High Diffraction Efficiency Research Articles

Computing Liquid‐Crystal Photonics Platform Enabled Wavefront Sensing

AbstractDetecting the wavefront of photons in an efficient and accurate way is crucial to retrieve the information of an optical field. Liquid crystal (LC) possesses strong ability to manipulate the phase and polarization of photons with high diffraction efficiency and therefore offers a promising avenue toward this aim. In this paper, LC performing analog optical processing is explored and a computing LC platform is proposed to implement the wavefront sensing via tuned bias retardation. It is demonstrated that precisely introducing a tiny Pancharatnam–Berry phase by LC can achieve flexible separation of two spin components with high resolution. Modulating the retardation by rotation of an analyzer, the computing LC platform is used to reconstruct wavefronts with phase‐contrast patterns. In particular, combining this scheme with microscopy can precisely retrieve a micro‐scale phase distribution. The result suggests that the computing LC platform can perform as a wavefront sensor with high efficiency and may open up new opportunities in the applications of precision metrology, real‐time analog optical processing, and high‐contrast microscopy.

Miniaturized two-channel broadband spectrometer based on variable-spacing concave blazed gratings

Recently, it becomes a tendency for cost-effective, portable spectrometers to have more applications from scientific research to daily life, e.g., in food safety and air pollution analysis. While most spectrometers utilize plane gratings, we demonstrate a more miniaturized, two-channel, broadband spectrometer based on variable-spacing concave gratings, combining the functionality of imaging optics and diffraction grating in one component. The added degree of design freedom in the micro-sized grating spacing further corrects most optical aberrations, thus the design achieves a tiny volume of <26 × 12 × 10 mm3 with a high spectral resolution. Simulation results show an optical resolution of <1.6 nm in the VIS-channel (400 to 790 nm) and <3.1 nm in the NIR-channel (760 to 1520 nm). The blazed structure of grating grooves provides a high overall diffraction efficiency in the whole spectral range, more than 50% on average. To further validate the feasibility for mass production, we successfully manufactured the variable-spacing concave gratings by using diamond tooling for fabricating the master mold and hot embossing for replication. Our fabricated variable-spacing grating replicas have a diffraction efficiency up to 70% in the VIS-channel and up to 60% in the NIR-channel. We built the prototype with fabricated concave gratings, and experimental results show a good match (error < 7 % ) in spectral resolution with the nominal design.

Two-dimensional polarization beam splitter based on cylindrical nano grating

In this paper, we report on a two-dimensional (2D) reflective grating with excellent polarization selectivity and high diffraction efficiency (DE). Based on silver cylindrical nano arrays, the grating is calculated by rigorous coupled wave analysis (RCWA). Optimized results show that the energy of transverse electric (TE) polarization is diffracted into (0, ±1) reflection orders and the energy of transverse magnetic (TM) polarization is diffracted into (±1, 0) reflection orders under normal incidence at 780 nm incident wavelength, with extinction ratio (ER) over 30 dB. To the best of our knowledge, this is the first time that a two-dimensional polarization beam splitter (2D-PBS) based on 2D grating is proposed, which opens up a new direction for the development of PBS. The holographic exposure technology, dry etching, and electron beam evaporation were used to fabricate the 2D-PBS. Preliminary experiments indicate that the 2D grating is sensitive to the polarization states of incident light and can be applied in the wide field of optical communication, high precision displacement measurement, and so on.

Off-Axis Polarization Volume Lens for Diffractive Waveguide

In augmented reality diffractive waveguide technology, the light field needs to be collimated before being transmitted into the diffractive waveguide. Conventional schemes usually require additional collimating optics to collimate the light from the micro-image source and guide it into the waveguide in-coupling elements. In order to meet the needs of head-mounted devices and further miniaturize the equipment, this paper proposes a waveguide device that combines collimation and coupling by using a reflective polarization volume lens (PVL). A related model is also established and simulated to calculate the diffraction and transmission characteristics of the PVL element, and is then improved to fit the experiment. The diffraction lens studied in this paper has high diffraction efficiency with a large off-axis angle, which can fold the optical path and reduce considerably the volume of the optical system when applied to the waveguide system.

