Anisotropic Gratings Research Articles

Strong enhancement of near-field radiative heat transfer between anisotropic magneto-optical gratings by a weak magnetic field

The highly tunable surface magneto-plasmon polaritons supported by magneto-optical materials provide reliable means for active adjusting of near-field radiative heat transfer (NFRHT). In our study, we proposed an anisotropic system consisting of two graphene-covered magneto-optical gratings, and theoretically investigate the NFRHT between them controlled by a weak magnetic field. We find that an external magnetic field open a broad-spectrum photo tunneling channel and modify the energy distribution of surface electromagnetic modes in the wave-vector space. The near-field heat flux between two magneto-optical gratings can achieve a remarkable maximum enhancement of 6 times at the magnetic field H=0.9T compared to that without a magnetic field. Owing to the coupling between highly tunable surface magneto-plasmon polaritons of graphene and surface modes of InSb, the regulation ratio of the heat flux exceeds 10 from 0.5 T to 0.9 T. In addition, the effects of the separation distance, chemical potential of graphene and structural configuration of two gratings are also investigated. We look forward to the proposed system being used as a thermal switch, providing convenience for tuning thermal radiation and improving the efficiency of energy-utilization devices.

Design of two-dimensional multilevel optical anisotropic diffraction gratings with a generative adversarial network.

This study uses a generative adversarial network to design multilevel optical anisotropic diffraction gratings with specific customizable characteristics. As input, this method uses the far electric field of polarization and intensity in each diffracted light through the gratings to design. Using the finite-difference time-domain method, the designed structures are numerically evaluated, confirming that they can be created with the intended parameters. Multilevel optical anisotropic diffraction gratings created this way can be used in various fields to develop improved optical elements.

Two-Dimensional Polarization Holographic Gratings in Azopolymer Thin Films: Polarization Properties in the Presence or Absence of Surface Relief

During polarization holographic recording in azopolymer thin films, usually together with the volume anisotropic grating, a surface relief grating (SRG) is also formed. By using two consecutive exposures, it is possible to obtain a two-dimensional (2D) grating. To the best of our knowledge, the polarization properties of such gratings have not been studied yet. To determine the influence of the surface relief on the polarization selectivity of the 2D gratings, we propose two methods to suppress the SRG formation: by varying the recording conditions or varying the sample structure. In these experiments we have used the commercially available azopolymer PAZO, poly[1-4-(3-carboxy-4-hydrophenylazo) benzene sulfonamido]-1,2-ethanediyl, sodium salt] to perform the polarization holographic recording using a 442 nm He-Cd laser. As indicated by our results, when the surface relief is present, it strongly dominates the response of the 2D grating and it behaves almost as a scalar polarization insensitive grating. Conversely, when the SRG formation is suppressed, the polarization properties of the 2D grating in all four diffracted orders are very well pronounced. In this way, we demonstrate that we can easily control SRG formation and, if desired, obtain 2D grating with high surface relief modulation, or alternatively record polarization-selective 2D gratings with virtually no surface relief.

Waves in frequency-independent single-mode 2-D artificial media with forced refraction

Investigations of the waves in planar structures with anisotropic gratings proved that structures of this type demonstrated properties of 2D media with forced refraction. Parasitic modes and limited frequency range were typical for them. This study investigated the propagation of guided waves along metal grating lying between two metal screens inside parallel plate waveguide (PPW). Dominant T-mode was the focus of this study. This mode was studied theoretically by the method of integral equations (IE) and numerically with the help of an electromagnetic Ansys HFSS simulation. Both methods demonstrated that the investigated structure had the properties of a 2D medium with forced refraction independent of frequency. The modified structure contained metal grating and an additional 2D array of metal cylinders was also studied numerically. The numerical simulation demonstrated that an array of metal cylinders suppressed parasitic PPW mode and thus modified structure demonstrated properties of a single-mode frequency independent artificial media with forced refraction. The results have the potential for application in microwave and millimeter-wave planar quasi-optical lens antennas.

Detection band expansion by independently tunable double resonances in a long-wavelength dual-color QWIP.

