Blazed Gratings Research Articles

Holographically fabricated out-of-plane blazed gratings and channel waveguides in silica for integrated free-space beam delivery

Grating couplers are widely used in integrated optics to generate free-space beams and facilitate localized interactions with systems such as atom or ion traps. However, etched devices often exhibit small-scale inconsistencies; exacerbated by the high index contrast of the devices, this can lead to phase errors, limiting devices to a sub-millimeter scale. Here we present the first demonstration, to our knowledge, of tilted, out-of-plane blazed gratings in planar silica fabricated by UV inscription using a 213 nm laser. Our devices deliver collimated and focusing beams into free space from a waveguide input without the need for additional optics such as beam expanders.

Emitter design for efficient waveguide spectral lens

Abstract As an alternative to arrayed waveguide gratings (AWGs), the waveguide spectral lens (WSL) stands out with the ability to focus light in free space, thereby eliminating the need for relay optics between the chip and the camera. This becomes convenient in constructing a truly compact instrument for astronomical spectroscopic analysis. Besides dispersion and focusing, WSL offers another important function: the envelope of the diffraction orders can be manipulated via the output emitter, i.e., the waveguide array at the facet. Through careful emitter design, the diffraction efficiency can be largely improved because the side orders are well suppressed, and light is concentrated to the selected order. This feature, though particularly important for the photon-hungry astronomical application, has not been well explored in the previous works. Here, we come up with four emitter designs and evaluate their performance, including linear taper, parabolic taper, multimode interference (MMI), and slot MMI. A figure of merit (FoM) considering both diffraction efficiency and uniformity is introduced to identify the optimal structure. Experimental results agree well with the simulation and confirmed that the optimal parabolic taper can achieve a diffraction efficiency of 90.9%, making it the most attractive design. This work highlights the potential of WSLs for astronomical spectroscopy with an efficiency that rivals conventional blazed gratings. It may also inspire emitter designs for side-lobe suppression in optical phase array applications.

Performance improved for two-photon multi-focus microscopy based on a spatial light modulator by eliminating the zero-order beam

Two-photon microscopy (TPM) has a wide range of applications in the biomedical field. Two-photon multi-focus microscopy (TPMM) greatly improves the imaging speed by combining TPM with multi-focus technology. Therefore, TPMM based on spatial light modulator (SLM) has greater advantages in generating multi-focus point (MFP) with uniform intensity and flexible position than to other schemes. However, the zero-order beam (ZOB) produced by SLM in TPMM causes imperfection of the imaging system. For example, some sample areas are scanned repeatedly or illuminated unexpectedly. In this article, we proposed a TPMM system with improved performance by eliminating the ZOB. Apart from the target MFP, we utilize a phase-only SLM to generate another corrective beam (CB) with controllable amplitude, phase, and position precisely. The CB can produce destructive interference completely with the ZOB generated by the dead areas of the SLM. The method has a larger field of view, higher efficiency, and better uniformity in generating MFP compared to the method of overlaying the blaze grating. Simulations and experiments demonstrate the advantages of this approach.

Topology optimization of blazed gratings under conical incidence

A topology optimization method is presented and applied to a blazed diffraction grating in reflection under conical incidence. This type of grating is meant to disperse the incident light on one particular diffraction order, and this property is fundamental in spectroscopy. Conventionally, a blazed metallic grating is made of a sawtooth profile designed to work with the ±1st diffraction order in reflection. In this paper, we question this intuitive triangular pattern and look for optimal opto-geometric characteristics using topology optimization based on finite element modelling of Maxwell’s equations. In practical contexts, the grating geometry is mono-periodic, but it is enlightened by a 3D plane wave with a wave vector outside of the plane of invariance. Consequently, this study deals with the resolution of direct and inverse problems using the finite element method in this intermediate state between 2D and 3D: the so-called conical incidence. A multi-wavelength objective is used in order to obtain a broadband blazed effect. Finally, several numerical experiments are detailed. Our numerical results show that it is possible to reach a 98% diffraction efficiency on the −1st diffraction order if the optimization is performed on a single wavelength, and that the reflection integrated over the [400,1500] nm wavelength range can be 29% higher in absolute terms, 56% in relative terms, than that of the sawtooth blazed grating when using a multi-wavelength optimization criterion (from 52% to 81%).

