Double-layer Grating Research Articles

Giant Goos-Hänchen shifts with high reflection driven by Fabry-Perot quasibound states in the continuum in double-layer gratings

Giant Goos-Hänchen shifts with high reflection driven by Fabry-Perot quasibound states in the continuum in double-layer gratings

The enhancement of nonreciprocal radiation for light near to normal incidence with double-layer grating

The enhancement of nonreciprocal radiation for light near to normal incidence with double-layer grating

Realization of large transmitted Goos-Hänchen shifts with high (near 100%) transmittance based on a coupled double-layer grating system.

Achieving Goos-Hänchen shift enhancement with high transmittance or reflectance based on the resonance effect is challenging due to the drop in the resonance region. This Letter demonstrates the realization of large transmitted Goos-Hänchen shifts with high (near 100%) transmittance based on a coupled double-layer grating system. The double-layer grating is composed of two parallel and misaligned subwavelength dielectric gratings. By changing the distance and the relative dislocation between the two dielectric gratings, the coupling of the double-layer grating can be flexibly tuned. The transmittance of the double-layer grating can be close to 1 in the entire resonance angle region, and the gradient of the transmissive phase is also preserved. The Goos-Hänchen shift of the double-layer grating reaches ∼30 times the wavelength, approaching 1.3 times the radius of the beam waist, which can be observed directly.

Plasmonic Surface Lattice Resonances in Suspended Symmetric Double-Layer Gratings

Surface lattice resonances (SLRs) with high-quality factors supported by metal nanoparticle arrays are useful for plasmonic nanolasers, biochemical sensors, and surface-enhanced Raman spectroscopy. Most nanoparticle arrays are fabricated on a substrate, and the refractive index mismatch between the substrate and superstrate suppresses the performance of SLRs. In this work, we propose unique SLRs excited in suspended, self-aligned symmetric double-layer gratings with index-matched environment. The self-aligned double-layer gratings are fabricated using a single-step electron beam lithography and exhibit a Fano-like spectra resulting from interference between out-of-plane plasmonic resonances and diffraction modes. By changing the incident angle and refractive index of the surrounding medium, the SLRs can be tuned from visible to near-infrared regions with a high-quality factor of 120.

Taper-tip double-layer grating antenna based on SiN-on-SOI with large scale-scanning range for LiDAR

In this article, with the objective of reducing the emission loss, enhancing the emission directionality and scale-scanning range, without increasing the complexity of the LIDAR system, a CMOS-compatible silicon nitride taper-tip double-layer auxiliary grating antenna is proposed and investigated systematically. Compared to conventional double layer grating, the directionality exceeds 72% for wavelengths from 1450 nm to 1600 nm, with a maximum directionality of 97.5% at 1550 nm. Therefore, the far-field divergence angle of 0.0081° × 0.03° and a large field of view of 108.45° × 23.43° can be achieved when the antenna aperture is 1.6 cm × 2.5 cm. These improvements come mainly from the increasing unidirectional emission performance of the phased array by adjusting the key structural parameters of the inverted taper-tip double-layer structure.

Micro-opto-electro-mechanical systems accelerometer based on the Talbot effect of double-layer diffraction gratings.

In this paper, we present the design, fabrication, and test of a micro-opto-electro-mechanical systems (MOEMS) accelerometer based on the Talbot effect of double-layer diffraction gratings. The detection of acceleration is realized by using the highly sensitive displacement characteristic of Talbot imaging of near-field diffraction with double-layer gratings. For the purpose of obtaining optimal contrast of the optical interferometric detection, the parameters of the gratings are optimized by the finite-difference time-domain (FDTD) simulation. The experimental results indicate that this MOEMS accelerometer with the proposed design can achieve a resolution of 246 µg, sensitivity of 6.1 V/g, and bias stability of 0.02 mg. The proposed accelerometer can be operated at higher accelerations (∼80g), which shows significant potential for being used in applications that require detection of strong and fast vibrations, especially in vibration sensing of vehicles and geophysical seismic sensing in real time.

Enhanced and tunable circular dichroism in the visible waveband by coupling of the waveguide mode and local surface plasmon resonances in double-layer asymmetric metal grating

Circular dichroism (CD) has shown very interesting possibilities as a means to characterize the chiral signal of a chiral structure. Here, we theoretically demonstrated enhanced and tunable CD in the visible light regime using a composite structure consisting of a double-layer metal grating gaped by a dielectric waveguide layer. Based on the coupling of the waveguide modes and the localized plasmonic resonances, the CD could reach a maximum value as high as 0.52 at 635 nm, which is four times higher than the CD value obtained in a conventional double-layer grating without the waveguide coupling effect. Furthermore, the spectral positions of the enhanced CD bands could be easily tuned by controlling the structural parameters. The proposed hybrid double-grating and waveguide structures could have potential applications in chiral selective imaging, sensing and spectroscopy, especially where the transmission measurement is required.

Excitation of Surface Plasmon Polariton Modes with Double-Layer Gratings of Graphene.

