Metallic Gratings Research Articles

Tunable terahertz slow light of a cavity-integrated guided-mode resonance grating

We present a dynamically tunable anomalous electromagnetic induced transparency (EIT) of a cavity-integrated metallic grating by the coupling of guided-mode resonance (GMR) with cavity-mode resonance (CMR) in the terahertz regime. The strong group slowing effect of terahertz waves results from the EIT mechanism under simultaneous excitation of GMR and CMR at a degenerate state. With the introduction of graphene as a functional layer overlying the grating structure, the enhanced group delay can be achievable beyond 6.1 ps with stable operation frequency and signal efficiency by tuning the graphene Fermi level. The work could provide an efficient scheme to manipulate the group velocity of terahertz signals.

Direct Laser Writing for the Formation of Large‐Scale Gold Microbumps Arrays Generating Hybrid Lattice Plasmon Polaritons in Vis–NIR Range

AbstractSurface plasmon polaritons (SPPs) based light–matter interactions play a major role in photonic sensor applications. In order to implement such sensors in microchip devices or reduce the fabrication costs, a method with high flexibility and large‐scale capabilities must be developed. Here, the formation of large‐scale gold microbumps arrays by using a direct laser writing technique is presented. The fabricated arrays exhibit the hybrid lattice plasmon polaritons in the Vis–NIR range, in the same range as metallic gratings fabricated by lithography‐based techniques. The peak of hybrid lattice plasmon resonance depends on the period of the fabricated arrays, the thickness of the gold film, coupled light polarization, and sample orientation. The experimental results are in good agreement with the theoretical estimation. The demonstrated method shows relevant technological progress in the formation of large‐scale metallic gratings by using a cost‐effective laser‐based technique. It provides new opportunities for hybrid lattice plasmon polariton (HLPP)‐based applications in biotechnology, photonics, plasmonics, and so on.

Polarization property associated with surface plasmon resonance in a metal grating in a conical mounting and its application to refractive index sensing.

We focus on the normalized Stokes parameter s3 of the specularly reflected light from a metal grating in a conical mounting in order to accurately and sensitively detect the occurrence of surface plasmon resonance (SPR). Experimental and theoretical investigations into the incident angle dependence of s3 reveal a rapid change in s3 around the resonance angle θsp at which SPR occurs. The rapid change in s3 has features that are successfully applied to SPR sensing: the zero-crossing point of s3 determines θsp, and s3 in the vicinity of θsp varies markedly in response to a small change in the refractive index of a sample. An experiment using gas samples demonstrates that SPR sensing that considers the rapid change in s3 clearly detects a refractive index change on the order of 10-5.

Numerical analysis of a dual-wave band guided-mode resonance filter with embedded bilayer asymmetric metallic gratings

In this paper, a novel guided-mode resonances (GMRs) based embedded dual-wavelength filter, which had the bilayer gratings located at the two sides of dielectric substrate and arranged at asymmetric position, was investigated and studied theoretically. As compared to symmetric structures that usually provided single wavelength of transmission resonance, as one normal incidence was used for transverse magnetic polarized light, the designed asymmetric structure could generate two remarkable narrow band wavelengths of transmission resonance. The parameters to affect the resonance wavelengths and the transmission resonance Q value (Q = λ/Δλ) of two bands were the distance between the two metallic gratings, the relatively lateral positions of the upper and lower gratings, the number of structure period, the thicknesses of metallic gratings, and the thickness of dielectric film. By optimizing the designed parameters, a GMR device with two resonance wavelengths located at 1239 and 1302 nm showed relative optimal performance because they had high transmission depth (99.9% and 90%) and ultra-narrow transmission bandwidth (2.8 and 1.8 nm) at the two resonance wavelengths. The presented structure can offer a potential route towards dual-band narrow-band filters and refractive index sensors in the near infrared.

Guided Mode Resonance in a Low-Index Waveguide Layer

In this paper, we show that the guided mode resonance can exist in a low-index waveguide layer on top of a high-index substrate. With the help of the interaction of diffraction from a metal grating and total internal reflection effects, we verify that the guided mode can be supported in the low-index SU8 layer on a high-index substrate. Simulation and experiment show the resonant wavelength can be simply manipulated by controlling the geometrical parameters of the metal grating and waveguide layer. This structure extends the possibilities of guided-mode resonance to a broader class of functional materials and may boost its use in applications such as field enhancement, sensing and display.

