Wire Gratings Research Articles

Evolutionary Optimized, Monocrystalline Gold Double Wire Gratings as a Novel SERS Sensing Platform (Small 35/2024)

Evolutionary Optimized, Monocrystalline Gold Double Wire Gratings as a Novel SERS Sensing Platform (Small 35/2024)

Evolutionary Optimized, Monocrystalline Gold Double Wire Gratings as a Novel SERS Sensing Platform.

Achieving reliable and quantifiable performance in large-area surface-enhanced Raman spectroscopy (SERS) substrates poses a formidable challenge, demanding signal enhancement while ensuring response uniformity and reproducibility. Conventional SERS substrates often made of inhomogeneous materials with random resonator geometries, resulting in multiple or broadened plasmonic resonances, undesired absorptive losses, and uneven field enhancement. These limitations hamper reproducibility, making it difficult to conduct comparative studies with high sensitivity. This study introduces an innovative approach that addresses these challenges by utilizing monocrystalline gold flakes to fabricate well-defined plasmonic double-wire resonators through focused ion-beam lithography. Inspired by biological strategy, the double-wire grating substrate (DWGS) geometry is evolutionarily optimized to maximize the SERS signal by enhancing both excitation and emission processes. The use of monocrystalline material minimizes absorption losses and ensures shape fidelity during nanofabrication. DWGS demonstrates notable reproducibility (RSD=6.6%), repeatability (RSD=5.6%), and large-area homogeneity >104µm2. It provides a SERS enhancement for sub-monolayer coverage detection of 4-Aminothiophenol analyte. Furthermore, DWGS demonstrates reusability, long-term stability on the shelf, and sustained analyte signal stability over time. Validation with diverse analytes, across different states of matter, including biological macromolecules, confirms the sensitive and reproducible nature of DWGSs, thereby establishing them as a promising platform for future sensing applications.

Terahertz structured deep-subwavelength plasmonic grating metasurface for manipulating polarization-dependent coupling response

The complex electromagnetic anisotropy of terahertz (THz) metasurfaces with geometric symmetry breaking has attracted extensive attention. Typical effects arise from the coupling of polarization responses in orthogonal directions of various components of the metasurface structure, such as the electromagnetically induced transparency (EIT) effect. However, it is a challenge to precisely control or perfectly avoid the polarization-dependent coupling responses. In this work, deep-subwavelength plasmonic gratings (PGs) with a fine wire width of 1 μm at the order of deep subwavelengths of 1/100 THz wave are fabricated by electron beam lithography, and these wire gratings are graphically designed as a C-shaped metasurface pattern with a period of 100 μm in sub-wavelength scale. The complete anisotropic response in the single-oriented PG metasurface is demonstrated by both simulation and experiments, where the polarization-dependent coupling effect is eliminated. More interestingly, the hybrid-oriented PG metasurface exhibits narrowband and wideband EIT effects in the x and y polarization directions with the maximum polarization extinction ratio of 20 dB, respectively, indicating this mechanism can realize more flexible manipulation of polarization-dependent coupling. This patterned deep-subwavelength PG provides a new structure and mechanism for excitation, regulation, and restriction of polarization-dependent mode coupling, and has important applications in THz spectroscopic detection, polarization imaging, and wireless communication.

Graphene Saturable Absorber Mirrors for Silicon Photonic Platforms

We employ graphene saturable absorption for the theoretical demonstration of saturable absorption mirrors based on planar silicon photonic Bragg gratings. Two geometries are investigated, that of a silicon wire grating and a silicon slot grating, showcasing the increased light-matter interaction of the high-confinement slot waveguide. The gratings are designed in the linear regime for single mode operation, low footprint and broadband operation in the near infrared optical communications frequency range. The saturable absorption effect is introduced through the saturation of graphene’s interband surface conductivity, and we discuss the necessary biasing conditions and applicability of our CW approach on ps-long pulses. We also rigorously include other, possibly detrimental, nonlinear effects, such as silicon’s Kerr effect and two photon absorption, and graphene’s Kerr effect. These effects are proven to have a negligible impact on the operation of the graphene saturable absorber mirror, thanks to the much lower power threshold of graphene’s saturable absorption. Finally, we calculate the nonlinear reflectance and transmittance of the graphene-enhanced Bragg gratings and demonstrate that they can provide high modulation depths at low saturation powers, both highly valued characteristics of saturable absorber mirrors for mode locking applications in view of next-generation integrated pulsed photonic light sources.

