication Etching

We fabricated large-area metallic (Al and Au) nanoantenna arrays on Si substrates using cost-effective colloidal lithography with different micrometer-sized polystyrene spheres. Variation of the sphere size leads to tunable plasmon resonances in the middle infrared (MIR) range. The enhanced near-fields allow us to detect the surface phonon polaritons in the natural SiO2 thin layers. We demonstrated further tuning capability of the resonances by employing dry etching of the Si substrates with the nanoantennas acting as the etching masks. The effective refractive index of the nanoantenna surroundings is efficiently decreased giving rise to blueshifts of the resonances. In addition, partial removal of the Si substrates elevates the nanoantennas from the high-refractive-index substrates making more enhanced near-fields accessible for molecular sensing applications as demonstrated here with surface-enhanced infrared absorption (SEIRA) spectroscopy for a thin polymer film. We also directly compared the plasmonic enhancement from the Al and Au nanoantenna arrays.

Electric Field-Induced High Order Nonlinearity in Plasmonic Nanoparticles Retrieved with Time-Dependent Density Functional Theory

The nonlinear response of metallic nanoparticles is obtained from quantum time dependent density functional theory calculations. Without any aprioristic assumption our calculations allow us to identify high-order harmonic generation in canonical plasmonic structures such as spherical single particles and dimers. Furthermore, we demonstrate that under currently available experimental conditions, the application of an external polarizing field to the nanoparticles allows to actively control even-order harmonic generation in otherwise symmetry forbidden situations. Our quantum calculations provide quantitative access to the high-order response of metallic nanoantennas, which is of utmost importance in the design, control, and exploitation of optoelectronic devices as well as in the generation of extreme ultraviolet radiation.

Entangled light from bimodal optical nanoantennas

We suggest a hybrid plasmonic device consisting of a bimodal metallic nanoantenna coupled to an incoherently pumped quantum emitter. This hybrid device emits light into the two modes entangled in the number of photons. The process is a prime example where losses are turned from a nuisance into something beneficial, since, even though counterintuitively, the entanglement is enabled by strong incoherent processes, i.e. dominant scattering and absorption rates of the nanoantenna. Both, the high emission rate and the degree of entanglement of the emitted light are insensitive with respect to imperfections in the nanoantenna geometry, rendering the scheme feasible for an implementation.

Superresolution Focusing Using Metasurface with Circularly Arranged Nanoantennas

Circular lens composed of annularly arranged metal nanoantennas is proposed to achieve far field superresolution focusing. Light illuminating on the nanoantennas’ layer approximately acquires paraboloidal phase profile and then focuses into a spot. Lens constructed by monolayer nanoantennas achieve focusing with FWHM (full width at half maximum) of 924 nm and a focal length of 3385 nm, breaking the diffraction limit. Moreover, tri-layered lens realizes subwavelength focusing with FWHM of 320 nm (about 0.49λ) and the field intensity of focus is optimized to 0.97 a.u. (arbitrary unit). Our proposal shows advances in focusing performance compared with previous work, making it promising in many applications, such as nanolithography, dense storage, and integrated optics.

Modeling and Performance Analysis of Metallic Plasmonic Nano-Antennas for Wireless Optical Communication in Nanonetworks

In this paper, metallic plasmonic nano-antennas are modeled and analyzed for wireless optical communication. More specifically, a unified mathematical framework is developed to investigate the performance in transmission and reception of metallic nano-dipole antennas. This framework takes into account the metal properties, i.e., its dynamic complex conductivity and permittivity; the propagation properties of surface plasmon polariton waves on the nano-antenna, i.e., their confinement factor and propagation length; and the antenna geometry, i.e., length and radius. The generated plasmonic current in reception and the total radiated power and efficiency in transmission are analytically derived by utilizing the framework. In addition to numerical results, the analytical models are validated by means of simulations with COMSOL Multi-physics. The developed framework will guide the design and development of novel nano-antennas suited for wireless optical communication.

Enhanced infrared detectors using resonant structures combined with thin type-II superlattice absorbers

We examined the spectral responsivity of a 1.77 μm thick type-II superlattice based long-wave infrared detector in combination with metallic nanoantennas. Coupling between the Fabry-Pérot cavity formed by the semiconductor layer and the resonant nanoantennas on its surface enables spectral selectivity, while also increasing peak quantum efficiency to over 50%. Electromagnetic simulations reveal that this high responsivity is a direct result of field-enhancement in the absorber layer, enabling significant absorption in spite of the absorber's subwavelength thickness. Notably, thinning of the absorbing material could ultimately yield lower photodetector noise through a reduction in dark current while improving photocarrier collection efficiency. The temperature- and incident-angle-independent spectral response observed in these devices allows for operation over a wide range of temperatures and optical systems. This detector paradigm demonstrates potential benefits to device performance with applications throughout the infrared.

Monolithically integrated single quantum dots coupled to bowtie nanoantennas.

Deterministically integrating semiconductor quantum emitters with plasmonic nano-devices paves the way towards chip-scale integrable, true nanoscale quantum photonic technologies. For this purpose, stable and bright semiconductor emitters are needed, which moreover allow for CMOS-compatibility and optical activity in the telecommunication band. Here, we demonstrate strongly enhanced light-matter coupling of single near-surface (< 10 nm) InAs quantum dots monolithically integrated into electromagnetic hot-spots of sub-wavelength sized metal nanoantennas. The antenna strongly enhances the emission intensity of single quantum dots by up to ~ 16×, an effect accompanied by an up to 3.4× Purcell-enhanced spontaneous emission rate. Moreover, the emission is strongly polarised along the antenna axis with degrees of linear polarisation up to ~ 85 %. The results unambiguously demonstrate a pronounced coupling of individual quantum dots to state-of-the-art nanoantennas. Our work provides new perspectives for the realisation of quantum plasmonic sensors, step-changing photovoltaic devices, bright and ultrafast quantum light sources and efficient nano-lasers.