Periodic Plasmonic Structures Research Articles

Formation of Highly Tunable Periodic Plasmonic Structures on Gold Films Using Direct Laser Writing

AbstractDirect laser writing method is a promising technique for the large‐scale and cost‐effective fabrication of periodic nanostructure arrays exciting hybrid lattice plasmons. This type of electromagnetic mode manifests a narrow and deep resonance peak with a high dispersion whose precise controllability is crucial for practical applications in photonic devices. Here, the formation of differently shaped gold nanostructures using the direct laser writing method on Au layers of different thicknesses is presented. The resonance peak is demonstrated to be highly dependent on the shape of the structures in the array, thus its position in the spectra, as well as the quality, can be additionally modulated by changing the morphology. The shape of the structure and the resonance itself pertain not only on the laser pulse energy but also on the grating period. This overlapping effect occurring at distances smaller than the diameter of the focused laser beam is studied in detail. By taking advantage of the highly controllable plasmonic resonance, the fabricated gratings open up new opportunities for applications in sensing.

Topological electromagnetic waves in dispersive and lossy plasma crystals

Topological photonic crystals, which offer topologically protected and back-scattering-immune transport channels, have recently gained significant attention for both scientific and practical reasons. Although most current studies focus on dielectric materials with weak dispersions, this study focuses on topological phases in dispersive materials and presents a numerical study of Chern insulators in gaseous-phase plasma cylinder cells. We develop a numerical framework to address the complex material dispersion arising from the plasma medium and external magnetic fields and identify Chern insulator phases that are experimentally achievable. Using this numerical tool, we also explain the flat bands commonly observed in periodic plasmonic structures, via local resonances, and how edge states change as the edge termination is periodically modified. This work opens up opportunities for exploring band topology in new materials with non-trivial dispersions and has potential radio frequency (RF) applications, ranging from plasma-based lighting to plasma propulsion engines.

Transformation of Equivalent Graphene Plasmonics for Metagratings and Hyperbolic Metamaterials

Recent advances in graphene plasmonics offer numerous opportunities for enabling the design and manufacture of a variety of nanoscale optical devices. Here, a method of designing metagratings and hyperbolic metamaterials based on the geometrical transformation of the proposed equivalent graphene is reported. The physical mechanism underlying this method is the strongly enhanced light–matter interaction of equivalent graphene plasmonics, which can be characterized by effective conductance and remains constant in geometrical transformation. As proof of the method, we design and demonstrate a compact retroreflector that can anomalously reflect the incident wave along its original path utilizing the roll-up plasmonic structure. In addition, an air-medium hyperbolic metamaterial and hyperlens consisting of periodic plasmonic structures are also theoretically predicted and experimentally validated by anomalous diffraction. Compared with the existing approaches of designing these metadevices, the proposed method significantly lowers the requirement on their compositional materials. The concept of equivalent graphene effectively links optical and microwave plasmonics on the basis of the effective conductance model. In this case, the methodology of geometrical transformation can function in both the microwave and nanofields and serves as a platform for designing nanoscale and microwave focusing lenses and diverse on-chip optical wave control devices.

Enhancing Nonlinear Interactions by the Superposition of Plasmonic Lattices on χ(2)-Nonlinear Photonic Crystals

Plasmonic structures have been revealed as efficient units to enhance localized nonlinear phenomena generated at dielectric–metal interfaces. However, their effect on the nonlinear interactions provided by quasi-phase matching processes in χ(2) modulated dielectric crystals have been scarcely addressed, mainly due to the complexity in manufacturing appropriate periodic plasmonic structures overlying the χ(2) dielectric structure. Here, by a simple method we have fabricated a periodic structure based on the combination of two commensurate lattices: a periodic lattice of chains of Ag nanoparticles and a periodic lattice of χ(2)-modulation based on a ferroelectric domains structure. The hybrid system supports multiple surface plasmon lattice resonances (SLRs) at the technologically relevant NIR spectral region, which yield the enhancement of the nonlinear diffraction pattern generated by the χ(2) structure. The superposition of the plasmonic and the χ(2)-modulation lattice results in a 20-fold enhancement of the directional SHG due to the excitation of SLRs by the interacting waves involved in the nonlinear process. The results are obtained in lithium niobate, a widely used crystal in optoelectronics, and demonstrate the potential of the approach to design integrated solid-state platforms for on-chip optical steering, multiplexing or quantum technologies.