High-speed wafer surface defect detection with edge enhancement via optical spatial filtering in serial time-encoded imaging

With the rapid development of the chip and metasurface industries, high-speed defect detection of wafer during semiconductor batch manufacturing has become increasingly important for yield control. Here, we propose a high-speed, macro-scale wafer surface defect detection method with enhanced image edges, via optical spatial filtering in serial time-encoded imaging. By using blazed gratings to spatially project different wavelength components of pulsed light into a one-dimensional line, together with dispersive Fourier time stretching technology, a high-speed serial time-encoded imaging system based on a single pixel detector is constructed for testing the wafer surface defects, and optical spatial filtering is introduced to improve the imaging quality. Experiments show high-pass spatial filtering effectively restrains environmental interference and enhances image edge detection. Small target detection is achieved without a complex image algorithm. Our solution extends optical spatial filtering to serial time-encoded imaging, not only permitting wafer macro scale defect detection with edge enhancement, but also having potential significance for imaging lidar, imaging spectrometers, ghost imaging technology, and more.

Eyebox expansion of a lensless near-eye display using diverging spherical wave illumination and a multiplexed holographic optical element

We expand the eyebox size of a lensless holographic near-eye-display (NED) using passive eyebox replication technique that incorporates a spatial light modulator (SLM) and a holographic optical element (HOE). In holographic NEDs, the space-bandwidth product (SBP) of the SLM determines the exit pupil dimensions and corresponding eyebox size. The base eyebox size is replicated in the horizontal direction by using the horizontal high-order diffractions of an SLM under spherical wave illumination and a multiplexed HOE combiner. When a digital blazed grating and a digital lens phase are added to the computed phase hologram sent to the SLM, two spatially separated, horizontal high-order diffraction terms with identical intensity and information can be uniformly formed without the zero-order term. When the eyebox size is expanded, the field-of-view (FOV) is not sacrificed; spherical divergence wave illumination alleviates the need for a tradeoff between FOV and eyebox size. The astigmatism introduced during HOE fabrication is counterbalanced by pre-correcting the target image using a computer-generated, holographic computation algorithm. The prototype system shows simple and effective, distortion-free eyebox expansion of a wide-angle, lensless, holographic NED.

Simple strategy for the simulation of axially symmetric large-area metasurfaces

Metalenses are composed of nanostructures for focusing light and have been widely explored in many exciting applications. However, their expanding dimensions pose simulation challenges. We propose a method to simulate metalenses in a timely manner using vectorial wave and ray tracing models. We sample the metalens’s radial phase gradient and locally approximate the phase profile by a linear phase response. Each sampling point is modeled as a binary blazed grating, employing the chosen nanostructure, to build a transfer function set. The metalens transmission or reflection is then obtained by applying the corresponding transfer function to the incoming field on the regions surrounding each sampling point. Fourier optics is used to calculate the scattered fields under arbitrary illumination for the vectorial wave method, and a Monte Carlo algorithm is used in the ray tracing formalism. We validated our method against finite-difference time domain simulations at 632 nm, and we were able to simulate metalenses larger than 3000 wavelengths in diameter on a personal computer.

High quality holographic 3D display with enhanced focus cues based on multiple directional light reconstruction.

Holographic display faces the trade-off between image quality and focus cues, resulting from the specific choice of phase distribution. In this Letter, we propose a speckle-free holographic display with enhanced focus cues by multiple directional light reconstruction. The uniform phase hologram is first generated by the gradient descent optimization algorithm. The blazed grating phase is used to steer the object light to a specific direction. Multiple sub-holograms with different blazed gratings are refreshed fast to reconstruct the images from different directions. Thus, the defocus blur is improved due to the separation of multiple spots on the defocus plane. The multi-plane reconstruction is also realized by pre-processing the depth images to eliminate image separation. The proposed method provides apparent focus cues while maintaining high image qualities, which is expected to realize comfortable holographic near-eye display in the near future.

Fabrication of blazed gratings by tilted reactive ion beam etching with the side mask for augmented reality applications

A novel manufacturing method using the side mask and tilted reactive ion beam etching (RIBE) is proposed for the fabrication of blazed gratings. Electron beam lithography was carried out to pattern a groove with a nanoscale line width, and then appropriate mask materials were filled into the SiO2 trench by atomic layer deposition. After being etched by RIBE at specific tilted angles, the required blazed gratings were achieved. In contrast to the typical fabrication process with a patterned mask on top of the surface to be etched, V-shaped nano structures filled into the trenches were used as the side mask. During etching, the surface material located in the shadow of the side mask along incident ion beams was not etched. The depth and blazed angle of blazed gratings are determined by the height of the side mask and the mounting angle of the sample, respectively. In addition, the fabricated blazed gratings can be used as imprinting moulds to duplicate blazed gratings for augmented reality applications. Due to more options for side mask materials, this method is able to provide high repeatability and accurate controllability for fabricating blazed gratings.

