Metallic Gratings Research Articles

Low voltage, high modulation depth graphene THz modulator employing Fabry–Perot resonance in a metal/dielectric/graphene sandwich structure

In this article, a novel THz modulator for modulating signals in the frequency range of 0.5–10 THz is designed and numerically investigated with finite difference time domain method. The structure is composed of a graphene layer sandwiched between two complementary metallic gratings. The proposed structure has Fabry–Perot like behavior at which the resonant condition depends on the geometrical parameters of the device and graphene plasmonic mode characteristics. A major advantage of the presented modulator is its capability to function in various regimes such as resonant and non-resonant as well as transmission and reflection. Based on the proposed structure, a broadband modulator for THz signals in the frequency range of 0.5–3 THz is proposed with modulation depth of 90% (36%) at 0.5 THz (3 THz). Insertion loss of the modulator is 1.2 dB. Electro-optic bandwidth is more than 2 GHz depending on the device surface area. In addition to this, several low voltage and relatively narrowband modulators are designed either at central frequencies of 4.5, 5.5, and 6.5 THz with modulation depth of more than 75% and insertion loss of less than 2.5 dB at their central frequency.

Extra-narrowband metallic filters with an ultrathin single-layer metallic grating**Project supported by the National Key Research and Development Program of China (Grant Nos. 2018YFA0704401, 2017YFF0206103, and 2016YFA0203500), the National Natural Science Foundation of China (Grant Nos. 61922002, 91850103, 11674014, 61475005, 11527901, 11525414, and 91850111), and the Beijing Natural Science Foundation, China (Grant No. Z180015).

Narrowband and high-transmission optical filters are extensively used in color display technology, optical information processing, and high-sensitive sensing. Because of large ohmic losses in metallic nanostructures, metallic filters usually exhibit low transmittances and broad bandwidths. By employing both strong field enhancements in metallic nano-slits and the Wood’s anomaly in a periodic metallic grating, an extra-narrowband and high-transmission metallic filter is numerically predicted in an ultrathin single-layer metallic grating. Simulation results show that the Wood’s anomaly in the ultrathin (thickness H = 60 nm) single-layer metallic grating results in large field enhancements in the substrate and low losses in the metallic grating. As a result, the transmission bandwidth (transmittance T > 60%) at λ = 1200 nm is as small as ΔλFWHM = 1.6 nm, which is smaller than 4% of that in the previous thin dielectric and metallic filters. The corresponding quality factor is as high as Q = λ/ΔλFWHM = 750, which is 40 times greater than that in the previous reports. Moreover, the thickness of our metallic filter (H = 60 nm) is smaller than 40% of that in the previous reports, and its maximum transmittance can reach up to 80%. In experiments, a narrowband metallic filter with a bandwidth of about ΔλFWHM = 10 nm, which is smaller than 25% of that in the previous metallic filters, is demonstrated.

Ultra-Wide Spectral Bandwidth and Enhanced Absorption in a Metallic Compound Grating Covered by Graphene Monolayer

Graphene, a two-dimensional monatomic layer of carbon material, has demonstrated as a good candidate for applications of ultrafast photodetectors, transistors, transparent electrodes, and biosensors. Recently, many studies have shown that using metallic deep gratings could enhance the absorptance of graphene of 2.3% up to 80% in the near infrared region for applications in photon detection. This paper presents utilizing a nanograting structure, namely, a compound metallic grating could greatly enhance the absorptance of graphene up to 98% and widen its spectral bandwidth to 0.6 μm, which are greater than those of previous work. The study also showed that the absorptance spectrum is insensitive to angles of incidence. Furthermore, the proposed graphene-covered compound grating might bring a lot of benefits for graphene designs-based optical and optoelectronic devices.

