Metallic Gratings Research Articles

Spectral Dyadic Green’s Function for Multilayer Periodic Bi-Anisotropic Media

A computational technique for evaluating the spectral dyadic Green’s function utilized in the analysis of multilayer structures composed of periodic bi-anisotropic layers is presented. The Green’s function is achieved with the help of a fully vectorial rigorous matrix formulation based on a generalized transmission line (TL) formulation of Maxwell’s equations. This matrix formulation simplifies the mathematics involved in the modeling of the coupled diffraction orders within a periodic bi-anisotropic layer. To examine the soundness of this modeling, the extracted Green’s function is used in an integral equation for the analysis of metallic gratings on homogeneous/inhomogeneous anisotropic and chiral materials. The resulting integral equation is then solved by the Method of Moments (MoM) formulated in terms of sub-domain basis and Galerkin’s test functions. As verification, several examples are analyzed and the results obtained by this method are compared with those available in the literature or obtained by EM-solvers. The efficiency assessment of this method is carried out by considering its convergence rate and cost-time in terms of truncation orders. It is revealed not only the metallic grating with inhomogeneous periodic bi-anisotropic layers can be analyzed by this method but also analysis time, memory, and CPU occupancies are decreased in comparison with commercial EM-solvers.

3J2-3 Detection of GHz shear acoustic waves in picosecond laser ultrasonics assisted by two-dimensional metallic diffraction gratings

3J2-3 Detection of GHz shear acoustic waves in picosecond laser ultrasonics assisted by two-dimensional metallic diffraction gratings

Polarization dependent high refractive index metamaterial with metallic dielectric grating structure in infrared band

According to the theory of high refractive index of metamaterials, a composite structure of metal dielectric grating was designed to achieve high refractive index in infrared band. Based on the S-parameter inversion algorithm, we extracted the effective permittivity, the effective permeability, and the effective refractive index of the designed metamaterial. By changing the geometric parameters of the composite grating metamaterial structure, the effective refractive index of the designed metamaterial reaches more than 8.0 at the infrared resonance frequency. This is a high refractive index that many natural materials cannot achieve. It is noteworthy that the metamaterial structure has obvious polarization sensitivity. The metamaterial structure has both high refractive index and wide-band zero refractive index properties when different polarized light is incident. At the same time, we further investigate the influence of metamaterial geometric parameters on the effective refractive index of metamaterials. Also, we propose a double grating metamaterial structure to obtain more degrees of freedom of metamaterial on the effective refractive index.

On the application of MoS2 monolayer for enhanced performance in metallic grating based plasmonic sensor structure

On the application of MoS2 monolayer for enhanced performance in metallic grating based plasmonic sensor structure

2-D Artificial Media With Forced Refraction

The propagation of guided waves along two identical metal gratings symmetrically lying on both sides of a dielectric substrate is studied. Earlier, investigation of the properties of such waves has been restricted to odd waves equivalent to waves in grounded dielectric substrate. Averaged boundary conditions for the fields at the grating were used to simplify the analysis. Even waves equivalent to waves in a substrate grounded by a perfect magnetic wall are in the focus of this paper. The dominant even wave is studied with help of electromagnetic HFSS simulation and averaged boundary conditions. Both methods demonstrate that the investigated structure in some frequency range has properties of a 2D media with forced refraction. The results have potential application for microwave and millimeter wave planar quasi-optical lens antennas.

Broadband High-Efficiency Ultrathin Metasurfaces With Simultaneous Independent Control of Transmission and Reflection Amplitudes and Phases

We demonstrate a broadband near-100%-efficiency ultrathin metasurface operating at microwave and millimeter-wave frequencies. We develop and employ two orthogonally polarized metallic gratings to form a Fabry–Perot cavity and incorporate a subwavelength metallic double-split-ring resonator at the center of each unit cell. It allows arbitrary amplitude-phase combinations with no coupling between amplitude and phase or between transmitted and reflected waves, leading to the design of an ultrathin but highly efficient broadband metasurface with multiple functionalities. Furthermore, the proposed metasurface can generate diffractive beams with different orders and vortex beams with different orbital angular momentum (OAM) modes in reflection and transmission spaces simultaneously. Both numerical and experimental results verify that the proposed metasurface has superior performance to its counterparts that are based solely on phase control. The proposed metasurface presents a lightweight, low-cost, and easily deployable flat device for microwave and millimeter-wave applications.

Numerical Optimization of Refractive Index Sensors Based on Diffraction Gratings with High Aspect Ratio in Terahertz Range.

