High combiner efficiency and self-aligned compact double-sided diffractive waveguide based on linear surface relief gratings.

A diffractive waveguide based on surface relief gratings demonstrates significant potential for augmented reality owing to its ultra-thin and lightweight design, as well as its feasibility for mass production using nanoimprint technology. However, traditional waveguides suffer from low combiner efficiency. To address this issue, we propose what we believe to be a novel double-sided surface relief grating waveguide (abbreviated as double-sided waveguide) with a high combiner efficiency, which comprises a double-sided in-coupler, two single-sided turners, and a double-sided out-coupler.

Blazed gratings with both controllable blaze angle and anti-blaze angle fabricated by using a twice oblique ion beam etching method.

A twice oblique ion beam etching method is proposed to fabricate triangular blazed gratings that have controllable both blaze angle and anti-blaze angle. The anti-blaze angle is controlled by first obliquely etching the photoresist mask to obtain an asymmetrical trapezoidal grating, one sidewall of which then evolves into the anti-blaze facet in the second etch step. The blaze angle is controlled by obliquely etching the asymmetrical trapezoidal grating to obtain a triangular blazed grating.

Low-stray-light gratings fabricated with scanning exposure method based on Lloyd's mirror for a high-contrast near-eye display in augmented reality.

The stray light of gratings lowers the image contrast of augmented reality display devices based on lightguide gratings. We propose to reduce the stray light of gratings fabricated with the scanning exposure method in a Lloyd's mirror interferometer setup. The photoresist-coated substrate is moved in the longitudinal direction parallel to the exposure interference fringes during exposure to average out the laser speckle-induced noise.

Aberration-reduced spherical concave grating holographically recorded by a spherical wave and a toroidal wave for Rowland circle mounting.

We propose a new recording system that employs a spherical wave and a toroidal wave to holographically record an aberration-reduced Rowland-circle spherical grating. Two more variables in this recording system are available for the design compared with a conventional symmetric dual-plane-wave recording system, which allows both the astigmatism and sagittal coma of the grating to be corrected to minimum. We derive the aberration coefficients of such grating and give the universal design principles for the recording system.

Astigmatism-reduced spherical concave grating holographically recorded by a cylindrical wave and a plane wave for Rowland circle mounting.

We propose a new recording system that employs a cylindrical wave and a plane wave to holographically record astigmatism-reduced Rowland-circle spherical gratings. We find that the astigmatism of the grating can be sharply reduced, so that the spectral intensity is improved. Meanwhile, its defocus aberration and meridional coma remain zero throughout the working spectrum, meaning the spectral resolution is similar to that of a conventional grating recorded by symmetric dual plane waves.

Achieving unlimited recording length in interference lithography via broad-beam scanning exposure with self-referencing alignment.

Large-area holographic gratings are of great importance in diverse fields including long-range interference metrology, high-resolution astronomical telescopes, and chirped-pulse-amplification systems. However, in conventional interference lithography, the recording length is limited by the aperture of the collimating lenses. Here we propose broad-beam scanning exposure which employs the latent grating generated continuously during scanning for real-time dynamic fringe locking and thus achieves unlimited recording length.

Duty cycle estimate of photoresist gratings via monitoring TM/TE diffraction efficiency ratio during development.

We present an in situ duty cycle control method that relies on monitoring the TM/TE diffraction efficiency ratio of the -1st transmitted order during photoresist development. Owing to the anisotropic structure of a binary grating, at an appropriately chosen angle of incidence, diffraction efficiencies in TE and TM polarizations vary with groove depth proportionately, while they vary with duty cycle differently. Thus, measuring the TM/TE diffraction efficiency ratio can help estimate the duty cycle during development while eliminating the effect of photoresist thickness uncertainty.

Two-probe optical encoder for absolute positioning of precision stages by using an improved scale grating.

In this paper, a novel optical encoder enabling the simultaneous measurement of displacement and the position of precision stages is presented. The encoder is composed of an improved single-track scale grating and a compact two-probe reading head. In the scale grating, multiple reference codes are physically superimposed onto the incremental grooves, in contrast to conventional designs, where an additional track is necessary.

Controlling the duty cycle of holographic crossed gratings by in situ endpoint detection during development.

A method of in situ development endpoint detection is proposed to control the duty cycle of holographic crossed gratings. Based on the observation that after the developer first touches the substrate surface the topography of the crossed grating undergoes an evolution process from a hole array of increasing diameter to a pillar array of decreasing diameter, we set up a development model. In this model, the shapes of both holes and pillars are assumed to have square in-plane cross sections, rotated 45° with respect to the main periodic directions, and straight side walls perpendicular to the grating plane.

Analysis and minimization of spacing error of holographic gratings recorded with spherical collimation lenses.

This research proposes a feedback method to adjust the dual-beam exposure system with spherical collimation lenses to achieve gratings with low spacing error. Through theoretical analysis and numerical calculation, it is proved that the interference aberration can be analyzed with the Zernike polynomials and the adjustment errors can be estimated according to the linear relationship between the errors and the polynomial coefficients. Moreover moving the substrate along its normal is proposed to decrease the spacing error but keep the grating's period unchanged.

Fabrication of low-stray-light gratings by broad-beam scanning exposure in the direction perpendicular to the grating grooves.

Previous research on making low-stray-light gratings is mainly focused on process steps after the photoresist mask has been made. We propose to improve the quality of the photoresist mask directly in exposure. We present a broad-beam scanning exposure method along the grating vector (i.

Scatterometry specialized for a highly asymmetric triangular grating on a transparent substrate.