Aluminum nitride two-dimensional-resonant-rods

In the last few decades, bulk-acoustic-wave filters have been essential components of 3G-to-4G radios. These devices rely on the high electromechanical coupling coefficient (kt2 ∼ 7%), attained by aluminum nitride (AlN) film-bulk-acoustic-resonators (FBARs), to achieve a wideband and low-loss frequency response. As the resonance frequency of FBARs is set by their thickness, the integration of multiple FBARs, to form filters, can only be attained through the adoption of frequency tuning fabrication steps, such as mass loading or trimming. However, as the ability to reliably control these steps significantly decays for thinner (or higher frequency) FBARs, manufacturing FBAR-based filters, addressing the needs of emerging IoT and 5G applications, is becoming more and more challenging. Consequently, there is a quest for new acoustic resonant components, simultaneously exhibiting high-kt2 and a lithographic frequency tunability. In this work, a novel class of AlN resonators is presented. These radio frequency devices, labeled as two-dimensional-resonant-rods (2DRRs), exploit, for the first time, the unconventional acoustic behavior exhibited by a forest of locally resonant rods, built in the body of a profiled AlN layer that is sandwiched between a bottom un-patterned metal plate and a top metallic grating. 2DRRs exhibit unexplored modal features that make them able to achieve high-kt2, a significant lithographic frequency tunability, and a relaxed lithographic resolution, while relying on an optimal AlN crystalline orientation. The operation of 2DRRs is discussed, in this work, by means of analytical and finite-element-methods. The measured performance of the first fabricated 2DRR, operating around 2.4 GHz and showing a kt2 in excess of 7.4%, is also reported.

Integration of plasmonic heating and on-chip temperature sensor for nucleic acid amplification assays.

Nucleic acid tests have been widely used for diagnosis of diseases by detecting the relevant genetic markers that are usually amplified using polymerase chain reaction (PCR). This work reports the use of a plasmonic device as an efficient and low-cost PCR thermocycler to facilitate nucleic acid-based diagnosis. The thermoplasmonic device, consisting of a one-dimensional metal grating, exploited the strong light absorption of plasmonic resonance modes to heat up PCR reagents using a near-infrared laser source. The plasmonic device also integrated a thin-film thermocouple on the metal grating to monitor the sample temperature. The plasmonic thermocycler is capable of performing a PCR amplification cycle in ~2.5 minutes. We successfully demonstrated the multiplex and real-time PCR amplifications of the antibiotic resistance genes using the genomic DNAs extracted from Acinetobacter baumannii, Klebsiella pneumonia, Escherichia coli and Campylobacter.

Further improvement of the light efficiency of LCDs via the asymmetric reflection of metallic nanowire gratings

Metallic nanowire gratings have been proposed for use as transmitted-type non-absorptive colorfilters and polarizers that take the place of the conventional absorptive ones in liquid crystal displays (LCDs), which can improve the light efficiency by recycling the reflected lights. To achieve a high recycling rate, the designed reflected light should be as high as possible, meaning absorption should be as low as possible. In this work, we find that higher reflection and lower loss can be obtained for the light incident to the grating side than to the substrate side in bi-layered aluminum nanowire gratings (BANGs), by decreasing light localization and waveguiding loss in the substrate. Taking full advantage of the reflection characteristics, we firstly demonstrate that when a BANG-based integrated polarizer and colorfilter is placed with its grating side facing the backlight in LCDs, more than a 30% light enhancement is obtained than the case with the substrate side facing the backlight. This work affords an essential guide for the design of eco-displays by using MNGs.

Correction to: Polarization Transformation of Electromagnetic Radiation Upon Excitation of Surface Plasmon-Polaritons on a Metallic Grating

To the article “Polarization Transformation of Electromagnetic Radiation Upon Excitation of Surface Plasmon-Polaritons on a Metallic Grating,” by I. Z. Indutnyi, V. I. Mynko, M. V. Sopinskyy, and K. V. Svezhentsova, Vol. 86, No. 6, pp. 1058–1063, January, 2020.