Terahertz surface plasmon resonance (SPR) sensors have been regarded as a promising technology in biomedicine due to their real-time, label-free, and ultrasensitive monitoring features. Different authors have suggested a lot of SPR sensors, including those based on 2D and 3D metamaterials, subwavelength gratings, graphene, and graphene nanotube, as well as others. However, one of the traditional approaches to realize high sensitivity SPR sensors based on metal diffraction gratings has been studied poorly in the terahertz frequency range. In this article, a linear metal rectangular diffraction grating with high aspect ratio is studied. The influence of the grating structure parameters on the sensor sensitivity is simulated. Effects arising from different ratios of depth and width were discovered and explained. The results show that the sensitivity can be increased to 2.26 THz/RIU when the refractive index range of the gas to measure is between 1 and 1.002 with the resolution .

Tunable and enhanced Goos–Hänchen shifts in a monolayer graphene-based metallic grating structure

The Goos–Hänchen (GH) effect, a lateral displacement of the reflection beam, enables rich applications in sensors and detectors. Metallic grating structures have gained great attention due to their unique properties, such as enhanced optical transmission and surface enhanced Raman scattering. Here, instead of using the dielectric grating, we demonstrate an enhanced GH shift in the structure with the metallic grating layer, monolayer graphene, dielectric spacer, and metallic substrate. We find that the amplitude of the GH shift is as high as 3616 times of the incident wavelength at the specific working frequency. This enhancement can be understood from the generation of the coupled surface plasmon polaritons between two surfaces of the dielectric space layer. Furthermore, we show efficient control of the GH shift by modulating the geometric parameters of the proposed structure and chemical potential of the monolayer graphene. Our result establishes an alternative metallic grating structure for tunable and enhanced GH shifts in graphene-based nanostructures.

Digital Multiplexing of Rectangular Gratings for Multiple Independent Resonance Modes

Multiplexed gratings can provide multiple independent resonance modes for control of light–matter interactions. Here, we propose a method to digitally multiplexing rectangular metal gratings that can be fabricated with planar processing technologies, and adaptable to various structure and material systems. As different logic operators (OR, AND, XOR) may be applied in the multiplexation, we investigate their influences on plasmonic resonance properties of as–multiplexed metallic gratings, as well as dependences of their performances on structure features of the constitutive gratings. It’s concluded that, generally, the OR–operation on constitutive rectangular metal gratings with a medium ratio of ridge–width to period generates optimal multiplexed gratings that have excellent plasmonic resonance figures and are less critical for fabrication. The proposed structures may be applied in developing sensors and optoelectronic devices for improved performances or novel functions.

Optical amplification by surface-plasmon-resonant Au grating substrates: Monolayer MoS2 with 170-fold second harmonic generation and 3-fold (off-resonance) Raman scattering

Nanoplasmonics is a potential game-changer in the development of next-generation on-chip photonic devices and computers, owing to the geometrically controlled and amplified linear and nonlinear optical processes. For instance, it resolves the limited light-matter interaction of the unique two-dimensional (2D) crystalline materials like semiconducting monolayer molybdenum disulfide (1L-MoS 2 ). Metal grating (MG) substrates excel at this because their surface plasmons (SPs) can lead to stark field confinement near the surface. This work studies optical amplification of 1L-MoS 2 on the gold (Au) MG substrate, which was designed to operate in a glycerol environment with SP resonance (SPR) at 850 nm excitation wavelength. Its design was verified by simulated and experimental reflectances, and topographically inspected by atomic force microscopy (AFM). Two advanced imaging modalities, second harmonic generation (SHG) and confocal Raman microscopy (CRM) were used to evaluate its 170-fold SHG on- and 3-fold CRM off-resonance optical amplifications, respectively. Some MoS 2 -to-grating adhesion issues due to trapped liquid showed as image nonuniformities. Possible improvements to limitations like surface roughness were also discussed. These Au MG substrates can boost conventional linear and nonlinear backscattering microscopies because they are tunable in the visible and near-infrared range by selecting geometry, metal, and environment. • Plasmonic optical amplification study of monolayer MoS 2 on Au/Si-grating substrate. • 170× on- and 3× off-resonance gains in second harmonic generation and Raman signals. • These nanogratings can boost both linear and nonlinear backscattering microscopies. • Tune by geometry, metal, and environment within the visible and near-infrared range. • Improve light-matter interaction (with less power and less light-induced damage).

Shaping the spontaneous emission of extended incoherent sources into composite radial vector beams

It is well known that concentric diffraction gratings are capable of beaming the spontaneous emission of large extended incoherent light sources (e.g., hot radiating surfaces and luminescent materials). Here, we reveal additional properties of such beams using layers of colloidal PbS nanocrystals coated onto metallic spiraling gratings as an example. We observe and explain the formation of multiple beams with a simple model when the spirals are deformed. We also point out an aspect of the light emission that does not seem to have been discussed so far—namely, that the polarization of the directional beams has a radial distribution. These findings are not restricted to our experimental configuration, suggesting a simple way to build incandescent and electroluminescent sources with non-trivial polarization states. The price to pay is an isotropic emission background due to the composite nature of the beams, which result from the incoherent superposition of a continuum of diffracted plasmons everywhere above the surface.