Scattering characteristics of metal circular wire gratings in microwave and terahertz ranges

Scattering characteristics of metal circular wire gratings in microwave and terahertz ranges

THE RESONANT SYSTEM OF A SUB-TERAHERTZ LOCAL OSCILLATOR

Purpose. The excitation efficiency is investigated of the first higher-order axially asymmetric oscillation mode (TEM10q) excited in a hemispherical open resonator (OR) at the frequencies of the fundamental and second-order harmonics of the Gunn diode in the 4-mm and 2-mm wavelength ranges. The hemispherical resonator is coupled to its input waveguide via aperture-type coupling elements. The diameter 2a of the OR mirror apertures is 38 mm, while the curvature radius of the spherical reflector is R = 39 mm and the normalized distance between the mirrors is L/R = 0.593. Two aperture coupling elements of dimensions a× b = 6.9 × 9.6 mm are used to excite the OR. They permit controlling separately the functions of field-to-field matching (modes in the resonator and in the waveguide) and volume- to-volume coupling of the structural elements (the resonator and the waveguide). They are located at the center of the planar mirror. The field matching is determined by the geometric dimensions of the coupling elements, whereas the coupling matching is determined by the period of the one-dimensional E-polarized grating in their apertures. The Gunn diodes are used as generators, operating at the frequencies of the fundamental (75 GHz) andthe second-order (150 GHz) harmonics. The excitation efficiency of the TEM1011 oscillation in the OR of the geometry specified here, using aperture-type coupling elements as described, is 81.5%. Design/methodology/approach. The excitation efficiency of higher-order oscillation modes ТЕМ10q in the OR being driven by an incident ТЕ10 mode that arrives via two rectangular guides, is evalua-ted using the antenna surface utilization factor. The reflection coefficient from the OR and the loaded Q-factor are estimated in the familiar technique of partial reflection coefficients summation. Findings. As has been shown, in an OR of parameters 2а= 38 mm, R= 78 mm, and L/R= 0.287 TEM1022 oscillations are excited at the frequency of the Gunn diode’s second-order harmonic (i.e., 150 GHz) with an efficiency of 84%. In that same resonator, the excitation efficiency of the TEM1011 mode at the fundamental Gunn diode’s harmonic (frequency of 75 GHz) equals 54%. By placing one-dimensional (E-polarized) wire gratings in the aperture of the coupling elements it proves possible to match the resonator with the waveguide. It has been found that in the case of a l = 0.2 mm spatial period of the wire grating and matched excitation of the resonator at f = 150 GHz (i.e. Г150 = 0), the reflection coefficient Г75 from the OR at f = 75 GHz equals 0.637. Upon excitation in the OR of oscillations in the TEM1022 mode, the total loss at f = 150 GHz is –1.23 dB. With TEM1011 oscillations excited in the same resonator at a frequency of 75 GHz, the total losses increase up to –5.4 dB. Conclusions. The analysis has shown that an OR implementing the proposed method of excitation of higher-order axially asymmetric for constructing a subterahertz range local oscillator. Moreover, such a resonant system may be considered both as a power combiner and a diplexer (filter).

