Uniform Diffraction Efficiency Research Articles

Equalization of RGB coupling efficiencies of metasurface waveguide coupler by adjusting imaginary part of refractive index

Optical metasurfaces offer high-efficiency and flexible wavefront shaping for near-eye displays, especially in wideband waveguide couplers accommodating RGB primary colors. By leveraging the resonance characteristics of sub-wavelength periodic nanostructures, metasurfaces surpass the limitations of traditional optics that rely on multiple components and mediums. In this study, we propose adjustment of the imaginary parts of the material refractive indices as a new method to achieve balanced first-order diffraction efficiencies among RGB colors over a wide field of view (FOV) in an in-coupling metasurface waveguide coupler. Physical mechanism is investigated deeply and systematically in theory. It is found that nanostructure resonances deflect the wavefront and Poynting vector, significantly enhancing first-order diffraction efficiency, while resonance-enhanced absorption plays a crucial role in balancing the diffraction efficiency of RGB primary colors. First experimental demonstration well confirms the practical feasibility of this method and a uniform first-order diffraction efficiency of approximately 20% is achieved among RGB colors across a FOV as large as ∼30° over a single-piece glass substrate. This research provides insights into the design and mechanisms of metasurface waveguide couplers, advancing our understanding of metasurface-based RGB displays and facilitating further advancements in this field.

Chromatic aberration measurement of liquid crystal Pancharatnam Berry phase lens by a RGB full-Stokes imaging polarimeter

Pancharatnam Berry (PB) phase optical elements can manipulate local polarization state of light in the cross-sectional plane by addressing an additional PB phase relating to the orientation of the optical axis of the waveplate-like structure. However, chromatic aberration including spectrum-dependent retardation and diffraction efficiency limits their applications in the field of visible broadband imaging and display. In this article, the chromatic aberration of a liquid crystal (LC) PB phase lens was measured by a home-built RGB full-Stokes imaging polarimeter. The chirality conversion efficiency and depolarization at RGB wavelengths can be calculated from the measured Stokes vectors. Our experiments show that even the spectrum-dependent retardation exists, the measured PB phase lens has a uniform and high diffraction efficiency in the visible band. Affected by the local LC molecule, the input polarization of RGB light was rotated in varying degrees, but the focusing or defocusing manipulation to the right- or left-hand circular polarization, originated from the PB phase, results in fixed focal lengths for different colors. Our experiments show that the LC-based PB phase lens is suitable for the visible broadband applications.

Structural and optical properties of triple-layer for high-uniformity beam division

In this paper, a triple-layer grating beam splitter with three-port output is designed. Through the combination of rigorous coupled-wave analysis (RCWA) and simulated annealing algorithms (SAA), the calculation and optimization of each parameter of the grating is carried out to make the diffraction efficiency of the grating meet the conditions. At the incident wavelength of 800[Formula: see text]nm, the diffraction efficiencies of 0th order and ±1st order under TE polarization are 31.6% and 31.4%, respectively. The diffraction efficiencies of 0th order and ±1st order under TM polarization are 32.7% and 32.5%, respectively. It can be seen that not only the total diffraction efficiency under TE polarization and TM polarization exceeds 90%, but also the uniformity of diffraction efficiency at each order exceeds 99%. Finally, the influence of grating parameters and incident wave characteristics on diffraction efficiency is discussed.

A new scheme for seven-port division via reflective arrays on a connecting slab

A seven-port division is described via the connecting-slab-based reflective grating in this paper. The rigorous coupled wave analysis (RCWA) and simulated annealing algorithm (SAA) are chosen to analyze and optimize the performance of the grating. The total diffraction efficiency of TE and TM polarization is 96.75% and 95.53%, respectively. Besides, the target grating has good uniformity errors, which are reduced to 3.69% and 1.76% for TE and TM polarization, respectively. The modal method provides a physical explanation for the energy transformations within the target grating. Furthermore, the manufacturing tolerance is discussed for the seven-port division grating. Through analysis and discussion, various performances of the grating can be better demonstrated. Compared with most seven-port grating splitters reported previously, the grating in this paper achieves better diffraction efficiency uniformity and higher efficiency.