Terahertz non-label subwavelength imaging with composite photonics-plasmonics structured illumination.

Inspired by the capability of structured illumination microscopy (SIM) in subwavelength imaging, many researchers devoted themselves to investigating this methodology. However, due to the free-propagating feature of the traditional structured illumination fields, the resolution can be only improved up to two-fold of the diffraction-limited microscopy. Besides, most of the previous studies, relying on incoherent illumination sources, are restricted to fluorescent samples. In this work, a subwavelength non-fluorescent imaging method is proposed based on the illumination of terahertz traveling waves and plasmonics. Excited along with a metal grating, the spoof surface plasmons (SSPs) are employed as one of the illuminating sources. When the scattering waves with the SSPs illumination are captured, the sample's high-order spatial frequencies (SF) components are already encoded into the obtainable low-order ones. Then, a modified post-processing algorithm is exploited to shift the modulated SF components to their actual positions in the SF domain. In this manner, the fine information of samples is introduced to reconstruct the desired imaging, leading to an enhancement of the resolution up to 0.12λ0. Encouragingly, the resolution can be further enhanced by attaching extra illumination of SSPs with an elaborately selected frequency. This method holds promise for some important applications in terahertz non-fluorescent microscopy and sample detection with weak scattering.

Intensity-tunable terahertz bandpass filters based on liquid crystal integrated metamaterials.

In this paper, we demonstrate an intensity-tunable THz bandpass filter by introducing liquid crystal (LC) integrated with asymmetric frequency selective surface (FSS) and subwavelength metal gratings. Here, the tunable THz filter is derived from the inner polarization state conversion in composited devices, and the incident linear polarization can be converted into 90° orthogonal components. By controlling the LC orientation under the applied electric field with the metamaterial electrodes, the polarization conversion process can be actively modulated; thus, the polarization-dependent and tunable THz bandpass filter is achieved. Based on the multilayer design and the inner Fabry-Perot-like resonance mechanism, the LC-integrated metamaterials filter presents better filtering performance than the single FSS filter, and the Q-value is improved from 7.7 to 13.8 at the working frequency. Our simulated work paves the way for the design of new and efficient THz filters.

Tunable nonreciprocal thermal emitter based on metal grating and graphene

Various nonreciprocal thermal emitters, which provide opportunities for higher energy conversion efficiency, have been proposed to completely violate the traditional Kirchhoff's law. However, the tunable nonreciprocal thermal emitters remain barely investigated. In this paper, by sandwiching a graphene monolayer between a top metallic grating and a bottom magneto-optical film backed with a metallic mirror, we achieve tunable nonreciprocal radiation effect. It is shown that strong nonreciprocal radiation for graphene under initial state is realized at wavelength around 14.845 μm when the incident angle is 30° and the external magnetic field is 3 T. The physical origin behind such phenomenon is disclosed by investigating both the distribution of magnetic field at the resonant peak and the coupled-mode theory. Besides, the nonreciprocal radiation performance remains perfectly within certain range of structure parameters, which is benefit for practical fabrication and applications. More importantly, the strong nonreciprocal radiation of the proposed scheme can be flexibly tuned by the gate voltage of graphene without redesigning and refabricating the structure. We believe that this work will provide new approaches for the design of novel energy harvesting systems and nonreciprocal thermal emitters.

Strong dual-band nonreciprocal radiation based on a four-part periodic metal grating

Nonreciprocal radiation possesses many potential applications, such as solar photovoltaic system and military camouflage. However, the previous studies mainly focused on single-band nonreciprocal radiation. In this paper, a novel thermal emitter, in the form of a magneto-optical film sandwiched between a four-part periodic metal grating and a metal mirror, is designed and investigated. Numerical results show that the proposed emitter can achieve strong dual-band nonreciprocal radiation at wavelengths of 13.25 μm and 15.858 μm under a 3 T magnetic field when the angle of incidence is 25°. The physical mechanism can be explained by the distributions of magnetic field at the resonant wavelengths. Furthermore, the nonreciprocal radiation is robust against the geometric dimensions, which greatly reduces the difficulty of fabrication. We believe that this work is promising in providing a new approach for the design of novel energy conversion devices and frequency selective detectors.

Low-Threshold Nanolaser Based on Hybrid Plasmonic Waveguide Mode Supported by Metallic Grating Waveguide Structure.