A long-range surface plasmon polariton (SPP) waveguide, composed of double-layer graphene, can be pivotal in transferring and handling mid-infrared electromagnetic waves. However, one of the key challenges for this type of waveguide is how to excite the SPP modes through an incident light beam. In this study, our proposed design of a novel grating, consisting of a graphene-based cylindrical long-range SPP waveguide array, successfully addresses this issue using finite-difference time-domain simulations. The results show that two types of symmetric coupling modes (SCMs) are excited through a normal incident light. The transmission characteristics of the two SCMs can be manipulated by changing the interaction of the double-layer gratings of graphene as well as by varying various parameters of the device. Similarly, four SCMs can be excited and controlled by an oblique incident light because this light source is equivalent to two orthogonal beams of light. Furthermore, this grating can be utilized in the fabrication of mid-infrared optical devices, such as filters and refractive index sensors. This grating, with double-layer graphene arrays, has the potential to excite and manipulate the mid-infrared electromagnetic waves in future photonic integrated circuits.

High-Q localized surface plasmon resonance based on bound states in the continuum for enhanced refractive index sensing.

Nanophotonics based on localized surface plasmon resonance (LSPR) has emerged as a vibrant arena for research into enhanced light-matter interactions with potential applications in imaging, sensing, and computing. However, the low quality (Q) factor of LSPR is a significant barrier to comprehensive device applications. Here, we demonstrate that coupling the LSPR of a gold nanowire array with the optical bound states in the continuum (BIC) of a dielectric double-layer grating can significantly increase the Q factor of LSPR. We realize two hybrid modes with Q factors of up to 111 at 558 nm and 83 at 582 nm, which are about 14 and 10 times larger than those of an uncoupled gold nanowire array. Based on temporal coupled-mode theory, we further show that the resonance frequencies and Q factors of the hybrid modes can be modulated and optimized by varying relevant structural parameters. This coupled system provides a new platform for improving the figures of merit (FoMs) of LSPR-based refractive index sensors, and the concept of LSPR-BIC coupling can be extended to other similar nanosystems.

Broadband near-infrared reflector based on double-layer subwavelength gratings

Broadband near-infrared reflector based on double-layer subwavelength gratings

Method of singular integral equations in scattering by double-layer infinite strip grating with several strips removed in every layer

The H-polarized plane wave scattering by the infinite double-layer strip grating with several strips removed in every layer is considered. The scattered field is represented as a sum of the field of currents on the ideally periodic double-layer grating and field of correction currents excited due to the absence of strips. The Fourier amplitudes of these fields are obtained from the singular integral equation with additional conditions. The solution is rigorous. Numerical results for the near and far fields are presented.

Terahertz range resonances of metasurface formed by double-layer grating of microsize graphene strips inside dielectric slab

We analyse, using integral equations and a previously developed in-house numerical algorithm, the scattering and absorption of the H -polarized plane wave by a metasurface consisting of a double-layer grating of flat graphene strips placed into a lossless dielectric slab. The algorithm is meshless and its convergence is guaranteed mathematically. It is a version of the method of analytical preconditioning; namely, it uses the set of weighted Chebyshev polynomials as expansion functions in the discretization of a hypersingular electric field integral equation for the on-strip current. Then the computational error is controlled by the matrix size and can be reduced to machine precision. Using this advanced tool, we plot the frequency dependences, in a huge range from 1 GHz to 10 THz, of the transmittance, reflectance and absorbance of such a metasurface. This accurate analysis reveals resonances on several types of natural modes, best understood via visualization of in-resonance near-fields. In addition to plasmon-mode resonances, special attention is paid to the ultra-high- Q resonances on the lattice modes, which are absent on the free-standing graphene strip gratings.

Generation of three-port splitter by double-layer grating in second-order Littrow configuration

In this paper, a novel double-layer three-port grating is described. The incident grating structure is in the second-order Littrow configuration. The grating region is composed of fused silica and Ta2O5. The designed grating beam splitter has high efficiency under TE polarization and TM polarization, respectively. The efficiency of two polarizations is more than 90%. In addition, compared with a single-layer three-port grating, this new beam splitter has good fabrication tolerance and incident bandwidth. Therefore, the optimized structure has a good application value.

Embedded dual-layered dielectric slit arrays under second Bragg incidence for 1×3 splitting field propagation

Embedded double-layer transmission grating beam splitter under the second Bragg incidence is presented in this paper. With a given duty cycle of 0.5 and a period of 940 nm, we use rigorous coupled-wave analysis method to investigate this beam splitter. The grating parameters are optimized for high-efficiency output. Each efficiency of the three orders under polarizations of TE and TM is close to 33.3 % respectively. Compared with the single-layer fused-silica grating, the three-port double-layer grating is flexible with complex structure and stable in performance. The field distribution modal method are given, which can well explain the propagation process and coupling mechanism.