Multi-resonance light coupling management is a promising way to expand the operating spectral ranges of optoelectronic devices. The classical strategies are either lack of independent tunability for each resonance or involved with complex fabrication. Here, we propose a new scheme for expanding the operating spectral range of an optoelectronic device through a dual-color active material integrated with a simple resonant waveguide structure. The TM waveguide mode and the SPP mode of the resonant waveguide structure are regulated to match the two active regions of the dual-color material both spectrally and spatially. Applying this scheme to a long-wavelength infrared quantum well photodetector, the absorption efficiencies at the two peak detection wavelengths of the dual-color quantum wells are both enhanced by more than 10 times compared with the case of a standard 45° edge facet coupled device with the same detection material. The simple light coupling structure is easy to accomplish and compatible with focal plane arrays. For thermal radiation detection, the absorption efficiency of the 300 K blackbody radiation by our dual-color detector is 83.8% higher than that by a single-color detector with the optimized structural parameters. Moreover, either polarization sensitive or polarization insensitive detection could be achieved in this dual-color infrared quantum well photodetector by using anisotropic or isotropic gratings.

Two-dimensional guided-mode resonance gratings with an etch-stop layer and high tolerance to fabrication errors.

Guided-mode resonance (GMR) bandpass filters have many important applications. The tolerance of fabrication errors that easily cause the transmission wavelength to shift has been well studied for one-dimensional (1D) anisotropic GMR gratings. However, the tolerance of two-dimensional (2D) GMR gratings, especially for different design architectures, has rarely been explored, which prevents the achievement of a high-tolerance unpolarized design. Here, GMR filters with common 2D zero-contrast gratings (ZCGs) were first investigated to reveal their differences from 1D gratings in fabrication tolerance. We demonstrated that 2D ZCGs are highly sensitive to errors in the grating linewidth against the case of 1D gratings, and the linewidth orthogonal to a certain polarization direction has much more influence than that parallel to the polarization. By analyzing the electromagnetic fields, we found that there was an obvious field enhancement inside the gratings, which could have a strong effect on the modes in the waveguide layer through the field overlap. Therefore, we proposed the introduction of an etch-stop (ES) layer between the gratings and the waveguide-layer, which can effectively suppress the interaction between the gratings and modal evanescent fields, resulting in 4-fold increased tolerance to the errors in the grating linewidth. Finally, the proposed etch-stop ZCGs (ES-ZCGs) GMR filters were experimentally fabricated to verify the error robustness.

High-performance silicon TE-pass polarizer assisted by anisotropic metamaterials.

The polarizer is a key component for integrated photonics to deal with the strong waveguide birefringence, especially for silicon photonics. A high-performance silicon TE-pass polarizer covering all optical communication bands with low insertion loss (IL) and high polarization extinction ratio (PER) is proposed here. This polarizer is based on anisotropic subwavelength grating (SWG) metamaterials, which maintain the fundamental TE mode as a guided mode but make the fundamental TM mode leaky. Furthermore, based on this working mechanism, the proposed polarizer can work well for any upper cladding material, including air and silicon dioxide (SiO2). The numerical results show that our proposed TE-pass polarizer has a remarkable performance with IL < 0.34 dB over 420 nm (PER > 23.5 dB) or 380 nm (PER > 30 dB) for the air cladding, and IL < 0.3 dB over 420 nm (PER > 25 dB) or 320 nm (PER > 30 dB) for the SiO2 cladding. The fabricated polarizer shows IL < 0.8 dB and PER > 23 dB for the bandwidths of 1.26-1.36 µm and 1.52-1.58 µm (other bandwidths were not measured due to the limited instrument in our research center, but it still covers the most important O-band and C-band).

Polarization information landscapes expanded from single-shot images of ring-like diffraction patterns

In this paper, we propose a ring-type grating in which grating vectors formed by periodic molecular alignments are superimposed radially. Depending on incident polarization states, the anisotropic gratings generate polarization dependencies for the ring-like first order diffraction. To determine the polarization states of incident light for both single and multi-wavelengths, we attempted to extract the optical intensity distribution on the ring-like diffraction from each single-shot diffraction image. In addition, a system that enables the detection of spatial modulation of optical birefringence is proposed. Finally, we concluded that the optical intensity distribution on the circumference of the ring-like diffraction has sufficient potential to be available.

Broadband and Tunable Light Harvesting in Nanorippled MoS2 Ultrathin Films.

