High Diffraction Efficiency Research Articles

Diffraction characteristics of a non-mechanical beam steering system with liquid crystal polarization gratings.

The non-mechanical beam steering system is composed of multiple liquid crystal polarization gratings (LCPGs) cascaded by binary or ternary technologies. However, cascading multiple LCPGs cause the beam from one LCPG to obliquely enter the subsequent LCPGs, changing their diffraction efficiency and working voltage at different steering angles. This paper uses the elastic continuum theory of liquid crystals to simulate the tilt angle of liquid crystal molecules under different voltages. The transmission process of the beam in the system at oblique incidence is described with an extended Jones matrix, and the highest diffraction efficiency and working voltage of each LCPG at different steering angles are calculated using vector diffraction theory. It is convenient to calibrate the LCPGs' working voltage and analyze the system's diffraction characteristics. In addition, we used an improved binary cascade technology to design a LCPG non-mechanical beam steering system with a steering angle of ±10° and an angular resolution of 0.67°. Compared with binary cascade, this technology can effectively reduce the number of cascaded devices and increase the system throughput under the same maximum beam steering angle and angular resolution.

Nano-cylindrical hole metamaterials for high-efficiency ultra-bandwidth two-dimensional diffraction

Two-dimensional (2D) grating as the core component of the 2D grating displacement measurement system has a more demanding performance in recent years. In this paper, a 2D metal grating with an array of nano-cylindrical holes is proposed, which can achieve a high diffraction efficiency (DE) output of 2 × 2 at a wavelength of 780 nm, where the DEs of (0, ±1) and (±1, 0) orders are over 23% with a nonuniformity of 0.23%. Meanwhile, the results of the finite-difference time-domain method are in good agreement with those of the finite element method. Notably, the proposed grating has a wavelength bandwidth of 44 nm and a large angular bandwidth of 12° when the DE of all main diffraction orders is chosen to be above 20% as the target condition. This angular bandwidth is a significant improvement relative to previous reports. Moreover, the results show that the proposed grating has good manufacturing tolerances while ensuring high performance, demonstrating its feasibility in practical manufacturing. Therefore, this grating has significant potential in the field of 2D grating displacement measurement due to its simplicity, high efficiency, good uniformity, wide bandwidth and good process tolerance.

Color-selective geometric-phase lenses for focusing and imaging based on liquid crystal polymer films.

The geometric-phase lens (GPLs) with small form factor compared to traditional refractive lenses has been identified as a compelling solution in augmented-/virtual-/mixed-reality (AR/VR/MR) headsets. Formed either with liquid crystals (LCs) or metasurfaces, the GPL is a type of emerging leading technology that implements the arbitrary aspheric phase to realize low loss and minimal ghosting. However, the inherent chromatic abberation (CA) of GPLs can significantly degrade the image quality. A possible solution is the independent spectral phase implementation for RGB. In this work, we propose the design of three types of multi-twist LC based color-selective GPLs (CS-GPLs), exhibiting highly chromatic efficiency spectra with diameter 30 mm, focal length around 41.2~mm, and F -number 1.37. Through theoretical and experimental validation, each type of CS-GPL manifests high diffraction efficiency (>91%) on respective primary color of orthogonal polarization and high transmission on the complementary color of input polarization. The triplet composed by RGB CS-GPLs demonstrates relative contrast ratio and minimal ghosting. The strong color and polarization dependency of CS-GPLs not only provide a novel technique to mitigate CA but also offer more design freedom in the AR/VR/MR polarization and imaging system.

Metagrating solutions for full color single-plate waveguide combiner

In this work we propose several full-color metagrating solutions for single waveguide-based Augmented and Virtual Reality near-eye display systems. The presented solutions are based on a combination of reflective and/or transmissive diffraction gratings inside or outside a waveguide. The proposed in-coupler designs have high diffraction efficiency across a wide angular range. Applying our new grating combination solution, we can provide good gathering of diffracted rays for the different colors. We demonstrate that by using a dual-mode symmetrical in-coupling system and angular pupil tiling, we can extend the overall horizontal FoV for three RGB colors. The new characteristics of the full single waveguide system including Eye Pupil Expander and out-coupling components compatible with the proposed in-coupling solutions are discussed. We show that a new nonsymmetrical design of metagratings can be used to change its diffraction properties improving the diffraction efficiency and diffraction uniformity of the optical components.

Регистрация высокочастотных рельефно-фазовых структур на галоидосеребряном фотоматериале ПФГ-01

A new variant of manufacturing high–frequency holographic gratings with a spatial frequency up to 1200 mm-1 on silver halide photographic material for holography PFG-01 is proposed, the processing technology of which is tanning bleaching in bleach containing dichromates and etching in water or in solutions of glacial acetic acid in isopropyl alcohol. The key point is the destructive effect on gelatin of short-wave UV radiation with a wavelength of less than 270 nm. During the experiments, relief-phase holographic gratings with high diffraction efficiency up to a value of 42% were obtained.

