Metallic Gratings Research Articles

Polarized optical properties in liquid crystals devices with photoaligned metal nanoparticle gratings

Liquid crystals (LCs) alignment can be controlled by the local enhanced light fields of the metallic nanostructures in the localized surface plasmonic resonant (LSPR) state. Here, a nanostructured metallic grating patterns with groove widths of 180 nm is introduced to align the nematic LC molecules on the glass prism surface to construct the reflective hybrid LC cell accompanied by a rubbed ITO glass slide. Using the polarized light microscopy (PLM) characterization, the optical orientation effect of the silver nanostructured grating patterns on nematic LCs has been investigated. As a result, the silver nanostructured grating patterns has been found to regulate the orientation of the LC molecules when illuminated by linearly polarized white light and determine the light transmission and reflection. The results demonstrated that the silver nanostructured grating patterns serves for the topological alignment of LCs which is usually aligned by the rubbed substrate. Thus, our hybrid LC cell with topological rubbing and light controlled variation of LCs alignment could provide new opportunities for photonic applications.

Modified rigorous coupled-wave analysis for grating-based plasmonic structures with a delta-thin conductive channel: far- and near-field study.

The modified rigorous coupled-wave analysis technique is developed to describe the optical characteristics of the plasmonic structures with the grating-gated delta-thin conductive channel in the far- and near-field zones of electromagnetic waves. The technique was applied for analysis of the resonant properties of AlGaN/GaN heterostructures combined with a deeply subwavelength metallic grating, which facilitates the excitation of the two-dimensional plasmons in the terahertz (THz) frequency range. The convergence of the calculations at the frequencies near the plasmon resonances is discussed. The impact of the grating's parameters, including filling factor and thickness of the grating, on resonant absorption of the structure was investigated in detail. The spatial distributions of the electromagnetic field in a near-field zone were used for the evaluation of total absorption of the plasmonic structures separating contributions of the grating-gated two-dimensional electron gas and the grating coupler.

人工微结构薄层红外探测器的研究进展（特邀）

The thin layer of infrared detection material guarantees the uniformity of the materials and reduces the signal noise in infrared detection. The absorption of infrared detector is limited by the thin layer of infrared detection material attributing to small volume. According to the characteristics of different infrared detection materials, artificial microstructure can effectively improve the performance of infrared detector. The strategies of enhancing the absorption of thin-layer infrared detection materials were introduced. The strategies were based on metal back plate, metal grating and asymmetric Fabry-Perot cavity. They could have an excellent performance in their own adaptive scenarios. Meanwhile, the mechanism of adjusting the absorption peak height and width by artificial microstructure was also elaborated briefly. The application of artificial microstructure in several infrared detectors was demonstrated. Finally, an artificial microstructure HgCdTe infrared detector was designed, which could achieve broadband absorption in 3.5-5.5 μm atmospheric window. The absorption peak reached 91.8% and the relative peak width was 41.8%. In most of frequency in the atmospheric window, the absorption enhancement is higher than 6. The development of artificial microstructure opens up the design idea of traditional infrared devices, and provides theoretical basis and guidance for new infrared devices.

Antenna radome based on metal-dielectric gratings as angular filter at microwaves

This article is devoted to the analysis of numerical study results of printed angular filters scattering characteristics. It is shown these angular filters may be used as antenna radomes. Numerical results are obtained by full-wave simulation of frequency-selective surfaces with dielectric covers. The numerical research results of the scattering characteristics of a printed angular filter as antenna radome based on metal-dielectric gratings are presented. A comprehensive numerical study of microwave angular filters based on multi-element multilayer printed reflect arrays has been carried out. Constructive solutions for metal-dielectric structures in integral design, realizing the functions of angular filters, are found. These solutions are based on performed numerical studies. The problems of constructive implementation of multilayer planar spatially selective as angular filters are considered. These angular filters are integrated into the radiators and feeders of modern multi-element printed phased arrays. The problems connected with creation of such arrays are also considered. The numerical simulation results for angular filters based on metal gratings with dielectric covers are obtained. These results can be used to select the most rational options for the topology of metal-dielectric gratings. Such solutions may be useful for design of multifunctional radomes in microwave antenna systems. Based on the obtained numerical data, the possibilities of using flat gratings as angular filters in the composition of antenna radomes are considered. The spatial frequency-selective structures proposed in this work are performed as multi-planar printed gratings. These gratings are designed to ensure electromagnetic compatibility of closely spaced radio electronic sets. These radio electronic sets operate in close frequency ranges. They contain antenna arrays. These arrays are placed under the antenna radomes. At the same time, the installation of angular filters in the antenna radomes makes it possible to eliminate the appearance of unwanted grating lobes in the radiation patterns of sparse antenna arrays of promising radioengineering sets at microwaves.