Reconfigurable terahertz rainbow deflector

Dynamic terahertz (THz) beam steering is a promising technique for numerous applications, including communication, high-resolution imaging, and radar. However, advances in this technology are considerably limited by inadequate broadband and active phase shifters at THz frequencies. In this paper, we developed a type of VO2-based device capable of broadband THz beam steering. The effect of the tunable Brewster angle of the VO2 thin film along with the insulator-to-metal transition process was verified. We demonstrated that the VO2 grating could realize a dispersive beam deflector in a broad frequency band. Furthermore, upon heating, the VO2 grating could switch between three states of mirror reflector, “rainbow” deflector, and metallic grating. We believe that the proposed reconfigurable and broadband THz beam deflector offers a powerful avenue for manipulating THz beams.

Long-Range Spoof Surface Plasmons(LRSSP) on the Asymmetric Double Metal Gratings

Spoof surface plasmons (SSP) on the corrugated metal surfaces have been widely studied over the past decade in both microwave and Terahertz (THz) band. Previous studies show that SSP mode on the metal grating suffers from high dissipative losses in high THz band especially near asymptotic frequency which may limit its real applications. The issue can be addressed by utilizing long-range SSP mode (LRSSP). LRSSP is a special low-loss symmetric propagating surface wave mode along double-layer corrugated metallic waveguides. In this paper, the asymmetric double metal gratings are proposed to support LRSSP which can significantly reduce the damping losses of the other short-range SSP mode (SRSSP) on the conventional doubly-corrugated metallic surfaces, thus enabling new possibilities to utilizing this SRSSP mode within long-range manner. According to our dispersion theory and the numerical calculations, the propagation loss of LRSSP is still lower than that of SRSSP within asymmetric regime, which demonstrates good tolerance to the asymmetry of the double-layer corrugated structure. However, its long-range propagation superiority of LRSSP vanishes gradually as the degree of asymmetry is enlarged. This work provides an alternative approach to increase the propagation length of SRSSP mode on the doubly-corrugated metallic surfaces and can open up new avenues to develop some novel LRSSP-based plasmonic devices such as waveguides, filters, sensors and active powerful radiation sources induced by energy beam below optical spectrum.

Near-unity double-mode absorber integrating graphene-dielectric-stack and metal grating

A terahertz absorber is designed by integrating the graphene dielectric stack (GDS) above and the metal grating below. The GDS is formed through arranging the graphene monolayer and dielectric film periodically. In the case of impedance matching, two near-unity absorption modes arise as the coupling result of Fabry-Perot (FP) resonance and surface plasmon polaritons (SPP) resonance. Tuning the chemical potential or the period of GDS, both modes can be shifted. Changing the period or the duty cycle of metal grating, all of the locations and amplitudes of modes along with the peak-peak distance between modes are controllable. Altering the thickness of dielectric slab, these modes move towards the same direction linearly almost. Deviating the normal incidence, the number of absorption modes can be increased. The designed graphene absorber should be useful to multiple targets sensing, photoelectric detection and frequency conversion of nonlinear system.

High-Q quadrupolar plasmonic lattice resonances in horizontal metal-insulator-metal gratings.

We propose a plasmonic platform for achieving out-of-plane quadrupolar plasmonic surface lattice resonances (SLRs) with large quality factors. The proposed platform is composed of a horizontal metal-insulator-metal (MIM) grating embedded in a homogeneous dielectric environment. Numerical results based on rigorous coupled-wave analysis show that under oblique incidences, high-Q out-of-plane quadrupolar SLRs can be excited at wavelengths of 1242 nm over a wide range of insulator widths, and the quality factor can reach 1036. As a comparison, under the same conditions, only dipolar SLRs with much lower quality factors of ∼300 can be excited in a vertical MIM grating, which has the same period and a quarter-turned unit cell. We expect that the proposed high-Q quadrupolar SLR platform will find applications in light-matter interactions on the nanoscale.

Bound states in the continuum on perfect conducting reflection gratings* *Project supported by the National Natural Science Foundation of China (Grant No. 12074049) and the Fundamental Research Funds for the Central Universities, China (Grant Nos. 2020CDJQY-Z006 and 2019CDXZWL002).

Bound states can be supported on the surface of a periodically corrugated perfect conductor known as spoof surface plasmon polaritons with their dispersion curves reside below the light line. Here we show that bound states in the continuum (BICs) can also be achieved in such systems. Two types of grating structures are proposed to suppress the radiation leakage and hence generate bound states. The first one is a simple grating with broad grooves in which multiple cavity modes are accommodated. Due to the symmetry incompatibility and the destructive interaction mainly from the TM0 and TM1 modes, BICs at the Γ point and at off-Γ points are both realized. The second one is a dimerized grating with two grooves in each unit cell. The destructive interaction between the modes in the two grooves can suppresses the radiation and BICs at the Γ point are observed. The Q factors of the whole bands can be further tuned by the dimerization strength effectively. This work may offer new opportunity for the applications of metallic grating in the low frequency bands.