Registration of high-frequency relief-phase structures on silver halide photographic material PFG-01

A new variant of manufacturing high-frequency holographic gratings with a spatial frequency up to 1200 mm-1 on silver halide photographic material for holography PFG-01 is proposed, the processing technology of which is tanning bleaching in bleach containing dichromates and etching in water or in solutions of glacial acetic acid in isopropyl alcohol. The key point is the destructive effect on gelatin of short-wave UV radiation with a wavelength of less than 270 nm. During the experiments, relief-phase holographic gratings with high diffraction efficiency up to a value of 42% were obtained. Keywords: holographic diffraction gratings, silver halide photoemulsions, PFG-01 photographic plates, short-wave UV radiation, surface relief, diffraction efficiency, glacial acetic acid, isopropyl alcohol.

Improvement of the technology for manufacturing relief holographic gratings on dichromated gelatin irradiated with short-wave UV radiation

A new variant of manufacturing high-frequency holographic gratings with a spatial frequency up to 1500 mm-1 on layers of dichromated gelatin (DCG), in the processing technology of which water procedures are completely excluded, is proposed. The method is based on the use of glacial acetic acid after the photodestruction of layers by exposure to short-wave UV radiation instead of the standard method of processing holographic structures registered by coherent He-Cd laser radiation. In the course of the experiments, relief-phase holographic gratings with high diffraction efficiency up to 64% were obtained, regardless of the thickness of the DCG layers. Keywords: holographic diffraction gratings, dichromated gelatin, short-wave UV radiation, surface relief, diffraction efficiency, glacial acetic acid, isopropanol.

Effect of Post Heat-treatment on Spectral Shifts Photopolymer Reflection Holograms and Its Use for Creating Color Holograms

The effect of post heat-treatment on the spectral properties of photopolymer reflection holograms scheme has been studied. It has been established that holograms with protective film retain a high diffraction efficiency of ~ 90% and do not have additional shrinkage during heat treatment for several days at a temperature of 80-90oC. The holograms without a protective film, removed after exposition, when heated in the temperature range of 40-110oC, show effective shrinkage up to 17% without compromising diffraction efficiency. Addiction the degree of shrinkage from the time of keeping the hologram at a given temperature is described by the function biexponential growth with saturation. The parameters of the approximating function are calculated. The characteristic shrinkage times in the temperature range of 40-110oC decrease from hours to minutes for fast components and from tens of days to tens of minutes for the slow component. Effect large shift of spectral response was used to create color-separated holograms. Color-separated holograms were obtained in two ways: two-layer version with shrinkage of one of the layers, single-layer version using photoresistive pattern and subsequent shrinkage. Keywords: reflection photopolymer holograms, thermal shrinkage, spectral shifts, color holograms

Time-Scheduled Exposure Method for Full-Color High Diffraction Efficiency and Uniformity of a Photopolymer

Time-Scheduled Exposure Method for Full-Color High Diffraction Efficiency and Uniformity of a Photopolymer

Влияние термической пост-обработки на спектральные сдвиги фотополимерных отражательных голограмм и её использование для получения цветоделённых изображений

The effect of post heat-treatment on the spectral properties of photopolymer reflection holograms scheme has been studied. It has been established that holograms with protective film retain a high diffraction efficiency of ∼ 90% and do not have additional shrinkage during heat treatment for several days at a temperature of 80–90°C. The holograms without a protective film, removed after exposition, when heated in the temperature range of 40–110°C, show effective shrinkage up to 17% without compromising diffraction efficiency. Addiction the degree of shrinkage from the time of keeping the hologram at a given temperature is described by the function biexponential growth with saturation. The parameters of the approximating function are calculated. The characteristic shrinkage times in the temperature range of 40–110°C decrease from hours to minutes for fast components and from tens of days to tens of minutes for the slow component. Effect large shift of spectral response was used to create color-separated holograms. Color-separated holograms were obtained in two ways: two-layer version with shrinkage of one of the layers, single-layer version using photoresistive pattern and subsequent shrinkage.