2D electrons and 2D plasmons in AlGaN/GaN nanostructure under highly non-equilibrium conditions

We report on studies of electrically excited non-equilibrium 2D electrons and 2D plasmons in an AlGaN/GaN nanostructure. Optical access to 2D plasmons is provided by means of a metal grating fabricated at the nanostructure surface, while the properties of 2D electrons are examined in the samples without metal grating. The paper focuses on the creation of highly non-equilibrium conditions when the effective temperature of 2D electrons is much higher than the crystal lattice temperature. Such conditions are realized by applying short electrical pulses with a low repetition frequency. A method has been developed for independently determining the temperature of hot electrons and the temperature of the crystal lattice under an applied electric field. It has been shown that under highly non-equilibrium conditions the spectral density of terahertz electroluminescence of 2D plasmons can significantly exceed that of 2D electrons at a certain frequency.

Resonance absorption of visible light by subwavelength gratings

Diffraction of light of a visible spectral range by subwavelength metal gratings is investigated theoretically and experimentally. It is demonstrated that under certain conditions an effect of plasmon resonance occurs, at which a complete absorption of the incident radiation takes place. It is shown that the influence of the incident beam width on the diffraction efficiency is significant for the incident angles, at which the plasmon resonance occurs.

Large positive and negative Goos-Hänchen shifts near the surface plasmon resonance in subwavelength grating.

Diffraction of light of a visible spectral range by subwavelength metal gratings has been investigated experimentally and theoretically using rigorous electromagnetic calculations. It has been demonstrated that an effect of surface plasmon resonance (SPR), at which total absorption of light by metal grating can be observed, occurs under certain conditions. Large positive and negative Goos-Hänchen (GH) shifts occur near the SPR. It has been shown that the reflected beam is split into two parts, the relative powers of which depend on the incident beam width and the grating depth. The dependence of the GH shifts on the grating period and grating depth has been investigated for different incident beam widths. The high sensitivity of the GH shift on the incident angle of a light beam near the SPR has been demonstrated.

Anti-stain algorithm of angular displacement based on a single image sensor.

The displacement measuring technique is prone to failure within the industrial environment due to the influence of dust, oil, and other contaminants that stain the equipment. There is urgent demand for new anti-stain techniques. In today's image angular displacement measurement technology, the pixel array is used instead of the traditional photoelectric conversion element; this creates room for anti-stain improvement based on the image processing components. Based on a previous study on image-type angular displacement measurement technology, a single head image-type anti-stain algorithm is proposed in this paper that can remove the interference of small stains and ensure correct measurement value outputs. The influence of the stain on the calibration grating is first assessed based on the principle of image angular displacement measurement technology. An anti-stain algorithm based on the metal grating and multi-line fusion is proposed accordingly. The proposed algorithm is then tested on a circular grating with 38 mm diameter and ${{2}^{N}}\; = \;{256}$2N=256 lines in the circle. The results show that angle measurement output accuracy can be guaranteed when the number of lines covered by the stains is less than half of the coding-bits. This work may provide a technical basis for enhancing stain resistance in high-performance displacement measurement technology.

Au grating on SiC substrate: simulation of high performance plasmonic Schottky barrier photodetector in visible and NIR regions

Plasmonically enhanced light absorption in Schottky barrier photodetectors (PDs) can be achieved by engineering metallic nanostructures. In this context, gold (Au) gratings on titanium (Ti)/silicon carbide (SiC) based Schottky barrier is proposed in this work for photodetection application. Further, the effects of various grating parameters and incident angle on absorbance are demonstrated. The grating based structure is able to provide ~85% incident light absorption leading to a significantly high responsivity value as compared with that of conventional (without metallic grating) structure. High unbiased responsivity values of 4.396 mA W−1 and 2.496 mA W−1 are obtained for Au/Ti/6H-SiC Schottky barrier PD at wavelengths of 694.5 nm and 1035 nm, respectively based on Fowler’s model. In view of advanced properties of SiC, the present work will open new opportunities for plasmonic PDs based on SiC platform.