Development of a Novel Metal Grating and Its Applications of Terahertz Spectroscopic Detection of CuSO4 in Fruit

As a novel detection method of CuSO4, terahertz (THz) spectroscopy detection has great application prospects, but the terahertz detection with the tablet method has low sensitivity, and with the metamaterials has complicated manufacture. This work has proposed a high-sensitivity pesticide THz spectroscopic detection method with a novel metal grating. The metal grating was composed of a circular hole array. It was made by laser micromachining technology to provide a simplified but improved measurement and had a strong transmission peak at the 0.4–1.0 THz band. The results indicated that the addition of copper sulfate (CuSO4) solutions to different concentrations caused the transmission peak frequency to shift red. The detection limit was about 103 times higher than that of CuSO4 measured on the polyethylene (PE) film substrate, which reached 0.375 mg/L. The method was applied for detecting CuSO4 in apple and grape, and the relative error was less than 5.8%. The metal grating combined with Terahertz time-domain spectroscopy (THz-TDS) can improve the sensitivity of sample detection and realize the detection and analysis of trace elements.

Multi-band terahertz linear polarization converter based on carbon nanotube integrated metamaterial

Herein, we fabricated and investigated the carbon nanotube (CNT) integrated metamaterial for orthogonal polarization control in the THz regime, which is composed of a sandwiched CNT layer with the adjacent metal gratings in the sub-wavelength integration. Under the mechanism of multilayer polarization selection and multiple reflections in CNT constructed micro-cavity, the perfect orthogonal polarization conversion is achieved and the transmittance spectrum presents multi-band peaks and valleys, which coincide with the theoretical Fabry-Perot resonance. Besides, by controlling the layer number and orientations of the middle CNT, the active modulation of the amplitude and phase in compound metamaterials are realized. Based on the simulation of CNT in the grating model, it obtains a good agreement with the experimental results, and the simulated electric field distribution also confirmed the inner polarization conversion mechanism. This work combines nanomaterials with optical microstructures and successfully applies them to the THz polarization control, which will bring new ideas for design novel THz devices.

Efficient excitation of hybrid modes on a double-layer graphene with metallic slit grating.

The excitation of double-layer hybrid plasmonic modes is investigated by the finite element method. The hybrid modes, verified as the standing even order of both symmetric and anti-symmetric modes, are effectively generated. There are several advances in comparison with using the Si grating: the metallic grating not only compensates phase mismatch, but also acts as a magnetic polariton. The dependences of each hybrid mode on the geometric parameters are analyzed respectively. Interestingly, a second spectra splitting occurs at each hybrid resonant mode with an obliquely incident light. At last, the excitation efficiency can be further enhanced to 90% using the Salisbury screen. The proposed hybrid system can be utilized to design various double-layer graphene-based plasmonic devices, including tunable optical switches, thermal emitters, multiband absorbers, sensors, etc.

Dynamically adjustable and high-contrast asymmetric optical transmission based on bilateral compound metallic gratings

A dual-band asymmetric transmission (AT) device with high contrast ratio based on bilateral compound metallic gratings is proposed. The AT structure can be manufactured through connecting silver gratings on the outer surface and inner inverted T-shaped grating with an Ag film. Dual-band forward transmission arises from unidirectional surface plasmon polaritons (SPPs) excitation by the upper metallic grating when the modulated forward wave vector matches the wave vector of SPPs. The device can achieve a high transmittance of 0.67 and 0.82 with a contrast ratio of 0.997 and 0.99 at operation wavelengths 627 nm and 1238 nm, respectively. Since the SPPs coupling between two metallic structures can be used to suppress the forward transmission in specific frequency band, single-band AT function also can be realized. By adjusting those underlying parameters, the designed structure having AT capability at 624 nm and band-pass filter function at 1234 nm. The proposed structure is simple and feasible, and has great potential applications in the field of integrated optoelectronics.

The effect of gold quantum dots/grating-coupled surface plasmons in inverted organic solar cells.

We studied the effect of gold quantum dots (AuQDs)/grating-coupled surface plasmon resonance (GC-SPR) in inverted organic solar cells (OSCs). AuQDs are located within a GC-SPR evanescent field in inverted OSCs, indicating an interaction between GC-SPR and AuQDs' quantum effects, subsequently giving rise to improvement in the performance of inverted OSCs. The fabricated solar cell device comprises an ITO/TiO2/P3HT : PCBM/PEDOT : PSS : AuQD/silver grating structure. The AuQDs were loaded into a hole transport layer (PEDOT : PSS) of the inverted OSCs to increase absorption in the near-ultraviolet (UV) light region and to emit visible light into the neighbouring photoactive layer, thereby achieving light-harvesting improvement of the device. The grating structures were fabricated on P3HT:PCBM layers using a nanoimprinting technique to induce GC-SPR within the inverted OSCs. The AuQDs incorporated within the strongly enhanced GC-SPR evanescent electric field on metallic nanostructures in the inverted OSCs improved the short-circuit current and the efficiency of photovoltaic devices. In comparison with the reference OSC and OSCs with only green AuQDs or only metallic grating, the developed device indicates enhancement of up to 16% power conversion efficiency. This indicates that our light management approach allows for greater light utilization of the OSCs because of the synergistic effect of G-AuQDs and GC-SPR.