Dichromated gelatin, an efficient material for the fabrication of wavelength selective holographic solar concentrators for high-efficiency operation

The use of holographic concentrators for photovoltaic solar concentration is a very attractive proposition as the hologram can be designed to be wavelength selective i.e. it can be designed to disperse, diffract and concentrate specific spectral regions of the solar spectrum with maximum efficiency in a spectrum splitting solar cell system. The practical application of highly efficient holographic solar concentrators requires intensive as well as extensive research work on the choice of the appropriate recording material. Among the available materials for recording holographic concentrators, Dichromated gelatin (DCG) is considered to be one of the best materials because maximum diffraction efficiency and high value of modulation in refractive index (reported value is up to 0.08) can be achievable in DCG film. Also, DCG has high resolution and shows low scattering noise.In present paper theoretical study on refractive index modulation capability of dichromated gelatin (DCG) for same and different thickness of DCG film have been performed to design a holographic concentrator for high-efficiency wavelength selective performance. To validate the theoretical results, holographic concentrators have been recorded in DCG film with different values of index modulation, and the typically recorded holographic concentrators were reconstructed with different wavelengths of the laser source, and corresponding diffracted power is measured. A comparison between experimental results and theoretical results are presented. Experimental and theoretical study reveals that for a constant film thickness low value of modulation in the refractive index gives maximum efficiency in lower wavelength regions and high values of index modulation are desired for maximum efficiency in higher wavelength regions. Further for a constant value of index modulation maximum efficiency gets shifted towards higher wavelength regions with an increase in film thickness. Theoretically simulated curves show that by appropriate selection of index modulation and film thickness, holographic concentrator may be fabricated to use almost entire useful portion of the solar radiation with reasonably high diffraction efficiency.

Splitting Arbitrary Polarized Light with a Flat Dielectric Metagrating

The beam splitter is one of the basic optical components. Traditional beam splitters use bulky cubes or plates to realize beam splitting and thereby can hardly be used in a compact optical system. Recently, the beam splitter based on phase gradient metasurfaces has been proposed to split beam with a compact size. However, it is facing challenges on the efficiency and manufacturability. Herein, a beam splitter based on single‐sized flat dielectric metagrating for arbitrary polarizations is proposed. The diffraction efficiency of the fabricated beam splitter reaches as high as 80% for both TM and TE polarizations at peak wavelength. Moreover, it has a broad working wavelength range (455–550 nm) at visible wavelength where the diffraction efficiency is much higher than 60%. Apart from the broadband and high diffraction efficiency, the steering angle can be simply adjusted by varying the periodicity. Splitting light with a flat metagrating enriches the field of flat photonics, opening up new possibilities in applications such as augment reality displays, imaging systems, compact spectrometers, and lidars.

Stack-based grating for wideband polarization splitter in terahertz

A novel wideband terahertz polarization beam splitter with special diffraction orders working at terahertz band is described in this paper. The polarizer can achieve high diffraction efficiency and uniformity in the 2.50–2.56 THz band. Based on rigorous coupled-wave analysis (RCWA) and simulated annealing algorithm, we proposed an efficient algorithm to optimize the polarizer. After calculations, 98.45% single-port high-efficiency reflection for transverse electric (TE) polarization and 42.33%/42.57% highly uniform dual-port beam splitting for transverse magnetic (TM) polarization were finally obtained. In addition, through RCWA and simplified modal method, the electromagnetic field distributions of TE and TM polarizations are shown visually and described quantitatively. Moreover, the results displayed in section 3 prove that the grating possesses the characteristics of relatively large bandwidth and insensitivity to the incident angle. Therefore, the novel scheme in this paper has great reference value for the research of terahertz modulation devices and the integration of terahertz communication systems.

Low cost equipment for astronomical spectroscopy

The purpose of this paper is to show how we can obtain spectra for different astronomical objects using low coat equipment. Where a high-efficiency diffraction grating named “The Star Analyzer” was used by the International Astronomical Center (IAC) in Abu Dhabi, UAE to get the spectrum of different astronomical objects. Balmer series was readily visible when observing an “A” type star. TiO absorptions lines were distinguished by observing an “M” type star. Methane absorption lines were visible by observing Uranus and Neptune. Whereas HI and HeI emission lines were detected by observing a blue hypergiant. In addition, C2 Swan band absorption lines were identified by observing a red giant carbon star. This type of observation is very interesting for public outreach as well as university students, because it shows astrophysical principles for public and students practically and by using low cost equipment.

High-efficiency diffraction gratings for EUV and soft x-rays using spin-on-carbon underlayers

We report on the fabrication and characterization of high-resolution gratings with high efficiency in the extreme ultraviolet (EUV) and soft x-ray ranges using spin-on-carbon (SOC) underlayers. We demonstrate the fabrication of diffraction gratings down to 20 nm half-pitch (HP) on Si3N4 membranes with a bilayer of hydrogen silsesquioxane (HSQ) and spin-on-carbon and show their performance as a grating mask for extreme ultraviolet interference lithography (EUV-IL). High-resolution patterning of HSQ is possible only for thin films due to pattern collapse. The combination of this high-resolution resist with SOC circumvents this problem and enables the fabrication of high aspect ratio nanostructures. Rigorous coupled-wave analysis shows that the bilayer gratings exhibit higher diffraction efficiency than what is feasible with a grating made of HSQ. We also demonstrate a simple and accurate method to experimentally measure the diffraction efficiency of high-resolution gratings by measuring the relative ratio of the dose-to-clear curves of the photoresist. The measured diffraction efficiencies are in good agreement with the theoretically predicted values. Furthermore, we verify our calculations and measurements by printing line/space patterns in chemically amplified resists down to 10 nm HP with both HSQ and bilayer grating masks using EUV-IL. The improved diffraction efficiency of the bilayers is expected to have applications not only in gratings for interference lithography, but also in Fresnel zone plates and gratings for spectroscopy in the EUV and soft x-ray ranges.