LRTM effect and electronic crystal imaging on silicon surface

Some interesting phenomena have been observed in the laser reflecting Talbot magnification (LRTM) effect discovered at first, in which the high-order nonlinear imaging and the plasmonic structures imaging occur. The LRTM effect images were obtained on the 1D and 2D photonic crystals fabricated by using nanosecond pulsed laser etching on silicon surface, where the high-order nonlinear imaging on the 1D and 2D photonic crystals was observed interestingly. The theory result is consistent with the experimental one, which exhibits that the suitable wave-front shape of injection beam selected in optical route can effectively enlarge the magnification rate and elevate the resolution of the Talbot image. Especially the periodic plasmonic structures on silicon surface have been observed in the LRTM effect images, which have a good application in the online detection of pulsed laser etching process. The temporary reflecting Talbot images exhibit that the electrons following with photonic frequency float on plasma surface to form electronic crystal observed on silicon at first, which is similar with the Wigner crystal structure.

Influence of gold nanorods on the structure and photonic bandgap in a twist grain boundary phase with smectic C* blocks

We describe the first report of the influence of gold nanorods (GNR) on the induced twist grain boundary smectic C* (TGBC*) phase in a binary mixture of achiral bent-core and chiral linear liquid crystals. The GNR concentration-dependent phase diagram of these nanocomposites shows that the thermal range of this twist grain boundary phase having smectic C* blocks phase increases by 50% for an intermediate composition compared to that for the host binary mixture without nanorods. The inclusion of the nanorods is seen to have substantial effect on the structural and photonic bandgap features of the TGBC* phase. For example, the helical periodicity gets altered in all the three dimensions: while those within the block undergo a huge increase, the one which is orthogonal to the blocks, shrinks. The spacing of the square grid pattern arising normal to the TGB helix direction increases for the nanocomposites getting even doubled for a certain composition, a feature evidenced by optical microscopy as well as optical diffraction. Xray diffraction clearly brings out the feature that the presence of GNR alters the thermal character of the transition between the TGBC* and the cholesteric phase. Quantitative analysis of the data indicates that the system would remain in the vicinity of a possible tricritical point, a behavior having wider ramifications to understand the underlying critical phenomenon. Based on the experimental observations, and capturing the essence of the reported adaptive defect core targeting mechanism we propose a model wherein GNRs get confined in the grain boundary region. This feature offers a potential to have periodic and anisotropic plasmonic structure arising out of the synergetic interactions between the metal nanorod and the twisted grain boundary structure.

Identification of the absorption processes in periodic plasmonic structures using energy absorption interferometry.

Power dissipation in electromagnetic absorbers is a quadratic function of the incident fields. To characterize an absorber, one needs to deal with the coupling that may occur between different excitations. Energy absorption interferometry (EAI) is a technique that highlights the independent degrees of freedom through which a structure can absorb energy: the natural absorption modes of the structure. The coupling between these modes vanishes. In this paper, we use the EAI formalism to analyze different kinds of plasmonic periodic absorbers while rigorously accounting for the coupling: resonant golden patches on a grounded dielectric slab, parallel free-standing silver wires, and a silver slab of finite thickness. The EAI formalism is used to identify the physical processes that mediate absorption in the near and far field. First, we demonstrate that the angular absorption, which is classically used to characterize periodic absorbers in the far field and which neglects the coupling between different plane waves, is only valid under stringent conditions (subwavelength periodicity, far-field excitation, and negligible coupling between the two possible polarizations). Using EAI, we show how the dominant absorption channels can be identified through the signature of the absorption modes of the structure, while rigorously accounting for the coupling. We then exploit these channels to improve absorption. We show that long-range processes can be exploited to enhance the spatial selectivity, while short-range processes can be exploited to improve absorptivity over wide angles of incidence. Finally, we show that by simply adding scatterers with the proper periodicity on top of the absorber, the absorption can be increased by more than 1 order of magnitude.

Observation of Polarization Vortices in Momentum Space.