A large-size and polarization-independent two dimensional grating fabricated by scanned reactive-ion-beam etching

Abstract Scanned reactive-ion-beam etching method was proposed to transfer two-dimensional mask patterns into quartz substrate, which would produce a larger-size and polarization-independent two-dimensional grating. This method was realized by moving grating substrate in a unidimensional scanning manner and adjusting ion beam density in the vertical scanning direction. Graphite plates between the ion beam source and the substrate were used to correct the beam density. The original Gaussian ion beam density was changed to a uniform distribution to establish a knife-edge shape around the vertical scanning direction. Therefore, a large-area pattern with consistent depth and duty cycle would be engraved into a quartz substrate. A two-dimensional, 1200 groves/mm grating with an 85-mm × 85-mm area was fabricated under scanned reactive-ion-beam etching method and exhibited a 0.197λ (λ = 632.8 nm) diffraction wave front. At 780 nm, the efficiency nonuniformity was less than 9%, and the average diffraction efficiencies of transverse-magnetic and transverse-electric polarized light were 57.2 and 58.0%, respectively. The large-size two-dimensional grating with uniform diffraction efficiency and polarization independence enabled grating displacement measurement with high resolution, long measurement range, multiple degrees of freedom, and potential miniaturization.

Two‐Dimensional Angular Bandwidth Broadening of Metasurface Grating

Waveguide‐type near‐eye displays (NED) have attracted increasing attention in the market of augmented reality (AR) and virtual reality (VR) devices, thanks to their outstanding performance within ultra‐compact size. The performance of waveguide‐type NED mainly depends on the waveguide coupler that transmits the incident image to the user's eye through the total internal reflection (TIR) inside the waveguide. However, the waveguide couplers, that have been widely used, exhibit nonuniform diffraction efficiency within the transferable angular range, i.e., field of view (FoV) of waveguide‐type NED, so that the image intensity distribution distortion is induced. Thus, it is needed that many researchers focus on broadening angular bandwidth to overcome the non‐uniformity of the diffraction efficiency. Herein, the 2D angular bandwidth broadening metasurface grating that exhibits uniform and high diffraction efficiency for the 2D FoV range is proposed. Specifically, the proposed device is designed specifically, and the diffraction efficiency of this device shows uniform efficiency of about 75% with a variance of 0.25% for the FoV range, and it is demonstrated both numerically and experimentally. Consequently, it is expected that the proposed device can be a game‐changer in the market for waveguide‐type NED for the enhancement of image uniformity.

Time-scheduled exposure method for full-color high diffraction efficiency and uniformity of a photopolymer

A full-color reflection holographic optical element (HOE) with uniform diffraction efficiency, achieved via time-scheduled exposure, is proposed in which 660, 532, and 473 nm wavelengths are used for recording. To enhance the diffraction efficiency, the exposure energy of three wavelengths should be controlled to react uniformly in the photopolymer. The photopolymer’s optical characteristics present that the absorptance of the three wavelengths differs, moreover, the reactivity of the photopolymer to exposures is variant at the three wavelengths. The high-efficiency full-color HOE is implemented by determining the pre-exposure times of the three lasers for a set beam intensity. In order to verify the proposed method, we conduct repetitive optical experiments and analyze the results. Finally, a full-color reflection hologram is manufactured with an average efficiency of 61.6 % (standard deviation, 0.5) when the beam intensity of the three lasers is 0.4 mW/cm2. The reconstructed images of the full-color hologram show good uniformity and color reproduction.

Fabrication of integrated spectrometer module based on optical waveguide platform with planar nano diffraction grating using UV imprint lithography

We proposed a simple and attractive integrated polymeric spectrometer module based on a planar optical waveguide platform that can be easily manufactured at a low cost. The proposed integrated spectrometer module comprises a planar optical waveguide with a concave mirror, a planar nano diffraction grating with a non-uniform pitch, an optical fiber, and a photodiode array. The nano diffraction grating and optical waveguide platform were precisely fabricated using the UV imprint lithography suitable for mass production. To provide uniform diffraction efficiency in the wavelength range of 400–650 nm, the nano diffraction grating had a quasi-three-dimensional V-shaped groove with a blazing angle of 22.5° and non-uniform pitches, maximum of 0.406 μm and minimum of 0.393 μm. In addition, the optical waveguide platform focuses the optical source to the nano diffraction grating using the concave mirror. The alignment guide pattern is used to align the nano diffraction grating and optical fiber. Through the packaging process of nano grating device, photo diode array and fiber on the planar optical waveguide platform, we manufactured the polymeric integrated polymeric spectrometer module. We confirmed that the fabricated planar spectrometer module had a resolution of 5 nm or less for spectral bandwidth of 250 nm, and it can be used widely in portable spectrometer sensors.