Design of a Surface Plasmon Resonance-Enhanced ZnO Ultraviolet Photodetector Based on a Sub-wavelength Metal Grating Covered with a High-Refractive-Index Medium

To realize a surface plasmon resonance-enhanced zinc oxide (ZnO) ultraviolet photodetector based on a sub-wavelength metal grating, we take advantage of the sensitivity of the resonance condition of a sub-wavelength metal grating to the refractive index of the surrounding medium. We theoretically design a sub-wavelength Ag grating covered with a high-refractive-index medium layer and apply it to a ZnO ultraviolet photodetector. By optimizing the parameters (angle of incidence, grating period, grating spacing, grating thickness, high-refractive-index medium layer thickness, refractive index of the covering), the optical field is localized at the interface of the sub-wavelength Ag grating and the ZnO thin film; that is, surface plasmon resonance is realized within the device. Compared with the device without a high-refractive-index medium layer, the maximum absorption enhancement factor of the designed device can reach up to 108. This work will provide theoretical guidance to realize a surface plasmon resonance-enhanced ZnO ultraviolet photodetector with a sub-wavelength metal grating.

Terahertz Bragg Resonator Based on a Mechanical Assembly of Metal Grating and Metal Waveguide

Terahertz (THz) waveguide Bragg grating is experimentally demonstrated on the basis of a mechanical assembly that includes a metal-grating sheet and a metal parallel-plate waveguide (PPWG). Bragg frequency waves, which are also called phase-matching waves, are efficiently coupled from the free space on the basis of the designed parameters of the air gap width, the metal grating thickness, and the PPWG taper angle. These optimal parameters are critical to reflect the Bragg frequency waves with a transversely resonant field across the metal grating. For other THz waves that are not phase matching to the meal grating vector, transverse resonance simultaneously occurs. However, these waves propagate forward as waveguide modes. PPWG-assembled metal grating thus performs highly distinct Bragg frequencies in the waveguide transmittance when the noisy resonance field, not following Bragg law, is exactly constrained by the waveguide scheme. Given that all components of the waveguide Bragg grating are made of metal without dielectric perturbation, a series of strong Bragg resonant modes in a wide bandwidth and a hollow-core chamber is potentially facilitated for THz gas sensors.

Coupling and Decoupling of Surface Plasmon Polaritons with a Double Period Metallic Grating

Herein, a numerical and experimental study of the interaction of a directional beam of light with metallic surfaces with two periodic components is presented. The periods of these two components are chosen in such a way that, through one of them, light is coupled into surface plasmon polaritons (SPPs) and, through the other one, the SPPs are decoupled as a propagating beam in another direction. This interaction can produce a propagating beam of light that involves in its trajectory the passage through an SPP state, providing thus a means of studying the effects of SPP coupling on the state of coherence of the emerging field. These coupling/decoupling effects are studied numerically, and experimental evidence of the generation of such SPP decoupled beams is presented. The effect of this double scattering process on the temporal properties of short light pulses, reflected by a double period metallic grating, is also studied.

Optimization study of metallic sub-wavelength gratings as the polarizer in infrared wavelengths.

Despite the polarimetric detection in the infrared wavelengths of 8-10 µm being of great importance and broad applications, there has been limited addressing of the grating-based polarizers in this band. One of the main issues lies in the process incompatibility between the conventional nanofabrication technique and the II-VI materials such as HgCdTe, so that the direct integration of polarizers with sensors still remains a big challenge. This paper reports our recent work on optimizing the grating structures, materials, and nanofabrication processes for enhancing both the transmittance and the extinction ratio of polarizers on Si and/or ZnSe wafers, using numerical simulations for the grating design and electron beam lithography for the nanoscale pattern generation. By utilizing the finite-difference time-domain method, both the transmittance and the extinction ratio are maximized by optimizing the grating geometric dimensions and the duty cycle for two different grating materials of Al and Au for comparison. Based on the designed structures, nanofabrications of sub-wavelength gratings in both Al and Au are carried out, and the processes are compared for achieving high polarization performance. Optical characterizations of the fabricated polarizers demonstrate that both high transmittance and extinction ratio can be achieved in feasible parameters and the nano-process developed in this work.