The vortex, a fundamental topological excitation featuring the in-plane winding of a vector field, is important in various areas such as fluid dynamics, liquid crystals, and superconductors. Although commonly existing in nature, vortices were observed exclusively in real space. Here, we experimentally observed momentum-space vortices as the winding of far-field polarization vectors in the first Brillouin zone of periodic plasmonic structures. Using homemade polarization-resolved momentum-space imaging spectroscopy, we mapped out the dispersion, lifetime, and polarization of all radiative states at the visible wavelengths. The momentum-space vortices were experimentally identified by their winding patterns in the polarization-resolved isofrequency contours and their diverging radiative quality factors. Such polarization vortices can exist robustly on any periodic systems of vectorial fields, while they are not captured by the existing topological band theory developed for scalar fields. Our work provides a new way for designing high-Q plasmonic resonances, generating vector beams, and studying topological photonics in the momentum space.

A Plasmonic Sensor Array with Ultrahigh Figures of Merit and Resonance Linewidths down to 3 nm

Surface plasmon polaritons (SPPs) are extremely sensitive to the surrounding refractive index and have found important applications in ultrasensitive label-free sensing. Reducing the linewidth of an SPP mode is an effective way to improve the figure of merit (FOM) and hence the sensitivity of the plasmonic mode. Many efforts have been devoted to achieving a narrow linewidth by mode coupling, which inevitably results in an asymmetrical lineshape compromising the performance. Instead, the SPP modes are directly narrowed by elaborately engineering periodic plasmonic structures with minimized feature sizes to effectively reduce the radiative losses. A narrow linewidth smaller than 8 nm is achieved over a wide wavelength ranging from 600 to 960 nm and a minimum full width at half maximum of 3 nm at 960 nm. Benefiting from the almost perfect Lorentzian lineshape and the extremely narrow linewidth, a record FOM value of 730 is obtained. The sensor is capable of detecting bovine serum albumin with an ultralow concentration of 10-10 m. The sensor has great potential for practical application for its ultrahigh FOM, broad working wavelength, and ease of high-throughput fabrication.

Giant effect of terahertz-radiation rectification in periodic graphene plasmonic structures

The rectification of terahertz radiation due to plasmonic nonlinearities in a periodic graphene structure with a dual-grating gate and an asymmetric unit cell is considered. The influence of unit-cell asymmetry on the plasmonic rectification of the terahertz radiation near higher plasmonic resonances is considered for the cases of electron and hole conduction in parts of the graphene unit cell. The current responsivity of the rectification of terahertz radiation due to the plasmonic effects of differential charge-carrier drag and electron–hole ratchet is calculated.

Fabrication and optical properties of nanostructured plasmonic Al2O3/Au–Al2O3/Al2O3 metamaterials

Discontinuous multilayer (DML) thin films, which consist of nano-granular metals (NGMs) embedded in a dielectric matrix, have attracted significant interest as engineered plasmonic metamaterials. In this study, a systematic layer-by-layer deposition of three-dimensional sub-wavelength periodic plasmonic DML structures via the radio frequency sputtering of a composite target has been reported. The overall optical response of the DML films composed of Au–Al2O3 NGM homogenous layers, which are periodically sandwiched between two amorphous Al2O3 layers, are studied using reflection spectroscopic ellipsometry and transmission spectroscopy techniques. By applying the analytical optical approaches based on multiple Gaussian oscillators, ambient DML sub-wavelength structures have been successfully modeled. As a result, the effects of the size and shape of the Au nanoparticles as well as of the surrounding and interfacial media on their localized surface plasmon resonance (LSPR) are elucidated, and the related films thickness and effective optical constants are determined. Interestingly, during the examination of resonance frequencies and dielectric functions, the obtained DML structures exhibit unusual characteristics that are different from those of their NGM constituents due to the electromagnetic interactions of the NGM layers with the LSPR, which represent metamaterial features.

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.

Plasmonic Structure Integrated Single-Photon Detectors Optimized to Maximize Polarization Contrast

Numerical optimization was performed via COMSOL Multiphysics to maximize the polarization contrast of superconducting nanowire single photon detectors (SNSPDs). SNSPDs were integrated with four different types of one-dimensional periodic plasmonic structures capable of mediating p-polarized photon selectivity to the niobium-nitride superconducting nanowire pattern. Optimization with two different criteria regarding the maximal tilting resulted in wavelength-scaled periodic integrated structures, which have different geometrical parameters, and exhibit different polar angle dependent optical response and dispersion characteristics, as well as accompanying near-field phenomena at the extrema. Polarization contrast of 6.37*10^2 and 3.28*10^2 - 6.86*10^11 and 1.42*10^11 - 1.81*10^13 and 7.87*10^12 - 1.90*10^3 and 1.15*10^5 can be achieved in nanocavity-, nanocavity-deflector-, nanocavity-double-deflector-, nanocavity-trench-array-integrated P-SNSPDs optimized with 85 degree and 80 degree criterion regarding the maximal tilting.