倒梯形双层金属光栅式偏振分束器

倒梯形双层金属光栅式偏振分束器

Physical mechanism of beam splitting based on reflective embedded double-layer grating

It is not easy to achieve high performance for conventional beam splitters, such as high efficiency, good uniformity, polarization-independence, and wide bandwidth. A reflective embedded double-layer grating is described for beam splitting. With optimized grating profiles, the novel beam splitter can diffract both TE and TM polarizations into two orders with high performance. For the easy production, the fabrication tolerance is investigated and given. Most importantly, efficiencies more than 45% can be split into two orders within the wide bandwidth of 1412–1647 nm for TE polarization. The beam splitter based on multilayer coatings is sensitive to the incident angle and wavelength. And the bandwidth needs to be improved for the beam splitter based on simple grating. The design is of benefit for the performance improvement of the beam splitter by new grating configuration compared with the conventional simple grating.

Femtosecond-laser-assisted fabrication of double-layer diffraction gratings inside fused silica glass

We have fabricated double-layer diffraction gratings with a zigzag shape inside fused silica glass by using a femtosecond laser, the intention of which is to overcome the hindrance of shortening grating’s period. Our primary focus is to utilize double-layer gratings to achieve a diffraction pattern similar to that from a single-layer diffraction grating with the same period as the double-layer grating. We fabricated two kinds of double-layer gratings, where the grating’s period was either 6 μm or 2 μm for both layers. The first set of diffraction gratings was printed 70 μm inside the glass substrates. This layer was followed by the second layer grating engraved just below the first layer grating (The shifted grating’s position was shifted to the right by 3 μm for the 6-μm-period grating and by 1 μm for the 2-μm-period grating). By varying the layer gap, we have found the optimal position (layer gap: 500 nm) to achieve a diffraction pattern similar to that of a single-layer diffraction grating with half the grating’s period of the double-layer gratings.

Optimized Double-Layered Grating Structures for Chem/Biosensing in Midinfrared Range

Double-layered grating structures (DGSs) fabricated of gold (Au) and silver (Ag), respectively, are proposed for chem/biosensing in the midinfrared range. The geometry parameters of the DGS sensors are optimized using a rigorous coupled wave analysis method such that the minimum reflectance is obtained at an infrared wavelength of 4.8 μ or 6.7 μm. The magnetic field patterns and Poynting vector distributions within the optimized grating structures are examined using a finitedifference time-domain method. The simulation results show that for a resonant wavelength of 4.8 μm, the Au and Ag sensors both have a sensitivity of 2000 nm/RIU (refractive index unit) and 3000 nm/RIU theoretical sensing resolution of 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> RIU and of 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> RIU, respectively. Similarly, for a resonant wavelength of 6.7 μm, the two sensors have a sensitivity of around 200 nm/RIU and theoretical resolution of 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> RIU. In other words, the sensitivity of the DGS sensors optimized for an infrared wavelength of 4.8 μm is one order higher than that of the sensors optimized for a wavelength of 6.7 μm. In general, the results presented in this paper show that the proposed DGS sensors provide a simple and versatile solution for performing chem/biosensing at midinfrared wavelengths.

Robust existence of the broadband optical transmission effect in multiple-layer gratings

We numerically demonstrate the existence and robustness of the broadband optical transmission (BOT) effect in double-layer gratings (DLGs) with different longitudinal spacings and lateral displacements. For the two scenarios of large and ultrasmall longitudinal spacings, the different physical mechanisms by which BOT occurs through the gaps between the two layers are analyzed based on the surface charge densities. Besides the DLG structures, the robustness of the BOT phenomenon can also be extended to exist in multiple-layer gratings with more layers. This provides a highly controllable way to design novel devices, such as transparent multiple-layer metals, broadband metamaterials, and novel polarization filters.

Fabrication of flexible metallic wire grid polarizer using thermal NIL and lift-off processes

► Double layer metallic/dielectric gratings were fabricated for WGP components. ► Thermal NIL, aluminum lift-off, and dielectric etching were adopted in experiments. ► 192:1 Extinction ratio and 1.2× brightness gain were obtained with 240 nm pitch WGP. ► Designed optical performance and better process tolerance could be achieved. ► Low retardation COP films provide adequate optical performance. This work is dedicated to the fabrication of metallic/dielectric double layer gratings on polymeric films for wire grid polarizer (WGP) to obtain designed optical performance as well as better process tolerance. The fabrication process was based on thermal nanoimprint and aluminum lift-off techniques. Polymeric substrates PET (Polyethylene terephthalate) and COP (Cyclo-olefin polymer) were brought into comparison for the birefringence property. At the same time, by applying RCWA (Rigorous coupled-wave analysis) simulated optical designs, it would be also capable of minimizing difficulties of ultra-precision manufacturing. Experimental results revealed that using the 240 nm pitch stacked metallic/dielectric gratings performed 192:1 extinction ratio and 1.2× brightness enhancement, which allowed larger pitch replacing the smaller pitch of WGP gratings, but still possessed similar optical performance. Hence, these designs and process configuration not only make the continuous roll-to-roll process more feasible, but also relieve the difficulties of fabricating the imprint roller stamp.