Biosensor based on plasmonic Mach-Zehnder interferometer with metallic gratings

本文优化了一种基于表面等离激元银纳米链的马赫-曾德干涉式传感结构。该结构由参考臂、传 感臂及纳米线波导构成。纳米线波导是由硅纳米线包裹一定厚度的银来构成。引入两条银纳米链分 别作为马赫-曾德干涉仪的参考臂和传感臂，并研究所设计结构的传输特性，通过降低传输损耗以提 高所设计结构的精确度与灵敏度。相比于两条完全相同的银纳米线作为参考臂和传感臂的情况，在 参考臂和传感臂改为银纳米链后，传输特性有明显提高，单位长度损耗明显降低。这是由于银纳米 链中的单元结构之间的长程/库仑相互作用增强了结构中的电磁场，进而降低了传输损耗。我们将两 条银纳米链的晶格常数设置为不同的情况，我们研究发现，在较宽的硅包银纳米线的宽度与某些占 空比下，含有非对称的银纳米链结构的单位传输损耗小于含有对称的银纳米链结构。由此可以知道， 具有小损耗的银纳米颗粒链可以弥补大损耗的银纳米颗粒链的传输损失。利用这个特点，进一步优 化设计结构，将一侧银纳米链改为纳米线。改变另一侧银纳米链的晶格常数与占空比，我们发现大 多数情况下，这类结构传输特性优于含有两条银纳米链以及含有两条银纳米线的结构。我们的设计 结构可以大幅减小传统的马赫-曾德干涉仪的传输损耗，且在结构的制备过程中容错率高，在实际应 用中有巨大的潜在应用价值。

Frequency selective antenna radome based on metal-dielectric gratings and perforated screens

This article is devoted to the analysis of numerical study results of printed frequency selective surfaces scattering characteristics. It has been shown that these frequency selective surfaces may be used as antenna radomes. Numerical results have been obtained by full-wave simulation of frequency-selective surfaces with dielectric covers. The numerical research results of the scattering characteristics of printed frequency selective surfaces as antenna radomes based on metal-dielectric gratings and thick perforated screens have been presented. A comprehensive numerical study of microwave frequency selective surfaces based on multi-element multilayer printed reflectarrays and thick perforated screens has been carried out. Constructive solutions for metal-dielectric structures in integral design, realizing the functions of frequency selective surfaces, have been found. These solutions are based on performed numerical studies. The problems of constructive implementation of multilayer planar spatially selective as frequency selective surfaces have been considered. These frequency selective surfaces are integrated into radiation systems of modern multi-element printed phased arrays. The problems connected with creation of such arrays have been also considered. The numerical simulation results for frequency selective surfaces based on metal gratings with dielectric covers have been obtained. These results can be used to select the most rational options for the topology of metal-dielectric gratings. Such solutions may be useful for design of multifunctional radomes in microwave antenna systems. Based on the obtained numerical data, the possibilities of using flat gratings as frequency selective surfaces in the composition of antenna radomes have been considered. The spatial frequency-selective structures proposed in this work are performed as multi-planar printed gratings. These gratings are designed to ensure electromagnetic compatibility of closely spaced radio electronic sets. These radio electronic sets operate in close frequency ranges. They contain antenna arrays. These arrays are placed under the antenna radomes.

Surface-plasmonic sensor using a columnar thin film in the grating-coupled configuration [Invited

The excitation of a surface-plasmon-polariton (SPP) wave guided by a columnar thin film (CTF) deposited on a one-dimensional metallic surface-relief grating was investigated for sensing the refractive index of a fluid infiltrating that CTF. The Bruggemann homogenization formalism was used to determine the relative permittivity scalars of the CTF infiltrated by the fluid. The change in the refractive index of the fluid was sensed by determining the change in the incidence angle for which an SPP wave was excited on illumination by a p-polarized plane wave, when the plane of incidence was taken to coincide with the grating plane but not with the morphologically significant plane of the CTF. Multiple excitations of the same SPP wave were found to be possible, depending on the refractive index of the fluid, which can help increase the reliability of results by sensing the same fluid with more than one excitation of the SPP wave.