Tunable plasmonic thin magneto-optical wave plate

Planar periodic plasmonic structures encased in a magnetic host revealed an unexpected polarization transformation. In the proposed thin periodic gold–garnet layer, the near fields strongly modify (enhance) the magneto-optical response of garnet. We show that the configuration of near fields in the magnetized layer can be engineered so that the layer converts the linear polarization to elliptical (or circular) or rotates the plane of polarization over a large angle in the transmission (or Faraday) geometry. Since the helicity of elliptically (circularly) polarized light (or the polarization rotation angle) is altered by reversing the magnetization of garnet, the considered plasmonic structure acts as a tunable wave plate.

Spectral Solution for Scattering Analysis of Periodic Plasmonic Nano-antennas on Iso/Anisotropic Substrate

A fast solution for the scattering analysis of periodic plasmonic nano-antennas (PPNAs) on iso/anisotropic substrates is presented. This method is based on derivation of the spectral dyadic Green's function with taking into account the plasmonic effects. The obtained dyadic Green's function is then used in an integral equation formulated for computing the induced surface current densities on the PPNAs. The resultant integral equation is solved by means of Galerkin's Method of Moments with entire-domain basis and test functions. With the help of this computational technique, the scattering characteristics of several examples are analyzed. The computed results are validated by means of a full-wave EM solver based on finite element method. For showing the efficiency of this method, the convergence rate and computation time in terms of space harmonic orders are calculated. It is shown that the developed method is an efficient and speedy technique for the analysis of such periodic plasmonic structures. In addition, we study an array of PPNAs on a uniaxial anisotropic substrate where the scattering characteristics are calculated as a function of the optic-axis direction. It is found that using anisotropic materials as the substrate of PPNAs allows additional degrees of freedom for controlling the scattering information.

Electron tunneling through water layer in nanogaps probed by plasmon resonances

With an example of the periodic plasmonic dolmen structure we performed a theoretical study of the effect of the conducting water layer on the plasmon resonances of the system with narrow gaps. Using the scanning tunneling microscopy studies of the conductance of the water junctions as inputs, we show that water layer(s) should affect plasmon modes of the systems with nm and sub nm gaps in two ways. The frequency of the plasmon modes shifts because of the dielectric screening, as commonly used in plasmonic sensors, and the corresponding resonance in the optical spectra looses intensity and broadens because of the resistive tunneling current. The water layer in the junction lowers potential barrier for electron tunneling, so that quantum effects in plasmon response appear for the junction width at least twice larger as compared to the vacuum gaps.

Fourier plane colorimetric sensing using broadband imaging of surface plasmons and application to biosensing

We demonstrate an optical technique for refractive index and thickness sensing of sub-wavelength-thick dielectric analytes. The technique utilizes the broadband, multimode, directional leakage radiation arising from the excitation of hybrid mode surface plasmons (SP) on low aspect ratio periodic plasmonic substrates with period ≈λ. The approach requires relaxed fabrication tolerances compared to extra ordinary transmission-based sensing techniques, wherein minor shifts in the fabricated dimensions result in a very large change from the designed resonant wavelength. We show that refractive index perturbations due to about 10-nm-thick dielectric can be captured optically by the usage of carefully designed plasmonic substrates, a halogen lamp source, free-space optical components, polarizers, and a low-end, consumer-grade charge coupled device camera. The plasmonic substrates were designed for converting the signature of hybrid mode SP excitation into a transmission peak by utilizing a thin homogeneous metal layer sandwiched between the periodic plasmonic structures and the substrate. The resonance is highly sensitive to the refractive index and thickness of the analyte superstrate. The excitation of hybrid mode SP results in a polarization rotation of 90° of the leaked radiation at resonant wavelength. In order to eliminate the problem of image registration (i.e., placing the same feature in the same pixel of the image, for comparison before and after a change in refractive index) for sensing, we perform the color analysis in the Fourier plane. The change in color of the bright emitted spot with highest momentum, corresponding to the leakage of fundamental SP mode, was used to measure the changes in refractive index, whereas the number and color of spots of lower momenta, corresponding to higher-order Fabry Perot modes, was used to measure the variation in thickness. We further show that the Fourier plane analysis can also be used to sense the index of thicker dielectrics, where real plane image analysis may fail to sense index perturbations, simply due to superposition of different modes in the real plane images of such substrates. Control experiments and analysis revealed a refractive index resolution of 10–5 RIU. The results were correlated with simulations to establish the physical origin of the change in the fundamental mode and higher-order modes due to the refractive index and thickness of analyte. As a demonstration of an application and to test the limits of sensing, the substrates were used to image the surface functionalization using 2-nm-thick 11-mercaptoundecanoic acid and immobilization of 7-nm-thick mouse anti-human IgG antibody. In biological systems, where a priori knowledge about a process step is available, where accurate chemical composition testing is not necessary or possible, the presented method could be used to study the surface changes using a label-free sensing mechanism.