Influence of defects on the femtosecond laser damage resistance of multilayer dielectric gratings.

Multilayer dielectric (MLD) gratings with high diffraction efficiency and a high laser-induced damage (LID) threshold for pulse compressors are key to scaling the peak and average power of chirped pulse amplification lasers. However, surface defects introduced by manufacturing, storage, and handling processes can reduce the LID resistance of MLD gratings and impact the laser output. The underlying mechanisms of such defect-initiated LID remain unclear, especially in the femtosecond regime. In this Letter, we model dynamic processes in interactions of a 20-fs near-infrared (NIR) laser pulse and a MLD grating design in the presence of cylindrically symmetrical nodules and particle contaminants and cracks at the surface. Utilizing a dynamic model based on a 2D finite difference in time domain (FDTD) field solver coupled with photoionization, electron collision, and refractive index modification, we study the simulation results for the damage site distribution initiated by defects of various types and sizes and its impact on the LID threshold of the grating design.

Customized Design of Aperiodic Liquid Crystal Grating for Generation of Multiple Optical Patterns

Liquid crystal (LC) grating has drawn extensive attention due to its merits, including tunability, easy preparation process, cost effectiveness, and high diffraction efficiency. However, limited efforts have been made to explore LC grating for the generation of customized diffraction patterns, despite the fact that the patterning size of LC molecular director orientation has been tremendously reduced even to subwavelength scale via high-precision photoalignment techniques. Here, we propose a versatile method based on the binary particle swarm optimization algorithm to efficiently design aperiodic LC gratings for customized optical patterns generation, including needle beam, multi-foci, and hollow beam. The optical performance of the LC gratings was simulated using both the vectorial Rayleigh–Sommerfeld method (with discrete zero or π phase, in this article) and the actual director orientations. These results matched each other well, which proves the effectiveness of the optimization method based on discrete phase. This study paves the way for designing LC gratings with engineered functionalities, which may find applications in optical tweezers, laser delivery systems, and laser scanning imaging.

Convex-shaped grating as a polarization-selective TE-five/TM-four splitting

A new reflective polarization-selective grating with high diffraction efficiency is proposed based on a convex-shaped configuration in this paper. The efficiencies of 0th order, -1st order and -2nd order are 21.61 %, 19.65 % and 18.38 % for transverse electric (TE) polarization and the efficiencies of -1st order and -2nd order are 24.28 % and 24.32% for transverse magnetic (TM) polarization. When the incident wavelength (λ) and the period (d) of grating are in the range of 1415–1556 nm and 3395–3610 nm, efficiency of each order for TE polarization can be achieved more than 18 %. For TM polarization, efficiencies above 20 % can be obtained in all orders with λ range in 1474–1655 nm and d range in 3211–3596 nm. Compared with other gratings, the proposed grating in this paper has better performance with the broad incident wavelength bandwidth and period tolerance.

Inverse design of polarization-insensitive C-band Dammann grating based on dielectric metasurface

Conventional Dammann gratings control the phase profiles of light by varying the etching depths of the DOEs, suffering from a contradiction between the fabrication complexity and the output performance. Due to the advantage of the single-step fabrication procedure, the ability to locally manipulate light at a subwavelength resolution, and the possibility of vertical integration, metasurfaces have attracted extensive interest. Here, we propose polarization-independent metasurface-based Dammann gratings that can redistribute a collimated light into high efficiency wide-angle spot arrays with desired intensity distributions. We combine a hybrid optimization algorithm with the finite-difference time-domain method to optimize the supercells of the metasurfaces. To illustrate the effective control over the power distributions among the desired diffraction orders, we first optimize metasurfaces with uniform intensity distributions. As an extension of the finding, we furthermore demonstrate metasurfaces with an intensity proportion of 1: 1.5: 2 and 1: 2. The diffraction efficiency errors are 4.67 %, 3.85 %, 1.7 %, and 9.16 % with an overall efficiency reaching 89.9 %, 85 %, 90.2 %, and 77.6 % in the four designed metasurfaces. The maximal diffraction angles for 3 × 3 and 5 × 5 diffractive spot arrays are 43.2° and 49.3° from the center. The bandwidth performance analysis demonstrates that the proposed metasurfaces show high diffraction performance in the C-band fiber telecommunications window (1530–1565 nm). Owing to the accurate modulation over intensity distributions, high diffraction efficiency, large diffraction angle, wideband characteristics, polarization-insensitive property, and single-step fabrication procedure with high etching depth tolerance, the proposed gratings may have potential applications in various regions, including optical information processing, beam shaping, and depth detection.