Liquid crystal integrated metamaterial for multi-band terahertz linear polarization conversion

We experimentally investigate the linear polarization conversion for terahertz (THz) waves in liquid crystal (LC) integrated metamaterials, which consist of an LC layer sandwiched by two orthogonally arranged sub-wavelength metal gratings. A Fabry–Perot-like cavity is well constructed by the front and rear gratings, and it shows a strong local resonance mechanism, which greatly enhances the polarization conversion efficiency. Most importantly, the Fabry–Perot-like resonance can be actively tuned by modulating the refractive index of the middle LC layer under the external field. As a result, the integrated metamaterial achieves multi-band tunable linear polarization conversion.

Terahertz Broadband Polarization Conversion for Transmitted Waves Based on Graphene Plasmon Resonances.

We applied the harmonic oscillator model combined with the transfer matrix method to study the polarization conversion for transmitted waves in metallic grating/plasmon-excitation layer/metallic grating structure in the terahertz (THz) region. By comparing the calculated spectra and the simulated (by the finite-difference-time-domain method) ones, we found that they correspond well with each other. Both methods show that the Drude background absorption and the excited plasmon resonances are responsible for polarization conversion. The transmission is close to 0 when the distance between the top/bottom metallic gratings and gated graphene is an integer multiple of half the wavelength of the incident wave (in the dielectrics), at which points the plasmon resonances are greatly suppressed by the destructive interference between the backward/forward electromagnetic waves and that reflected by the top/bottom metallic gratings. Away from these points, the transmission can be higher than 80%. The electron density and the excitation efficiency of the plasmon-excitation layer were found to be important for the bandwidth of the polarization conversion window, while the scattering rate was found to influence mainly the polarization conversion rate. Multi-broadband polarization conversion is realized by exciting plasmon modes between the 0 transmission points in the THz region.

Methods for adjusting feedback in terahertz lasers

The optimal feedback coefficient in an active open resonator is an important condition for high lasing efficiency. For precise selection of the optimum communication and maintaining the optimum in all modes of laser operation, the ability to adjust smoothly the communication is required. Terahertz (THz) lasers have a number of features that one should take into account when choosing feedback control schemes. The paper presents a review and comparative analysis of the schemes of laser resonators (THz) in the range with a smoothly controlled radiation output. The authors consider both long known and widely used, as well as the latest schemes of laser resonators. Smooth adjustment can be realized in resonators formed by metal mirrors of total internal reflection and output mirrors in the form of metal mirrors with holes or one-dimensional metal gratings. The analysis of the advantages and disadvantages of each of the considered optical schemes of laser resonators is carried out.
 It is shown that the given resonator schemes make it possible to control and optimize the feedback in the laser during its operation. All of them are not very complex and can be realized by re-equipping existing lasers. The choice of a specific scheme should be made in accordance with the specifics of the laser application. The use of resonators with smooth coupling control makes it possible to achieve high efficiency of lasers at all energy operating modes.

Graphene metalenses with diverse electrical tunabilities at different terahertz frequencies

Graphene has attracted widespread attention in dynamic optoelectronic devices due to its tunable electrical and optical properties. But different modulation capabilities of the graphene-based designs at different frequencies are less studied. We study the electrical tunability of transmissive metalenses based on graphene when working at three frequencies 0.3, 1.25, and 2 THz, respectively. The constitutive meta-atoms are composed of graphene patches and metallic gratings for efficient phase shift in the orthogonal polarization. Although the conductivity of graphene is tunable at all the frequencies, responses of meta-atoms show weak and strong dependence on the Fermi level at the low and high frequencies, respectively. Therefore, the focal length of the metalens is not electrically tunable at 0.3 THz. In contrast, the metalenses designed at 1.25 and 2 THz show electrically adjustable focal lengths, and the tuning range of the focal length increases with frequency. The research here provides clear guidance for the design of graphene metalenses with different electrical tunabilities for a variety of application scenarios.