Fast Fourier factorization for differential method and RCWA: a powerful tool for the modeling of non-lamellar metallic diffraction gratings

The rigorous coupled-wave analysis (RCWA), also known as Fourier modal method, is one of the most popular methods for the modeling of diffraction gratings. It has been proven to be particularly effective for lamellar gratings. However, for non-lamellar metallic gratings, in TM polarization, the differential method (DM) or the RCWA need to be associated with the fast Fourier factorization (FFF) to provide more accurate solutions. In this study, we present the effectiveness of the FFF when it is associated with the DM and the RCWA for different grating’s profiles, and we show how to circumvent on the case where the FFF can diverge when the metal permittivity is close to a pure negative real value.

Broadly tunable laser based on novel metallic resonant leaky-mode diffraction grating

We present a fibre laser broadly tunable in a wavelength range from 1058 nm to 1640 nm based on a new type of metallic resonant leaky-mode diffraction grating and three fibre-pigtailed semiconductor optical amplifiers. For TM polarization in Littrow configuration, the grating has experimentally measured diffraction efficiency into the -1st reflected order of more than 90 % over a spectral range of 1417 nm to 1700 nm. The laser covered a spectral range of 331 nm within a tuning band of 558 nm without any adjustment of optics and its tuning range was limited by amplification bands of available semiconductor optical amplifiers.

Surface plasmon based 1D-grating device for efficient sensing using noble metals

This study presents the design and optimization of device with 1D nanostructured grating modeled on gold (Au), silver (Ag) and copper (Cu) for sensing a minor variation in refractive index (RI). This small change in the adjacent media to metallic grating is detected quite effectively using the unique characteristic of the surface plasmon polaritons (SPPs). The electric field (E-field) of the SPPs penetrates into the nearby media which is very sensitive to slight change in RI. The grating devices has been simulated by using COMSOL Multiphysics 5.3a with periodicity 700 nm, film thickness 50 nm and optimum slit width 300 nm in correspondence with fundamental plasmonic mode. The 0th order transmission spectra have been extracted from each grating device by illuminating (through the substrate side) with p-polarized light at normal incidence. The change in RI of analyte studied to have a significant effect on the resonance wavelength. The sensitivity has been calculated as 700, 731 and 722 nm/RIU for Au, Ag and Cu grating devices respectively which is remarkable. The stability and quick sensing of Au-device make it suitable for application besides some other deficiencies (e.g. slightly less sensitivity as compared to Ag) which should be compromised. Near field analysis has been performed to apprehend the underlying physics connected with each resonance. The highest value of electric and magnetic fields (E-field and H-field) are obtained in the case of Au grating indicating the most efficient excitation of SPPs. This confirms the reasoning behind the efficient sensitivity of Au grating device and finds applications in medicine.

Coupled optoelectronic simulation and optimization of thin-film photovoltaic solar cells

A design tool was formulated for optimizing the efficiency of inorganic, thin-film, photovoltaic solar cells. The solar cell can have multiple semiconductor layers in addition to antireflection coatings, passivation layers, and buffer layers. The solar cell is backed by a metallic grating which is periodic along a fixed direction. The rigorous coupled-wave approach is used to calculate the electron-hole-pair generation rate. The hybridizable discontinuous Galerkin method is used to solve the drift-diffusion equations that govern charge-carrier transport in the semiconductor layers. The chief output is the solar-cell efficiency which is maximized using the differential evolution algorithm to determine the optimal dimensions and bandgaps of the semiconductor layers.