We present a specialized scatterometry method to measure the groove profiles of highly asymmetric triangular gratings. Compared with the conventional scatterometry working in a specular way, this method utilizes diffraction spectra of the reflected ±1st orders and is good at measuring this kind of asymmetric grating with a higher sensitivity. In our work, diffraction efficiency angular spectra at a single wavelength are measured and passed on to a parameter optimization process to retrieve three profile defining parameters.

Fabrication of optical mosaic gratings: a self-referencing alignment method.

We propose and demonstrate a self-referencing alignment technique to conveniently enlarge fabricated grating area. The latent image gratings are used as the reference objects to align (adjust and lock) the attitude and position of the substrate relative to the exposure beams between and during consecutive exposures. The adjustment system and the fringe-locking system are combined into the exposure system, eliminating the drift errors between them and making the whole system low-cost and compact.

Fabrication of optical mosaic gratings with phase and attitude adjustments employing latent fringes and a red-wavelength dual-beam interferometer.

We present a method to make optical mosaic gratings that uses the exposure beams and the latent grating created by the previous exposure to adjust the lateral position and readjust the attitude of the substrate for the current exposure. As thus, it is a direct method without using any auxiliary reference grating(s) and it avoids the asynchronous drifts between otherwise independent exposure and alignment optical sub-systems. In addition, the method uses a red laser wavelength in the plane-mirror interferometers for the multi-dimensional attitude adjustment, so the adjustment can be done at leisure.

Measurement of period difference in grating pair based on compensation analysis of phase difference between diffraction beams.

We propose an experimental method for measuring the period difference in a grating pair. The method uses the relationship between the period difference and the phase difference of diffraction beams of the grating pair when gratings are translated along the grating-vector direction. The effect of the roll angular deviation of gratings can be eliminated by phase compensation with +1st- and -1st-order diffraction beams.

[Numerical simulation and experimental demonstration of error compensation between recording structure and use structure of flat-field holographic concave gratings].

A flat-field holographic concave grating can be the only optical element in a spectrometer. If the mutually compensating roles of the concave grating and the spectrometer are taken into account, the difficulty of fabricating concave grating can be reduced. When the holographic method is used to make concave gratings, it is very difficult to fix precisely the spatial distances between the two recording points and the vertex of concave gratings.

Routine turning maneuvers of koi carp Cyprinus carpio koi: effects of turning rate on kinematics and hydrodynamics.

Spontaneous swimming behaviors of koi carp Cyprinus carpio koi were recorded using a video tracking system. Routine single-beat turns were selected from the recorded image sequences for kinematic and hydrodynamic analysis. As with C-starts, the turns can be divided into two stages (stage 1 and stage 2), based on kinematics.

Grating mosaic based on image processing of far-field diffraction intensity patterns in two wavelengths.

A practical grating mosaic method is proposed based on quantitative image processing of three far-field diffraction intensity patterns in two wavelengths. This method aims at making a perfect mosaic of two planar gratings that can substitute for a single and larger grating without introducing wavefront aberration at any wavelength. The zeroth-order and first-order far-field patterns of one wavelength are analyzed for separating and eliminating the angular mosaic errors.

Kinematics, hydrodynamics and energetic advantages of burst-and-coast swimming of koi carps (Cyprinus carpio koi).

Koi carps frequently swim in burst-and-coast style, which consists of a burst phase and a coast phase. We quantify the swimming kinematics and the flow patterns generated by the carps in burst-and-coast swimming. In the burst phase, the carps burst in two modes: in the first, the tail beats for at least one cycle (multiple tail-beat mode); in the second, the tail beats for only a half-cycle (half tail-beat mode).

Optical mosaic gratings made by consecutive, phase-interlocked, holographic exposures using diffraction from latent fringes.

We propose and demonstrate a method for fabricating large optical mosaic gratings, i.e., gratings made by consecutive, phase-interlocked, holographic exposures on single substrates.

Method of making mosaic gratings by using a two-color heterodyne interferometer containing a reference grating.

We propose and demonstrate a two-color heterodyne interferometric method for making a perfect mosaic of two planar gratings that can substitute for a single and larger grating without introducing wavefront aberration at any wavelength. The lateral and longitudinal phase errors are separated and eliminated by use of two wavelengths in the interferometry. The accuracy of the phase difference measurement is improved by this heterodyne scheme.

Experimental verification of enhanced transmission through two-dimensionally corrugated metallic films without holes.

We present experimental results to verify extraordinary optical transmission through two-dimensionally periodic, corrugated metallic films without holes as predicted by Bonod et al. [Opt. Express 11, 482 (2003)].

Two-dimensional surface profile imaging technique based on a double-grating frequency shifter.

A two-dimensional surface profile imaging technique that uses a low-coherence heterodyne interferometer is proposed. A double-grating frequency shifter was used in a tandem interferometer to provide the achromatic frequency shift for low-coherence light. A chopper, together with a processing circuit, was implemented to modulate the interference fringes.

Measurement of duty cycles of photoresist grating masks made on top of multilayer dielectric stacks.

We present a novel and nondestructive method for measuring the duty cycles (ratio of ridge width to period) of submicrometer rectangular photoresist gratings made on top of multilayer dielectric stacks. The method exploits the fact that the effective index of the leaky mode that has a strong evanescent tail in the cladding changes with the duty cycle of the grating situated at the interface between the top dielectric layer and the cladding. By comparing measured coupling angles of the leaky mode with a theoretical or experimentally calibrated relationship between coupling angles and duty cycle, one can determine the duty cycle of the grating.