Nanofabrication of flat optic silica gratings conformally layered with two-dimensional (2D) MoS2 is demonstrated over large area (cm2), achieving a strong amplification of the photon absorption in the active 2D layer. The anisotropic subwavelength silica gratings induce a highly ordered periodic modulation of the MoS2 layer, promoting the excitation of Guided Mode Anomalies (GMA) at the interfaces of the 2D layer. We show the capability to achieve a broadband tuning of these lattice modes from the visible (VIS) to the near-infrared (NIR) by simply tailoring the illumination conditions and/or the period of the lattice. Remarkably, we demonstrate the possibility to strongly confine resonant and nonresonant light into the 2D MoS2 layers via GMA excitation, leading to a strong absorption enhancement as high as 240% relative to a flat continuous MoS2 film. Due to their broadband and tunable photon harvesting capabilities, these large area 2D MoS2 metastructures represent an ideal scalable platform for new generation devices in nanophotonics, photo- detection and -conversion, and quantum technologies.

Polarization holographic gratings in PAZO polymer films doped with particles of biometals

The paper presents a study of the diffraction efficiency of polarization holographic gratings recorded in thin films of the azopolymer PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]) doped with Cu(II) 3-amino-5,5′-dimethylhydantoin (CLP) and Ni(II) 3-amino-5,5′-dimethylhydantoin (NLP) at three different concentrations, namely 1, 2 and 5 wt.%. The influence of the dopants composition and concentration on the parameters of the polarization holographic gratings recorded in the thin composite films has been discussed. The gratings are recorded with a He-Cd gas laser with wavelength 442 nm. The polarization of the recording beams was left and right circular and the recording angle was 20°. Along with the anisotropic grating in the volume of the media, surface relief is also formed. The diffraction efficiency kinetics is probed at 635 nm and the height of the relief gratings is determined by AFM. Diffraction efficiency (η) higher than 30 % was achieved for the hybrid samples, as well as 585 nm surface relief height.

Spin-VCSELs with Local Optical Anisotropies: Toward Terahertz Polarization Modulation

Putting a different spin on photonics: Spin-injected vertical-cavity surface-emitting lasers (Spin-VCSELs) with strong anisotropies are important in particular for ultrafast and terahertz applications, but the field is being held back in part because of lack of sufficiently predictive physical models. This study uses temporal coupled-mode theory with the transfer-matrix formalism to make progress by extending the spin-flip model. Additionally, the authors propose a spin-VCSEL with an anisotropic intracavity grating. This approach points to engineering solutions for ultrafast optical communication and compact terahertz sources.

Polarization holographic gratings with enhanced parameters recorded in azopolymer based nanocomposite materials

We present study of the parameters of polarization holographic gratings recorded in thin films of the azopolymer PAZO (Poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]) and in a composite PAZO-based organic/inorganic material with incorporated TiO2 nanoparticles with size 21 nm and different concentrations. The gratings are inscribed by two plane waves with left and right circular polarization from a He-Cd gas laser (442 nm) at recording angle 20°. Along with the anisotropic grating in the volume of the media, surface relief is also formed. Gratings with different spatial frequencies are obtained by varying the recording angle. The diffraction efficiency kinetics is probed at 635 nm and the height of the relief gratings is determined by AFM. Our results indicate that the diffraction efficiency for the hybrid samples is enhanced with respect to the pure azopolymer films. Comparison is also made between polarization response of samples deposited from water and from methanol solution.

Two-dimentional diffractive optical structure based on patterned photoalignment of polymerizable liquid crystal

This paper is devoted to the investigation of the diffraction characteristics of a multilayer optical structure, represented by a two-dimensional phase lattice formed in thin-film layers of nematic polymerizable liquid crystal (PLC). In order to generate periodically ordered liquid-crystal (LC) domains with dimensions of 5–10 µm, which form an anisotropic LC grating, the layer-by-layer patterned photoalignment technology of the water-soluble azo dye AbA-2522 was used. The possibilities of spatial-polarization control of light beams by means of developed one- and two-dimensional diffractive LC structures have been studied experimentally. Our results are promising from the point of view of development and creation of LC-devices for solving current problems of polarization photonics.

Achromatic Cycloidal Diffraction Gratings

A theory of a broadband thin-film polarizing beam splitter based on an anisotropic cycloidal diffraction grating is proposed. The achromatization of the grating is provided by creating two-layer chiral structures with an opposite twist sense. Such a system has almost 100% efficiency. The calculations were carried out by the Jones matrix method. The resulting efficiency and bandwidth are higher than the previously known results.