Plasmon-to-exciton spin conversion in semiconductor-metal hybrid nanostructures

Optical control of electronic spins is the basis for ultrafast spintronics: circularly polarized light in combination with spin-orbit coupling of the electronic states allows for spin manipulation in condensed matter. However, the conventional approach is limited to spin orientation along one particular orientation that is dictated by the direction of photon propagation. Plasmonics opens new capabilities, allowing one to tailor the light polarization at the nanoscale. Here, we demonstrate ultrafast optical excitation of electron spin on femtosecond time scales via plasmon to exciton spin conversion. By time-resolving the THz spin dynamics in a hybrid (Cd,Mn)Te quantum well structure covered with a metallic grating, we unambiguously determine the orientation of the photoexcited electron spins which is locked to the propagation direction of surface plasmon-polaritons. Using the spin of the incident photons as additional degree of freedom, one can orient the photoexcited electron spin at will in a two-dimensional plane.

Strong Terahertz Absorption of Monolayer Graphene Embedded into a Microcavity.

Terahertz reflection behaviors of metallic-grating-dielectric-metal (MGDM) microcavity with a monolayer graphene embedded into the dielectric layer are theoretically investigated. A tunable wideband reflection dip at about the Fabry–Pérot resonant frequency of the structure is found. The reflectance at the dip frequency can be electrically tuned in the range of 96.5% and 8.8%. Because of the subwavelength distance between the metallic grating and the monolayer graphene, both of the evanescent grating slit waveguide modes and the evanescent Rayleigh modes play key roles in the strong absorption by the graphene layer. The dependence of reflection behaviors on the carrier scattering rate of graphene is analyzed. A prototype MGDM-graphene structure is fabricated to verify the theoretical analysis. Our investigations are helpful for the developments of electrically controlled terahertz modulators, switches, and reconfigurable antennas based on the MGDM-graphene structures.

Double-frequency metallic grating over large areas with ultrahigh Raman enhancement

Raman spectroscopy reveals molecular vibration and rotation modes and directly reflects the molecular structure of analytes. Surface enhanced Raman spectroscopy may be applied to practical applications because of the greater Raman scattering cross section. This paper proposes and demonstrates a large-area double frequency metallic grating based on surface enhanced Raman scattering substrate which is cost effective. By exciting two coupled resonance modes, a strong local electromagnetic field was formed. The Raman enhancement factor up to 108 and the minimum detectable concentration down to 10−15 M were achieved.

Terahertz characterization of two-dimensional low-conductive layers enabled by metal gratings

While terahertz spectroscopy can provide valuable information regarding the charge transport properties in semiconductors, its application for the characterization of low-conductive two-dimensional layers, i.e., σs < < 1 mS, remains elusive. This is primarily due to the low sensitivity of direct transmission measurements to such small sheet conductivity levels. In this work, we discuss harnessing the extraordinary optical transmission through gratings consisting of metallic stripes to characterize such low-conductive two-dimensional layers. We analyze the geometric tradeoffs in these structures and provide physical insights, ultimately leading to general design guidelines for experiments enabling non-contact, non-destructive, highly sensitive characterization of such layers.

A vacuum ultraviolet laser with a submicrometer spot for spatially resolved photoemission spectroscopy

Vacuum ultraviolet (VUV) lasers have demonstrated great potential as the light source for various spectroscopies, which, if they can be focused into a small beam spot, will not only allow investigation of mesoscopic materials and structures but also find application in the manufacture of nano-objects with excellent precision. In this work, we report the construction of a 177 nm VUV laser that can achieve a record-small (~0.76 μm) focal spot at a long focal length (~45 mm) by using a flat lens without spherical aberration. The size of the beam spot of this VUV laser was tested using a metal grating and exfoliated graphene flakes, and we demonstrated its application in a fluorescence spectroscopy study on pure and Tm3+-doped NaYF4 microcrystals, revealing a new emission band that cannot be observed in the traditional up-conversion process. In addition, this laser system would be an ideal light source for spatially and angle-resolved photoemission spectroscopy.