Tunable, Grating‐Gated, Graphene‐On‐Polyimide Terahertz Modulators

AbstractAn electrically switchable graphene terahertz (THz) modulator with a tunable‐by‐design optical bandwidth is presented and it is exploited to compensate the cavity dispersion of a quantum cascade laser (QCL). Electrostatic gating is achieved by a metal grating used as a gate electrode, with an HfO2/AlOx gate dielectric on top. This is patterned on a polyimide layer, which acts as a quarter wave resonance cavity, coupled with an Au reflector underneath. The authors achieve 90% modulation depth of the intensity, combined with a 20 kHz electrical bandwidth in the 1.9–2.7 THz range. The modulator is then integrated with a multimode THz QCL. By adjusting the modulator operational bandwidth, the authors demonstrate that the graphene modulator can partially compensate the QCL cavity dispersion, resulting in an integrated laser behaving as a stable frequency comb over 35% of the operational range, with 98 equidistant optical modes and a spectral coverage ~1.2 THz. This paves the way for applications in the terahertz, such as tunable transformation‐optics devices, active photonic components, adaptive and quantum optics, and metrological tools for spectroscopy at THz frequencies.

Spectral element method with modified Legendre polynomials for modal analysis of lamellar gratings.

We report on the derivation of a spectral element method whose originality comes from the use of a hierarchical basis built with modified Legendre polynomials. We restrict our work to TM polarization, which is the most challenging. The validation and convergence are carefully checked for metallic dielectric gratings. The method is shown to be highly efficient and remains stable for huge truncation numbers. All the necessary information is given so that non-specialists can implement the method.

Frequency-domain calculation of Smith-Purcell radiation for metallic and dielectric gratings.

The intensity of Smith-Purcell radiation from metallic and dielectric gratings (silicon, silica) is compared in a frequency-domain simulation. The numerical model is discussed and verified with the Frank-Tamm formula for Cherenkov radiation. For 30 keV electrons, rectangular dielectric gratings are less efficient than their metallic counterparts, by an order of magnitude for silicon, and two orders of magnitude for silica. For all gratings studied, radiation intensity oscillates with grating tooth height due to electromagnetic resonances in the grating. 3D and 2D numerical models are compared.

Tunability of optical responses in black phosphorus-dielectric heterostructures and black phosphorus covered metallic grating at terahertz frequencies

Exceptional properties of the terahertz (THz) radiation lead to growing research on its potential applications in diverse areas. In this work, we study two structures to enhance the polarization selectivity and absorption of the black phosphorus (BP)-based polarizer and absorber at the THz regime. For the first structure, we use the transfer matrix approach to design and optimize optical absorption in a heterostructure comprising BP sandwiched within a linear dielectric media. We model the impact of structural and material parameters and incident wave characteristics on the performance of the designed absorber. In the second design, BP covered metallic grating, we use the equivalent circuit model to show the realization of high extinction ratio polarizer and perfect absorber. The proposed polarizer has a simple structure and a large extinction ratio of around 40 dB. The working frequency of the designed polarizer and absorber is tunable by changing the geometric size of the structure and the electron concentration in the BP layer. We verify the accuracy of our results using the full-wave simulations carried out by the Lumerical finite-difference time domain solution.

Extraordinary optical transmission and enhanced magneto-optical Faraday effects in one-dimensional metallic gratings

The optical and magneto-optical (MO) properties of two different magneto-optic-plasmonic structures including double-layer and single-layer metallic gratings have been investigated. Numerical simulations based on the extended finite difference time domain method have revealed that, in the two proposed structures, apart from the extraordinary optical transmission effect, there arises high MO Faraday rotation in a broad spectrum spanning visible to near-infrared frequencies. Despite the easy fabrication process, the double-layer grating structure still shows an enhancement in transmittance, Faraday rotation, and figure of merit by two, five, and two times, respectively, compared to the single-layer structure, and this can be further improved by suitably modifying the configuration. The physical mechanisms of the physical phenomena are explained in detail.

High-resolution microscopy through optically opaque media using ultrafast photoacoustics.

We present a high-resolution microscope capable of imaging buried structures through optically opaque materials with micrometer transverse resolution and a nanometer-scale depth sensitivity. The ability to image through such materials is made possible by the use of laser ultrasonic techniques, where an ultrafast laser pulse launches acoustic waves inside an opaque layer and subsequent acoustic echoes from buried interfaces are detected optically by a time-delayed probe pulse. We show that the high frequency of the generated ultrasound waves enables imaging with a transverse resolution only limited by the optical detection system. We present the imaging system and signal analysis and demonstrate its imaging capability on complex microstructured objects through 200 nm thick metal layers and gratings through 500 nm thickness. Furthermore, we characterize the obtained imaging performance, achieving a diffraction-limited transverse resolution of 1.2 μm and a depth sensitivity better than 10 nm.