Dynamic Chiral Metasurfaces for Broadband Phase‐Gradient Holographic Displays

AbstractNext‐generation holographic displays have promising applications in medical science, augmented/virtual reality, smart security, data encryption, etc. Although metasurfaces emerged as the suitable choice to provide compact holographic displays, multifunctionality in metasurfaces at broadband optical wavelengths is inevitable for the abovementioned applications. Here, a metasurface is demonstrated based on chiral structures to introduce multifunctionality in terms of multiple wavefront information depending upon the polarization of incident light. The proposed metasurface integrated with a liquid crystal (LC) provides fast switching and dynamic optical response at broadband visible wavelengths in transmission mode. To avoid the phase distortion in multiple wavefront information embedded into a single planar metasurface, chiral z‐shaped meta‐atoms are used to provide high diffraction efficiency and phase chirality response for circularly polarized (CP) light illuminations. The phase mask of holographic information is encoded into the metasurface using a combination of dynamic and geometric phase modulation techniques. The experimental validation of the designed metasurface is performed to reproduce the spin‐dependent‐specific information at broadband visible wavelengths for changing the polarization of incident light. This research may pave the way toward designing highly efficient multifunctional metadevices to produce next‐generation holographic displays for promising applications in healthcare, media, smart security, and data encryption.

BaO–TeO2–MoO3 glass: excellent candidate for acousto-optic modulators with high diffraction efficiency, fast response, and stable operation

A novel tellurate glass with excellent acousto-optic properties has been prepared and can be used in the preparation of acousto-optic modulators with high diffraction efficiency and fast response.

Plasmon-enhanced parabolic nanostructures for broadband absorption in ultra-thin crystalline Si solar cells.

Sub-wavelength plasmonic light trapping nanostructures are promising candidates for achieving enhanced broadband absorption in ultra-thin silicon (Si) solar cells. In this work, we use finite-difference time-domain (FDTD) simulations to demonstrate the light harvesting properties of periodic and parabola shaped Si nanostructures, decorated with metallic gold (Au) nanoparticles (NPs). The active medium of absorption is a 2 μm thick crystalline-silicon (c-Si), on top of which the parabolic nanotextures couple incident sunlight into guided modes. The parabola shape provides a graded refractive index profile and high diffraction efficiencies at higher order modes leading to excellent antireflection effects. The Au NPs scatter light into the Si layer and offer strong localized surface plasmon resonance (LSPR) resulting in broadband absorption with high conversion efficiency. For wavelengths (λ) ranging between 300 nm and 1600 nm, the structure is optimized for maximum absorption by adjusting the geometry and periodicity of the nanostructures and the size of the Au NPs. For parabola coated with 40 nm Au NPs, the average absorption enhancements are 7% (between λ = 300 nm and 1600 nm) and 28% (between λ = 800 nm and 1600 nm) when compared with bare parabola. Furthermore, device simulations show that the proposed solar cell can achieve a power conversion efficiency (PCE) as high as 21.39%, paving the way for the next generation of highly efficient, ultra-thin and low-cost Si solar cells.