Polarization imaging using an anisotropic diffraction grating and liquid crystal retarders.

We present a simple yet versatile and practical polarization camera for imaging full Stokes parameters. The developed system consists of one anisotropic diffraction grating plate and two electro-switchable retarders and obtains in nearly real-time full Stokes images of the light scattered from the objects. The monochromatic S3 image is obtained by physically separating the right and left circularly polarized components with the aid of the anisotropic grating, and S1 and S2 images are obtained by fast electro-switching the retardation of each retarder. The simple polarization imaging system has possible applications in various types of imaging systems and especially should be incorporated into optical microscopes as well as imaging cameras in future work.

All-dielectric resonant waveguide based quantum well infrared photodetectors for hyperspectral detection

All-dielectric resonant waveguide based quantum well infrared photodetectors with high responsivity, fine spectral resolution and a wide spectral range are proposed. Without ohmic loss, the guided mode resonances lead to highly efficient absorption of the field power by the quantum wells and high Q factors for narrow spectral widths. Close to 100% resonant absorption with spectral resolution finer than 100 nm could be achieved in the long wavelength infrared region. When the incident wavelengths deviate from the intrinsic absorption range of the quantum wells, a decrease in absorption coefficient causes an increase in Q factor and in mode intensity that would compensate the absorption power. As a result, a three-fold extension of the absorption range is demonstrated. The infrared detectors with these advances simultaneously are well suited for hyperspectral detection. Further, the device could be either polarization discriminative or insensitive by using anisotropic or isotropic in-couple gratings. The good tolerance to structural deviation could facilitate the fabrication process.

Genetic algorithm-based design method for multilevel anisotropic diffraction gratings

We developed a method for the design of multilevel anisotropic diffraction gratings based on a genetic algorithm. The method is used to design the multilevel anisotropic diffraction gratings based on input data that represent the output from the required grating. The validity of the proposed method was evaluated by designing a multilevel anisotropic diffraction grating using the outputs from an orthogonal circular polarization grating. The design results corresponded to the orthogonal circular polarization grating structures that were used to provide outputs to act as the input data for the process. Comparison with existing design methods shows that the proposed method can reduce the number of human processes that are required to design multilevel anisotropic diffraction gratings. Additionally, the method will be able to design complex structures without any requirement for subsequent examination by a human designer. The method can contribute to the development of optical elements by designing multilevel anisotropic diffraction gratings.

Selective acquisition of multiple states on hybrid Poincare sphere

We experimentally demonstrate an approach to acquire multiple beams on a hybrid Poincare sphere, where different states are present at various diffraction orders. Moreover, the state in each diffraction order is controllable. Our scheme is accomplished by employing liquid crystal spatial light modulators, which act as anisotropic polarization diffraction gratings. The multiple states generated in the experiment, with various polarization distributions, are verified by a polarizer and a tilted lens, showing good agreement with the prediction. Such a proposed scheme has potential applications in the domains as laser machining, nanoparticle manipulations, and so on.

Polarization holographic recording in thin films of pure azopolymer and azopolymer based hybrid materials

Recently, a birefringence enhancement effect was observed in azopolymers doped with various nanoparticles. The paper presents comparison between the parameters of polarization holographic gratings recorded in a pure azopolymer PAZO (Poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]) and in a hybrid PAZO-based organic/inorganic material with incorporated ZnO nanoparticles of size less than 50 nm. Laser emitting at 491 nm is used for the holographic recording. Along with the anisotropic grating in the volume of the media, surface relief is also formed. Gratings with different spatial frequencies are obtained by varying the recording angle. The time dependence of the diffraction efficiency is probed at 635 nm and the height of the relief gratings is determined by AFM. Our results indicate that both the diffraction efficiency and the height of the surface relief for the hybrid samples are enhanced with respect to the pure azopolymer films.

Simultaneous generation of multiple perfect polarization vortices with selective spatial states in various diffraction orders.

We demonstrate an approach to generate multiple perfect polarization vortices (PPVs) with selective spatial polarization distribution in various diffraction orders. The key is the design of a hologram with an anisotropic polarization diffraction grating. In the experiment, a setup consisting of two spatial light modulators is built. By encoding the specially designed holograms, PPVs with various states are obtained in different diffraction orders simultaneously, for instance, in radial and azimuthal polarized states. These PPVs may have applications in laser processing, optical tweezers, and so on.