Double grating high efficiency nanostructured silicon-based ultra-thin solar cells

In this paper, the cylindrical, conical and parabolic nanostructures inherited from self-organized anodic aluminum oxide (AAO) are applied to silicon-based ultra-thin solar cells aiming for a new design concept for low-cost, high-efficiency double-grating solar cells. Numerical results reveal that the optimal bottom metal grating can enhance the absorption capacity of ultra-thin solar cell at 0.5 µm–1.1 µm waveband with a very small parasitic absorption. Meanwhile the surface grating with a period of 0.1 µm can strongly enhance the absorption of solar cell in 0.3 µm–0.5 µm waveband. Consequently, the silicon-based ultra-thin solar cell inherited their characteristics very well when both the optimal silicon surface grating and bottom metal grating are applied at the same time, thereby increasing the absorption rate of the solar cell in the entire waveband. Compared to the solar cell with single surface grating and the one with single bottom metal grating, the photocurrent density increments of the double-grating nanostructured solar cells can reach as high as 75.47% and 40.69%, respectively. Further results show that the conical and parabolic nanostructured gratings have higher light absorption efficiency and lower structure morphology-dependent sensitivity than the cylindrical nanostructured one, which is more conducive to the application of high efficiency ultra-thin solar cells.

Effect of ultra-shallow metallic gratings on sensitivity enhancement of Goos-Hänchen shift in SPR-based sensors

Metallic gratings have flexible and powerful modulation effects on electromagnetic waves. Reflection and transmission spectra of metallic gratings may become extremely narrow or ultra-wide depending on the grating parameters. In this paper, a surface plasmon resonance-based sensing system comprising of a metallic grating coupled with a prism configuration was proposed. The effects of all parameters of the sensing system on the Goos-Hänchen shift and sensitivity were studied in detail, and the results showed that ultra-shallow metallic gratings with large duty cycles, with no other materials involved, are more beneficial. In addition, a set of optimum parameters was obtained at the incident wavelength. Compared with the conventional SPR-based gold sensing chip, strongly coupled surface plasmon resonance around the grating enlarges the penetration depth, and enhances both Goos-Hänchen shift and the corresponding sensitivity by 5.6 times, with increasing refractive indices of the analyte solutions. These results indicate that the proposed ultra-shallow metal grating-based sensing structure is in favor of detecting ultralow concentrations of biomolecules in the analyte solutions.

Electron-driven photon sources for correlative electron-photon spectroscopy with electron microscopes

Abstract Electron beams in electron microscopes are efficient probes of optical near-fields, thanks to spectroscopy tools like electron energy-loss spectroscopy and cathodoluminescence spectroscopy. Nowadays, we can acquire multitudes of information about nanophotonic systems by applying space-resolved diffraction and time-resolved spectroscopy techniques. In addition, moving electrons interacting with metallic materials and optical gratings appear as coherent sources of radiation. A swift electron traversing metallic nanostructures induces polarization density waves in the form of electronic collective excitations, i.e., the so-called plasmon polariton. Propagating plasmon polariton waves normally do not contribute to the radiation; nevertheless, they diffract from natural and engineered defects and cause radiation. Additionally, electrons can emit coherent light waves due to transition radiation, diffraction radiation, and Smith-Purcell radiation. Some of the mechanisms of radiation from electron beams have so far been employed for designing tunable radiation sources, particularly in those energy ranges not easily accessible by the state-of-the-art laser technology, such as the THz regime. Here, we review various approaches for the design of coherent electron-driven photon sources. In particular, we introduce the theory and nanofabrication techniques and discuss the possibilities for designing and realizing electron-driven photon sources for on-demand radiation beam shaping in an ultrabroadband spectral range to be able to realize ultrafast few-photon sources. We also discuss our recent attempts for generating structured light from precisely fabricated nanostructures. Our outlook for the realization of a correlative electron-photon microscope/spectroscope, which utilizes the above-mentioned radiation sources, is also described.