Dual-band multifunctional coding metasurface with a mingled anisotropic aperture for polarized manipulation in full space

Integrated metasurfaces with diversified functionalities have demonstrated promising prospects for comprehensive implementations in compact 5G/6G communication systems by flexibly manipulating electromagnetic (EM) waves. Increasingly emerged multifunctional metasurfaces have successfully revealed integrated wavefront manipulations via phase gradient arrays, coding apertures, independent polarization control, asymmetric transmission/reflection, etc. However, multifunctional metasurfaces with more degrees of freedom in terms of multi-band/broadband operation frequencies, full-space coverage, and computable array factors are still in dire demand. As a step forward in extending manipulation dimensions, we propose and corroborate a dual-band multifunctional coding metasurface for anomalous reflection, radar cross-section reduction, and vortex beam generation through full-wave analysis and experiment. Our tri-layer meta-device comprises a shared coding aperture of split-ring and cross-shaped resonators sandwiched between two layers of orthogonal wire gratings. With an approach of independent control of a reflection–transmission wavefront under orthogonal polarization states and Fabry–Perot-like constructive interference, the low-cross-talk shared coding aperture features a smooth phase shift and high efficiency for 3-bit coding in the K-band and 1-bit coding in the Ka-band. Both numerical and measured results verify that the proposed coding metasurface can effectively realize full-space EM control and improve the capacity of the information channel, which could be developed for potential applications in multifunctional devices and integrated systems.

Multi-domain functional metasurface with selectivity of polarization in operation frequency and time

In this paper, we propose the design of a multi-domain functional metasurface (MDFM) that can achieve selectivity of polarization in operation frequency and time. The MDFM consists of a wire grating as the front layer, a frequency selective surface as the middle layer and an active orthogonal wire grating loaded with positive-intrinsic-negative (PIN) diodes as the back layer. The front wire grating serves as a polarization selective surface that only passes electromagnetic (EM) waves polarized perpendicular to the wire, whereas the middle frequency selective surfaces exerts a pass-band in the frequency-domain. To further obtain selectivity in the time-domain, another orthogonal wire grating loaded with PIN diodes is adopted to switch on/off the pass-band. Three functional layers are combined to integrate the three selective functions. To verify this design idea, a MDFM operating at 3.19 GHz was designed, with a total thickness about λ/11. A prototype was fabricated and measured to validate the design. The experimental results are in satisfactory agreement with the simulated ones. Due to multi-fold selectivity, the MDFM can achieve high rejection for unwanted signals and may find wide applications in areas such as wireless communication, EM compatibility and stealthy radomes.

Ultra-wideband tri-layer transmissive linear polarization converter for terahertz waves

Polarization control is shown to enhance functionalities of terahertz waves in various applications. As a step toward introducing efficient practical devices operating in the terahertz band, we propose and experimentally validate a free-standing three-layer polarization converter operating in transmission. The device can efficiently convert a linearly polarized terahertz wave to its orthogonal counterpart. Our device comprises split-ring and H-shaped resonators inserted between two layers of orthogonal wire gratings. The sole dielectric material is a cyclic olefin copolymer (COC) that has exceptionally low-loss to terahertz waves. The measurement demonstrates a conversion efficiency of >80%, over 0.2–1.0 THz, giving an ultra-wide bandwidth of 133%. The wide bandwidth is attributed to multiple resonances of the two resonators in a unit cell. Importantly, an unconventional fabrication process yields a multi-layer low-loss COC that leads to high efficiency. The proposed design can also be tailored for practical applications across the electromagnetic spectrum.

Direct metallic nanostructures transfer by flexible contact UV-curable nano-imprint lithography

We propose a novel flexible contact ultraviolet (UV)-curable nano-imprint lithography process which can directly transfer metallic nanostructures by UV curing bonding force. The flexible template evaporated with metal acts as a mask, allowing only the structural region to penetrate UV light and transfer surface metallic nanostructures. By this process, aluminum metal grating of different feature sizes had been transferred to abtain single-layer or even multi-layer metallic nanostructures. The minimum width and spacing of the wire grating were 50 nm and sub-20 nm, respectively. Aluminum grating polarizer had also been fabricated and tested demonstrating the practicability of the process. © 2019 The Japan Society of Applied Physics

Terahertz resonance switch induced by the polarization conversion of liquid crystal in compound metasurface.