Design of a uniform-illumination binocular waveguide display with diffraction gratings and freeform optics.

Uniform illuminance over the expanded exit pupil is an important requirement for waveguide display systems with a wide field of view (FOV). To address this issue, we develop a monochromatic binocular waveguide display in this paper. Two surface-relief diffraction gratings are designed as in-couplers and out-couplers. The parameters of the gratings are optimized to achieve uniform diffraction efficiency distributions over a broad angular range. The grating couplers enable the system to realize a diagonal FOV of 40°. A freeform surface prism is designed as the projection optics. The diameters of the two exit pupils are 12 mm in the expanding direction at an eye relief of 19 mm.

Design of a multiplexing grating for color holographic waveguide

Volume holographic gratings have been used in waveguide displays to implement full-color three-dimensional imaging. Among these, multiplexing gratings are advanced in low energy losses and simple manufacture technologies when used as couplers of color hologram waveguides. A multiplexing holographic grating is designed to realize a uniform red, green, and blue diffraction efficiency and eliminate stray light to the largest extent. Results indicate that the red, green, and blue light incident normal to the grating could be successfully in-coupled into the planar waveguide for total internal reflection with high peak diffraction efficiency, similar energy output, and little stray light. We also analyze the effect of the technical tolerance, including gating thickness, index modulation, grating period, slanted angle, and incident angle. This analysis could help to minimize the optical system and improve the color image quality of waveguide displays.

Full-color holographic diffuser using time-scheduled iterative exposure.

A compact wavelength multiplexing technique is proposed and experimentally investigated to improve the efficiency of a full-color holographic diffuser using photopolymer. The exposure responses of a monochromatic hologram and a three wavelength multiplexed hologram recorded in photopolymer film are presented. The time-scheduled exposure energies at wavelengths of 633, 532, and 473nm were chosen to optimize the uniform diffraction efficiency of the wavelength multiplexed hologram. These three wavelength iterative sequences of exposures are applied to achieve a specific color balance for a multicolor holographic diffuser. The experimental results confirm that the fabrication method is well suited to the manufacture of holographic diffusers for full-color display applications.

Implementation of uniform diffraction efficiency of partially overlapping holograms in photopolymers based on the photopolymerization of a free radical

The exposure schedule model for uniform diffraction efficiency is extended to be suitable for the partially overlapping multiplexing method. The proposed model is based on solving an optimization problem. Fifty holograms were multiplexed using the exposure schedule calculated with the extended model. The material used in the experiment is based on the photopolymerization of a free radical. By comparing the intensity of the reconstructed images during recording with that readout after recording, the calculated exposure schedule is verified to be effective to realize the uniform diffraction efficiency for the multiplexing holographic storage.

Exposure Schedule for Uniform Diffraction Efficiency of Partially-Overlapping Multiple Holograms in Dual Monomers Photopolymers

The exposure schedule model of uniform diffraction efficiency in single monomer photopolymer is extended to that in dual monomers with partially-overlapping multiplexing methods. The proposed model can be thought of solving an optimization problem. The exposure schedule for 51 holograms are calculated and verified by the numerical simulation. The result shows that the exposure schedule is effective to produce the uniform diffraction efficiency for the multiplexing holographic storage.