2D scanning system of the acousto-optical deflector with high diffraction efficiency

The paper considers advantages and special features of an acousto-optic method for the development of a functional device for spatial and temporal control of laser light and reports results of investigations to create a two-directional acousto-optic laser scanner with diffraction efficiency as high as 80 percent or better. An acousto-optic deflector on a paratellurite crystal operating under the Bragg diffraction condition with a lithium niobate piezoelectric acoustic transducer was developed and tested experimentally. The results of this work can serve as a basis for the development of prototype functional acousto-optic devices for spatial and temporal control of laser light and their application to create laser television images.

High-efficiency broadband polarization-independent binary fused silica gratings for large astronomical telescopes.

The binary fused silica gratings (BFSGs) with high diffraction efficiency are presented for large astronomical ground-based telescopes. Calculations demonstrate that the BFSGs could obtain high diffraction efficiency in a wider wavelength range and angle of incident (AOI) range compared with volume phase holographic gratings. Several gratings with a size of 60mm×60mm have been fabricated by holographic lithography and reactive ion-beam etching technology. The measured peak diffraction efficiency reaches 94%, and results show that there are 130 nm wavelength bandwidth and 12° AOI bandwidth in which diffraction efficiency is higher than 70%. The stray light causes the diffraction efficiency to decrease by about 0.48%. All measurements have indicated good consistency with the simulation results.

Compact and lightweight optical see-through holographic near-eye display based on holographic lens

Optical see-through holographic near-eye display (NED) is one of the most promising augmented reality (AR) technologies for commercialization in the future. However, the conventional optical elements used in holographic NED make the optical display huge and heavy. In this paper, an optical see-through holographic NED with a compact and lightweight structure is proposed, in which the traditional optical elements are replaced by reflection-type holographic lenses (HLs). The HLs are thin and lightweight compared with the conventional optical lenses. We fabricate the HLs according to the reflection volume holographic grating theory. The HLs have high transmittance at Bragg-mismatched beam and high diffraction efficiency at Bragg-matched beam. In addition, we analyze the reason for the distortion of the holographic image caused by the HL and perform the distortion pre-compensation. In the experiment, an HL-based optical see-through holographic NED is developed, which is compact and lightweight. It can present clear holographic images at different focus depths without the vergence-accommodation conflict.

Optical vortex switch based on multiplexed volume gratings with high diffraction efficiency

Systems of controllable orbital angular momentum (OAM) require more compact, higher conversion efficiency and more tolerable wavelength or polarization. We introduce an optical vortex switch based on a multiplexed volume grating (MVG). The MVG recorded in a piece of photo-thermo-refractive (PTR) glass exhibits high diffraction efficiency (DE, also known as conversion efficiency in transporting), sensitive angular selectivity, and polarization-insensitivity. The effects of the incident divergence angle and polarization on the DE and the far-field diffraction profiles are demonstrated and investigated. It turns out that the divergence angle of the probe beam can greatly affect the DE. The fluctuation of the DE caused by polarization variation is less than 1.59%. This switch can be potentially applied in vortex tweezers, optical communication, and high power systems.

A Multi-Plane Augmented Reality Head-Up Display System Based on Volume Holographic Optical Elements With Large Area

The traditional head-up display (HUD) system has the disadvantages of a small area and a single display plane, here we propose and design an augmented reality (AR) HUD system with multi-plane, large area, high diffraction efficiency and a single picture generation unit (PGU) based on holographic optical elements (HOEs). Since volume HOEs have excellent angle selectivity and wavelength selectivity, HOEs of different wavelengths can be designed to display images in different planes. Experimental and simulated results verify the feasibility of this method. Experimental results show that the diffraction efficiencies of the red, green and blue HOEs are 75.2%, 73.1% and 67.5%. And the size of HOEs is 20 cm × 15 cm. Moreover, the three HOEs of red, green and blue display images at different depths of 150 cm, 500 cm and 1000 cm, respectively. In addition, the field of view (FOV) and eye-box (EB) of the system are 12° × 10° and 9.5 cm × 11.2 cm. Furthermore, the light transmittance of the system has reached 60%. It is believed that this technique can be applied to the augmented reality navigation display of vehicles and aviation.