Simulation study of three-dimensional grayscale ice lithography on amorphous solid water for blazed gratings

Electron beam lithography (EBL) on amorphous solid water (ASW), termed as ice lithography (IL), has demonstrated promising capability in pattern transfer with unique advantages such as reduced proximity effect. So far, ice lithography for binary patterning has been proved a great success, however, application for three-dimensional (3D) profiling in nanoscale has still not been addressed to the best of our knowledge. This paper reports, for the first time, our progress in simulating study of three-dimensional ice lithography on ASW for linear blazed gratings, aiming to overcome the difficulty in replicating high quality blazed gratings with high diffraction efficiency. Systematic simulation of grayscale ice lithography for 3-D blazed grating templates with desired surface quality as the task was carried out, using Monte Carlo algorithm based on the measured contrast curves of ASW. For comparison, grayscale electron beam lithography on PMMA was also performed. The resultant profiles of blazed wavelengths around 1550 nm by grayscale IL show less flaws and higher diffraction efficiencies than by EBL. The successful simulation of 3D grayscale IL provides us with instructive guide for the fabrication of 3D nanostructures as a whole through the grayscale ice lithography on ASW.

Blazed Diffraction Gratings on Si—First Results

Blazed Diffraction Gratings on Si—First Results

Terahertz focusing blazed diffractive optical elements for frequency demultiplexing

This study presents the novel optical passive components for spatial frequency division demultiplexing of terahertz (THz) radiation. Four different diffractive optical elements (DOEs) were designed as the combination of phase kinoform lenses and phase blazed diffraction gratings. The designed structures were verified in numerical simulations and they showed the promising results. Subsequently, they were manufactured using fused deposition modeling (FDM) 3D printing technology from highly transparent cyclic olefin copolymer (COC). The manufactured structures were examined in the experimental setup. The results matched numerical simulations. Thus, eight frequencies in the range from 150 GHz to 220 GHz every 10 GHz were spatially separated. The novel design solution guaranteed 63% higher relative efficiency compared to the reference DOE. The presented study can be suitable as the application for 6G technology telecommunication systems as the spatial frequency division demultiplexing component for the THz radiation band.

Nanometer flat blazed x-ray gratings using ion beam figure correction.

With the development of nanometer accuracy stitching interferometry, ion beam figuring (IBF) of x-ray mirrors can now be achieved with unprecedented performance. However, the process of producing x-ray diffraction gratings on these surfaces may degrade the figure quality due to process errors introduced during the ruling of the grating grooves. To address this challenge, we have investigated the post-production correction of gratings using IBF, where stitching interferometry is used to provide in-process feedback. A concern with ion beam correction in this case is that ions will induce enough surface mobility of atoms to cause smoothing of the grating structure and degradation of diffraction efficiency. In this study we found however that it is possible to achieve a nanometer-level planarity of the global grating surface with IBF, while preserving the grating structure. The preservation was so good, that we could not detect a change in the diffraction efficiency after ion beam correction. This is of major importance in achieving ultra-high spectral resolution, and the preservation of brightness for coherent x-ray beams.

A correction for higher-order refraction in cathodoluminescence spectrometry

Cathodoluminescence (CL) is a developing analytical method in electron microscopy, because of its excellent energy resolution. Usually a Czerny–Turner type spectrometer is employed, having a blazed grating as analyzer. Unlike a prism analyzer, where the dispersion depends on the refractive index of the prism itself leading to a non-linear spectral distribution, the grating has the advantage that the spectral distribution depends linearly on the wavelength. As a draw-back, higher-order refraction alters the measured optical spectrum at larger wavelengths. In general, blazed gratings are used in order to minimize this effect in a certain spectral range. Nevertheless, the higher-order intensities can be still significant. In the present study we present a method for correcting the acquired optical spectra with respect to higher order diffraction intensities and apply it to CaO and GaN CL-spectra.