氧化铈抛光改善光束采样光栅衍射效率均匀性

氧化铈抛光改善光束采样光栅衍射效率均匀性

Design of angular magnifier based on multiplexed volume holographic grating

In order to design an angular magnifier based on multiplexed volume holographic grating, the physical model of multiplexed angular magnifier (MAM) is established. The design principle is summarized from two aspects, namely, uniform angular distribution and optimum diffraction efficiency. The effect of production error on MAM is studied. The effect of beam divergence on the performance of MAM is investigated. The results show that the desired angular distribution of MAM is achieved by controlling the spatial frequency and the tilted angle of the grating while the optimum diffraction efficiency is achieved by controlling the thickness and the amplitude of refractive index modulation of the gratings. The 10 VHGs can be multiplexed in the MAM at most. The raising of the grating tilted angle or the ratio between the wavelength of recording beam and working beam can weaken the effect of angular error of reference beam on the output angle distribution of MAM, and the reducing of the thickness is beneficial for reducing the effect of error of thickness and refractive index modulation on diffraction efficiency. When the beam divergence is greater than the angular half width, the optimum diffraction efficiency falls down to 50% or lower and the local minimum values disappear. The raising of the spatial frequency or thickness of the gratings can reduce the desired amplitude of refractive index modulation and multiplex more VHGs, but is not beneficial for achieving uniform diffraction efficiency distribution and weakening the effect of divergent beam.

Analysis of multilevel spiral phase plates using a Dammann vortex sensing grating

Optical vortices can be easily generated using a multilevel spiral phase plate (SPP). However the quality of the generated vortex beam depends on the number of phase segments. We review the theory for this multilevel SPP using a Fourier expansion of integer topological charged vortices. We then experimentally examine the validity of this expansion using a fabricated Dammann vortex diffraction grating spectrum analyzer. The Dammann vortex diffraction grating is fabricated using SU-8 photoresist on glass substrate and yields uniform diffraction efficiency across a desired number of diffracted orders. Experimental results show the extra angular harmonics of a multilevel SPP and agree with the theory.

Peristrophic multiplexing studies in silver doped photopolymer film

The peristrophic multiplexing technique with rotation of the film in a plane normal to the bisector of the incident beams was employed for recording plane-wave transmission gratings at the same location of silver doped acrylamide photopolymer film. Both constant and variable exposure scheduling methods were adopted for storing gratings using a 632.8 nm HeNe laser. The diffraction efficiency (DE) and M number (M/#) obtained from both methods were compared to determine which method enabled the greatest number of gratings to be recorded with uniform diffraction efficiencies. By the variable exposure energy scheduling method, 30 nearly uniform plane wave gratings with M/# equal to 4.7, could be recorded in a 130 µm thick photopolymer layer.

Process development for improved soft X-ray zone plates

We demonstrate two nanofabrication methods which improve the diffraction efficiency of high-resolution soft X-ray nickel zone plates. First, pulse electroplating is shown to result in uniform diffraction efficiency over the entire zone-plate area. A resulting enhancement of the total efficiency of 20% compared to conventional DC plating was measured. Second, we demonstrate that a high-resolution cold development process can be combined with efficiency-enhancing dry etching into an underlying germanium film. We present 16 nm half-pitch gratings composed of 50 nm nickel on top of 50 nm germanium.

Investigation on mechanism of multiple holographic recording with uniform diffraction efficiency in photopolymers

In this paper, our simplified model for grating formation in photopolymers is extended to describe the effects of uniform post-exposure on an existing hologram, and further to present a new model of holographic multiplexing for calculating the exposure schedule for multiplexed gratings with uniform diffraction efficiency. It is experimentally verified that the refractive-index modulation of the existing gratings continues to increase when a current grating is being recorded. Twenty gratings were multiplexed using the exposure schedule calculated with the uniform post-exposure model, and comparison with the result from traditional method confirmed the validity of this model.

Characterization of the diffraction efficiency of polymer-liquid-crystal-polymer-slices gratings at all incidence angles

Recently, a novel holographic diffraction grating made of polymer slices alternated to homogeneous films of nematic liquid crystal (POLICRYPS) was realized. We study the optical performance of the POLICRYPS gratings by both numerical simulations and experiments. Characterization of the grating at normal and conical reading mount are performed. The diffraction efficiency depends strongly on the angles of incidence. Besides, the characterization of the diffraction efficiency at Bragg angle incidence is studied. A uniform high diffraction efficiency is achieved when the incident wave satisfies the Bragg condition.