We experimentally demonstrate an active terahertz (THz) resonance switch induced by the polarization conversion in a compound metasurface, which is a LC layer sandwiched by a metallic wire grating and resonance metamaterial (LCGM). Here, the liquid crystal (LC) plays the role of polarization conversion, which can induce the TE resonance. Moreover, there exists a localized resonance between metallic grating and metamaterial layers, and then the excited resonance will be greatly enhanced. The results show that the high extinction ratio of the resonance switch exceeds 30dB at 0.82THz. This work will bring new ideas for the research in developing THz phase, polarization, and switch devices with LC and metasurface.

Graphene-loaded metal wire grating for deep and broadband THz modulation in total internal reflection geometry

We employed a metallic wire grating loaded with graphene and operating in total internal reflection (TIR) geometry to realize deep and broadband THz modulation. The non-resonant field enhancement effect of the evanescent wave in TIR geometry and in the subwavelength wire grating was combined to demonstrate a ∼77% modulation depth (MD) in the frequency range of 0.2–1.4 THz. This MD, achieved electrically with a SiO2/Si gated graphene device, was 4.5 times higher than that of the device without a metal grating in transmission geometry. By optimizing the parameters of the metallic wire grating, the required sheet conductivity of graphene for deep modulation was lowered to 0.87 mS. This work has potential applications in THz communication and real-time THz imaging.

Front dielectric and back plasmonic wire grating for efficient light trapping in perovskite solar cells

Thin film perovskite solar cells (PSCs) based on (CH3NH3PbI3) have been emerged as good alternatives to conventional silicon solar cells due to their low cost, low fabrication temperature, high carrier collection efficiency, and high-power conversion efficiency (PCE). However, the small thickness of thin film solar cells limits light absorption compared to thick solar cells. In this work, we proposed a theoretical design for enhancing light absorption to achieve maximum theoretical photocurrent using front dielectric and back plasmonic wire grating. Using finite element method (FEM) three-dimensional optical model, the optimum size and periodicity of the studied wire grating nanostructures were identified. Additionally, the electrical model revealed a satisfactory enhancement in PCE over that of the planar structure counterpart. The simulation results showed an average enhancement of 22.4% in total generation rate for the entire simulated wavelength, and more than 85% enhancement in narrow-band wavelength compared to the planar structure counterpart.

Highly selective transmission and absorption from metasurfaces of periodically corrugated cylindrical particles

Gratings of infinite wires make, perhaps, the simplest class of metasurfaces, which, however, are utilized for a variety of different objectives in wave manipulation. Importantly, due to their analytical solvability, they can be fully optimized in a fast and direct way. In this study, a lattice of periodically corrugated cylindrical particles under oblique plane-wave excitation is considered and treated rigorously. Several cases of particle radii, distances between two consecutive cylinders, and angles of illumination are examined; as a result, very high selectivity of the metasurface response in terms of line-of-sight transmission and absorption is reported. That abrupt change in the device output becomes even more dramatic in the parametric vicinity of the emergence of new refractive orders, which makes the proposed metasurface exceptionally fitting for switching, filtering, and sensing applications.

Exploiting a metal wire grating in total internal reflection geometry to achieve achromatic polarization conversion

We demonstrate how a metal wire grating can work as a 45° polarization converter, a quarter-wave retarder, and a half-wave retarder over a broadband terahertz range when set up in total internal reflection geometry. Classical electromagnetic theory is applied to understand the mechanism, and equations to calculate the polarization state of reflected light are derived. We use a metal grating with a period of 20 μm and width of 10 μm on a fused silica surface: linearly polarized terahertz light incident from fused silica with a supercritical incident angle of 52° is totally reflected by the metal grating and air. The polarization of the terahertz light is rotated by 45°, 90°, and circularly polarized by simply rotating the wire grating. The performance is achromatic over the measured range of 0.1–0.7 THz and comparable to commercial visible light wave retarders.

Rapidly convergent quasi-periodic Green functions for scattering by arrays of cylinders-including Wood anomalies.