Augmented reality system based on full-color holographic optical elements lens

Holographic optical element (HOE) lens is an imaging element fabricated through recording wavefront by interference. Because of its advantages of small form factor and wavelength, angle selectivity and arbitrary wavefront formation, it has a good application prospect in augmented reality display. To make the system more compact, the HOE lens is adopted as an off-axis optical element. At the same time, according to diffraction principle, its wavelength response is more sensitive than those of traditional refractive and reflective optical elements. Thus the fabrication and design of a full-color HOE lens is a challenge to optimizing the free-space head-up display system. To systematically analyze the HOE imaging system, the conjugate relation between the object and image is derived by scalar diffraction theory. Then the Gaussian conjugate imaging equation is obtained and the off-axis aberration of distortion and astigmatism in the HOE imaging system are analyzed. In addition, A head-up display with field of view (FOV) of 18° and eyebox of 10 mm is simulated and its imaging process is visualized through the geometric optics method of &lt;i&gt; &lt;b&gt;k&lt;/b&gt; &lt;/i&gt;-vector diagram and ray-tracing. A full-color HOE lens with high diffraction efficiency is fabricated by interference. Its average peak diffraction efficiency is 56.7%, reaching a high level in the world. A prototype of augmented reality system is established by integrating laser pico-projectior with HOE lens. The experimental results of distortion effect and astigmatism effect of the system are obtained, which are consistent with the simulation results. The modulation transfer function (MTF) parameter of the system is measured, and its definition basically meets the requirements of the human eyes for resolution. The aberration of the system is analyzed and the optimization method is proposed. To optimize the monochromatic image quality, an extra cylindrical lens is added to ensure the same optical power of meridian and sagittal plane to eliminate the astigmatism. Besides, a freeform wavefront is designed by the geometric construction method and forms a freeform HOE to deal with the distortion problem. The local recording freeform wavefront can be calculated by the imaging equation. When full-color HOE is applied to the display system, the images of three channels may separate in the space because of their different reconstruction wavelengths and angles. We propose a pre-compensation method of recording process to solve this problem. If these above-mentioned problems can be solved, due to its good image uniformity, sufficient field angle and eyebox area, the head-up display based on HOE lens with extra optical power will have a better application in augmented reality technology.

Design of Total Internal Reflection Immersed Gratings with High Diffraction Efficiency and Low Polarization Sensitivity

Design of Total Internal Reflection Immersed Gratings with High Diffraction Efficiency and Low Polarization Sensitivity

Fabrication error analysis of nonperiodic-multilayer-dielectric gratings

We analyzed fabrication errors of nonperiodic-multilayer-dielectric gratings that were designed to have a high reflective diffraction efficiency for one wavelength and a high transmittance for another wavelength at the same time. Significant deviations were found between the measured and calculated efficiency values although the groove profile parameters were very closed to the design values. The source of error was attributed to coating errors of the film stack. To explain the deviation well, we estimated the parameters of actual coating stack by reverse calculations and using scanning electron microscope. We recalculated the diffraction efficiencies and the results showed that the actual film stack parameters were inverted well. This provided a foundation for us to reoptimize grating groove parameters on the basis of actual coating stack.

Photorefraction of uranium-doped lithium niobate crystals at multiple visible wavelengths

Holographic storage with fast response speed, high saturation diffraction efficiency and high sensitivity are realized in LN:U at multiple visible wavelengths.

Adaptive mixed-constraint Gerchberg-Saxton algorithm for phase-only holographic display