A high Q-factor of the nanocavity can effectively reduce the threshold of nanolasers. In this paper, a modified nanostructure composed of a silver grating on a low-index dielectric layer (LID) and a high-index dielectric layer (HID) was proposed to realize a nanolaser with a lower lasing threshold. The nanostructure supports a hybrid plasmonic waveguide mode with a very-narrow line-width that can be reduced to about 1.79 nm by adjusting the thickness of the LID/HID layer or the duty ratio of grating, and the Q-factor can reach up to about 348. We theoretically demonstrated the lasing behavior of the modified nanostructures using the model of the combination of the classical electrodynamics and the four-level two-electron model of the gain material. The results demonstrated that the nanolaser based on the hybrid plasmonic waveguide mode can really reduce the lasing threshold to 0.042 mJ/cm2, which is about three times lower than the nanolaser based on the surface plasmon. The lasing action can be modulated by the thickness of the LID layer, the thickness of the HID layer and the duty cycle of grating. Our findings could provide a useful guideline to design low-threshold and highly-efficient miniaturized lasers.

Enhanced narrowband mid-IR thermal radiation enabled by plasmonic stacked gratings

We demonstrate tailored thermal radiation in the mid-infrared wavelength range with intriguing capabilities of high emissivity, narrowband spectra, and sharp angular response. The proposed thermal emitter consists of stacking a two-dimensional metallic grating on top of a one-dimensional dielectric Bragg grating (BG). It is interesting to find that the light interaction between the metallic grating and the BG gives rise to impedance matching at wavelengths located in the photonic bandgap of the BG, and allows enhanced polarization-dependent emissivity, selective resonance response, and high radiation directivity. We investigate the plasmonic resonance by exploring various properties and functions including tuning spectral selectivity and tailoring bandwidth at different light polarization and angle of incidence. The developed plasmonic stacked gratings could pave the way towards novel integrated infrared source platforms for various applications such as thermal analysis, imaging, security, biosensing, and medical diagnosis.

Optical spatiotemporal differentiator using a bilayer plasmonic grating.

As a key element in wave-based analog computation, optical differentiators have been implemented to directly perform information processing, such as edge detection and pulse shaping, in both spatial and temporal domains. Here, we propose an optical spatiotemporal differentiator, which simultaneously performs first-order spatial and temporal differentiation in transmission by breaking the mirror symmetry of a subwavelength bilayer metal grating. The spatial and temporal performance of the plasmonic differentiator is evaluated numerically using the output field profiles of an optical beam and pulse envelope, showing resolutions of ∼2µm and ∼50fs, respectively. Moreover, the function of spatiotemporal differentiation is demonstrated with input flat-top pulse fields. The proposed optical differentiator has potential applications in ultra-compact real-time optical multifunctional computing systems and parallel signal processing.

Miniaturized spectroscopy with tunable and sensitive plasmonic structures.

A broad linewidth and a lack of spectral analysis limit the applications of plasmonic sensors. In this Letter, a plasmonic sensor with a large sensitivity in the terahertz (THz) range is proposed based on high-quality factor (>1000) surface lattice resonance in subwavelength near-flat metallic gratings. Moreover, such a highly selective spectral manipulating scheme, plus the greatly localized plasmonic resonance, enables miniaturized spectroscopy based on a single detector by integrating an electro-optical material with the gratings. A spectral resolution of 0.1 GHz at a center frequency of 1.1 THz is predicted showing a four times improvement of measuring efficiency. This technique shows promising potential in on-site matter inspection and point-of-care testing.

Mechano-Optical Resonant Emission by Edge Angle Modulation of Wrinkled Graphene on Plasmonic Metal Gratings

Graphene coated on top of photonic-plasmonic metasurfaces can produce resonant radiative emission in the mid-infrared region. Narrowband emission peaks are observed through folding graphene into “origami” ridges over metal grating grooves, creating a complementary cavity mode above the trench. This geometrically tuned phenomenon of graphene surface plasmon excitation along the folded sheet enhances the emission when added to magnetic polariton (MP) resonance induced within the plasmonic grating groove. Our analytical models describe how this graphene surface plasmon polariton (SPPG) is a function of folded graphene geometric parameters, and most importantly, the graphene edge angle that distends from the grating surface. The frequency-dependent phase shift of SPPG is fitted to grating parameters, and a modified inductor-capacitor circuit model was developed for predicting MP resonance mode with graphene influence. It was found that the edge angle of wrinkled graphene blue-shifts the groove MP resonance and SPPG resonant emission peak in both wrinkled graphene alone and with the grating substrate. The understanding of geometrically modulated graphene adhered on plasmonic gratings impacts the design and capability of narrowband cavity emitters and contributes toward the development of mechanical-optical environmental sensors.