This paper presents a full-spectrum Green-function methodology (which is valid, in particular, at and around Wood-anomaly frequencies) for evaluation of scattering by periodic arrays of cylinders of arbitrary cross section-with application to wire gratings, particle arrays and reflectarrays and, indeed, general arrays of conducting or dielectric bounded obstacles under both transverse electric and transverse magnetic polarized illumination. The proposed method, which, for definiteness, is demonstrated here for arrays of perfectly conducting particles under transverse electric polarization, is based on the use of the shifted Green-function method introduced in a recent contribution (Bruno & Delourme 2014 J. Computat. Phys.262, 262-290 (doi:10.1016/j.jcp.2013.12.047)). A certain infinite term arises at Wood anomalies for the cylinder-array problems considered here that is not present in the previous rough-surface case. As shown in this paper, these infinite terms can be treated via an application of ideas related to the Woodbury-Sherman-Morrison formulae. The resulting approach, which is applicable to general arrays of obstacles even at and around Wood-anomaly frequencies, exhibits fast convergence and high accuracies. For example, a few hundreds of milliseconds suffice for the proposed approach to evaluate solutions throughout the resonance region (wavelengths comparable to the period and cylinder sizes) with full single-precision accuracy.

Ultrathin Terahertz Quarter-wave plate based on Split Ring Resonator and Wire Grating hybrid Metasurface.

Planar metasurface based quarter-wave plates offer various advantages over conventional waveplates in terms of compactness, flexibility and simple fabrication; however they offer very narrow bandwidth of operation. Here, we demonstrate a planar terahertz (THz) metasurface capable of linear to circular polarization conversion and vice versa in a wide frequency range. The proposed metasurface is based on horizontally connected split ring resonators and is realized on an ultrathin (0.05λ) zeonor substrate. The fabricated quarter waveplate realizes linear to circular polarization conversion in two broad frequency bands comprising 0.64–0.82 THz and 0.96–1.3 THz with an insertion loss ranging from −3.9 to −10 dB. By virtue of ultrathin sub wavelength thickness, the proposed waveplate design is well suited for application in near field THz optical systems. Additionally, the proposed metasurface design offers novel transmission phase characteristics that present further opportunities to realize dynamic polarization control of incident waves.

Waveguide‐Plasmon Polariton Enhanced Photochemistry

One key process in plasmon‐enhanced photocatalysis is the transfer of hot charge–carriers from metal nanostructures into photocatalytically active materials. This process is secondary to the initial step of light absorption by the metal nanostructures. Light absorption in these structures can be controlled by designing complex geometries with tailored optical cross‐sections. Here, a study on one of the simplest nanostructures exhibiting plasmon resonances is presented: 1D gratings of metal wires. Results on the effect of the periodicity of these arrays are presented on the resonant absorption of light and on the hot charge–carrier transfer to a supporting TiO2 thin film. This charge transfer process is monitored by following the photocatalytic decomposition of small organic molecules in solution. Strong enhancements in the rate of decomposition are found that exhibit a correlation with the measured optical extinction spectra of the plasmonic nanostructures.

Polarizing and angle-sensitive color filter in transmittance for security feature applications

Abstract Subwavelength gratings comprising a stack of two thin metallic wire gratings embedded in a dielectric exhibit distinctive color properties in transmittance. Its angle-dependent coloring can be easily noticed by human observers in ambient light. Moreover, it has pronounced polarizing properties in the visible wavelength range. The optical properties of these gratings have been numerically studied. The results have been confirmed by manufactured samples replicated by a simple nanoimprint process on plastic foils. The manufacturing process is predestined for industrial mass production. Applications as authentication devices are given.

A homogenization technique for obtaining generalized sheet transition conditions for an arbitrarily shaped coated wire grating

AbstractUsing a multiple‐scale homogenization method, we derive generalized sheet transition conditions (GSTCs) for an arbitrarily shaped coated wire grating. The parameters in these GSTCs are interpreted as effective electric and magnetic surface susceptibilities and surface porosities of the wire grating. We give expressions for determining these surface parameters for any arbitrarily shaped grating. We show that these GSTCs are a generalized form of the boundary conditions derived earlier by Wainstein and Sivov, who analyzed the case of symmetric, uncoated wire gratings. This work is also a first step in developing GSTCs for more general types of structures such as metasurfaces, metafilms, and metascreens.