At present, spatial light modulators are incapable of modulating both the amplitude and phase of the wavefront simultaneously. Therefore, when a spatial light modulator is used for holographic display, it is necessary to encode the complex amplitude of the object wave into amplitude-only or phase-only computer-generated-hologram. The phase-only holographic display has attracted much attention of researchers due to its characteristics of high diffraction efficiency and no conjugate image. However, current optimization algorithms for generating phase-only hologram have the problems of iterative divergence, slow convergence speed, and poor reconstruction quality, which is difficult to satisfy the requirements for high-quality holographic display. In this work, we propose an accurate adaptive mixed constraint Gerchberg-Saxton algorithm by constraining the frequency bandwidth in the hologram plane and adaptively constraining the amplitude of the reconstructed image in the image plane based on the angular spectrum propagation theory. Firstly, we use the angular spectrum propagation model without paraxial approximation to simulate the forward and backward propagation of the light wave for ensuring the accuracy of the wavefront propagation. Secondly, dividing the image plane into signal area and noise area according to the spatial distribution of target image, and different adaptive feedback strategies are set up for the two regions based on the optimized effect of the phase-only hologram. The adaptive feedback strategy is established, which can accelerate the convergence speed of the proposed algorithm and enhance the hologram of freedom of the optimization. Finally, the frequency bandwidth constraint strategy is introduced in the hologram plane to optimize the edge pixels and compensate for the frequency information loss of the phase-only computer-generated hologram, which improves the reconstruction quality of the hologram. After 100 iterations, the average correlation coefficient of the holographic reconstructed image of the proposed algorithm is about 0.9857, and the average peak signal-to-noise ratio is about 31.02 dB. The correlation coefficient and the peak signal-to-noise ratio of the reconstructed images of the proposed algorithm are better than those of the Gerchberg-Saxton algorithm with only frequency bandwidth constraint strategy, and the proposed algorithm has clearer texture and better display effect. The results of numerical simulations and optical experiments show the feasibility and effectiveness of the proposed method. The proposed adaptive mixed constraint Gerchberg-Saxton algorithm is a promising technology for high-quality holographic display.

Polarized Phase Holograms of High Diffraction Efficiency

Polarized Phase Holograms of High Diffraction Efficiency

Internal mechanism of perfect-reflector-backed dielectric gratings to achieve high diffraction efficiency

The work started 20 years ago [Appl. Opt. 42, 6255 (2003)] investigating the physical mechanism of multilayer dielectric reflection gratings to achieve 100% diffraction efficiency is extended to offer much deeper insight than before. How different diffraction amplitudes of the top surface corrugation contribute to the −1st-order efficiency of such a grating is shown analytically using a minimum set of real parameters. The two diffraction amplitudes transmitted through the corrugation play a dominant role in enabling DE = 100%. The necessary and sufficient condition for 100% efficiency is derived, and a very simple sufficient condition is also given. Moreover, the role of the reflection phase of the perfect-reflector, including the contribution due to optical path between the corrugation and the reflector, is emphasized.

High-efficiency broadband blazed metagrating working in visible light

A simple 1D blazed metagrating is proposed. The metagrating consists of SiO2 film sandwiched by Ag substrate and Ag nanostrips, which can achieve high-efficiency −1st-order diffraction in the range of 550 nm to 700 nm, and the peak efficiency is nearly 98%. The SiO2 dielectric layer in previous designs is used chiefly as a waveguide layer to support a guided mode. In comparison, it is introduced here to suppress the unwanted diffraction order (zero-order), which helps achieve high-efficiency diffraction at a high diffraction order. For analysis, the metagrating is disassembled into two parts, including a flat plate and a grating. By analysing the far-field radiation pattern of scattered waves and the reflection phase of a specific mode for these two parts, we conclude that the cause of high-efficiency blazing draws support from suppressing zero-order based on destructive interference. This work provides an intuitive physical image for this type of metagrating and an idea to design high-efficiency diffraction and beam deflection devices from the perspective of interference.

All-optical diffraction and ultrafast switching in a terahertz-driven quantized graphene system

Over the past few years, we have witnessed a substantial evolution in graphene optoelectronics. The tunability of the electronic structure of graphene has led to the next generation of carbon-based nano-photonic devices. Motivated by the unique optical properties of graphene, in the present work, we show that the landau-quantized graphene can be regarded as a platform for implementing a 2D laser-induced grating with high diffraction efficiency in the far-field regime. We consider a 2D doped graphene monolayer with a four-level system in Λ-configuration driven by a terahertz field. The electromagnetically induced Fraunhofer diffraction patterns are simulated for a weak probe field passing through the graphene nano-sheet. The optimum conditions for the maximum diffraction efficiency are investigated. Then, it is shown that the probe beam propagation can be coherently controlled via applying a terahertz field. The possibility of ultrafast all-optical switching of the probe beam is investigated. The proposed system may find applications in integrated nano-photonic devices for optical communications, infrared spectroscopy, astronomy, optical sensing, and also the realization of all-optical switching processes.