Fabrication of Periodic Plasmonic Structures Using Interference Lithography and Chalcogenide Photoresist.

This study reports on the employment of the interference lithography (IL) technique, using photoresist based on the chalcogenide glass (ChG) films, for fabrication of one-dimensional (gratings) and two-dimensional (arrays) periodic plasmonic structures on the surface of glass plates. The IL technique was optimized for patterning of the Au and Al layers and formation of gratings and arrays with a spatial frequency of 2000 mm−1. Optical properties of obtained structures were studied using measurements of spectral and angular dependence of transmission and reflection of polarized light. It was shown that the spectral and angular position of the surface plasmon polariton and local surface plasmon resonance, which are observed in these samples, can be adjusted over a wide range by selecting the geometric parameters of structures and technological modes of their manufacturing.

Imaging the Engineered Polarization States of Surface Plasmon Polaritons at Visible Wavelengths

We experimentally demonstrate and image the hybrid nature of surface plasmon polaritons (SPPs) at visible wavelengths excited in low aspect ratio 1D and 2D periodic plasmonic structures with periods nearly equal to the resonance wavelengths, fabricated on thin homogeneous metal-coated glass substrates. A thin homogeneous metal layer was sandwiched between the substrate and the periodic metal patterns to out-couple the SPPs as leakage radiation through the substrate. This resulted in the detection of SPP excitation as transmission peaks in a dark background, as opposed to transmission dips in a bright background in the structures without the metal layer reported earlier. Due to the periods being almost equal to the resonance wavelengths, the transmission peaks were not due to extraordinary transmission phenomenon, but because of the radiative leakage of wavelength selective SPPs excited on the surface. A cross-axis polarizer-analyzer was used in broadband leakage radiation microscopy to diminish the direct zeroth -order transmission and image the real and Fourier plane characteristics of the SPP transmission. The bright emission of different colors against a dark background corresponding to the transmission plasmonic resonances for different periods, in both real and Fourier plane revealed the hybrid nature of excited SPPs, when the polarizer was positioned at 45$^\circ$ with respect to the grating vector. The fabricated plasmonic substrates present interesting opportunities for imaging and sensing applications.

Terahertz Plasmonics: Good Results and Great Expectations

The terahertz (THz) range is the next frontier of electronics and optoelectronics with potential applications ranging from imaging, space communications, computing, quality control, and homeland security to biotechnology and medicine. At THz frequencies, the electron inertia becomes important, providing delay between the applied voltage and electron velocity and current. When the electron collisions with impurities and lattice vibrations are infrequent, this delay leads to oscillations of the electronic density (called plasma waves) with the transistor channels serving as resonant cavities for the plasma waves. In the collision-dominated regime, the plasma waves are overdamped but still play a role by dramatically changing the electron distribution in the device channels at THz frequencies. The resonant regime can be used to generate THz radiation. Both resonant and overdamped plasma waves enable other THz electronic devices, such as detectors, mixers, and phase shifters. Periodic (symmetrical and asymmetric) plasmonic structures are especially promising for generation and detection of THz radiation. In this article, we review the state of the art of the plasma-wave electronics for silicon, III-V, III-N, and graphene semiconductor devices and project future performance of plasma-wave THz devices.