Design and Study of a Reflector-Separated Light Dispersion-Compensated 3D Microscopy System.

The secondary-phase grating-based tomographic microscopy system, which is widely used in the biological and life sciences, can observe all the sample multilayer image information simultaneously because it has multifocal points. However, chromatic aberration exists in the grating diffraction, which seriously affects the observation of the image. To correct the chromatic aberration of the tomographic microscope system, this paper proposes a system that adopts blazed gratings and angle-variable reflectors as chromatic aberration correction devices according to the principle of dispersion compensation and Fourier phase-shift theory. A reflector-separated light dispersion-compensated 3D microscopy system is presented to achieve chromatic aberration correction while solving the problem of multilayer image overlap. The theoretical verification and optical design of the system were completed using ZEMAX software. The results show that the proposed system reduced the chromatic aberration of ordinary tomographic microscopy systems by more than 90%, retaining more wavelengths of light information. In addition, the system had a relatively wide range in the color difference compensation element installation position, reducing the difficulty of dispersion compensation element installation. Overall, the results indicate that the proposed system is effective in reducing chromatic aberration in grating diffraction.

New theoretical model of the evolution of blazed gratings in the ion beam etching process

Blazed gratings play a key role in advanced fields such as metaverse, AR and VR, etc. A good triangular cross section morphology is critical for its performance and applications. To investigate how triangular blazed gratings are evolved from rectangular masks, blazed gratings were fabricated by ion beam etching process. A new theoretical model called six-surface intermediate (SSI) model is proposed to explain the morphological evolution from rectangular homogeneous masks to triangular blazed gratings. The actual morphologies of blazed gratings with different process parameters were characterized by scanning electron microscopy. These observations confirm the correctness of the new model. Our research is of important guiding significance for the fabrication of blazed gratings with controllable morphology.

Time-varying gradient metasurface with applications in all-optical beam steering

Abstract Integrating the large, subpicosecond nonlinear optical response of epsilon-near-zero (ENZ) materials with the broad design freedoms of plasmonic metasurfaces shows potential for creating rapidly modulated optical devices with possible applications in telecommunications, sensing, and reactive beam steering. In this work, we experimentally investigate a metasurface consisting of a plasmonic gradient array on a thin layer of indium tin oxide (ITO), characterize how incident probe pulses diffract from a system as it is being dynamically modulated by a pump pulse at wavelengths near the ENZ region. Angular shifts in the diffraction orders are observed and can be principally attributed to the adiabatic wavelength conversion of the probe as it witnesses the temporal change of index induced by the pump. Of note, the asymmetric gradient metasurface, considered to be a blazed diffraction grating, shows significantly different dynamic responses for different diffraction orders. The free-space wavelength shift to +1 and −1 diffraction orders is 6 and 12 nm, resulting in steering angle changes of 0.65 and 1.5°, respectively.

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.

High-precision and high-efficiency fabrication of blazed grating by ultrasonic-assisted ultraprecision planing

Blazed gratings are optical components typically employed in modern optical engineering. To realize high-precision and high-efficiency fabrication of blazed gratings on reliable materials, an ultrasonic-assisted ultraprecision planing (UAUP) method using a round-nose diamond tool under elliptical vibration mode is proposed in this paper. High-phosphorus electroless nickel-phosphorus plating was adopted as the grating blank. The generation mechanism of the blazed grating texture was studied, and the relationship between the processing parameters and blazed grating characteristic dimensions was established. Through a combination of simulation and experiment, the theoretical and experimental results of the morphologies and optical performances of the blazed gratings were systematically compared and analyzed. The results show that the blazed grating configuration is an envelope of the diamond tool profile under ultrasonic vibration; the UAUP method has an extremely high machining efficiency owing to the high vibration frequency; and the blazed gratings fabricated using smaller vibration amplitudes exhibit high machining quality, good shape consistency, and favorable optical performance, thus verifying the feasibility of the UAUP method. • An ultrasonic-assisted ultraprecision planing (UAUP) method is proposed. • The grating configuration is precisely controlled by the processing parameters. • The machining efficiency of blazed gratings is extremely improved. • The fabricated blazed gratings exhibit favorable optical performance.

Ultra-precision Cutting Characteristics of Convex Blazed Gratings with Electroless NiP

Ultra-precision Cutting Characteristics of Convex Blazed Gratings with Electroless NiP