Reflection Phase Gratings Research Articles

Polarization Features of Diffraction of Light by Reflective Holographic Phase Gratings in a Plane-Parallel Layer

Polarization Features of Diffraction of Light by Reflective Holographic Phase Gratings in a Plane-Parallel Layer

Use of Type II sequence diffusers for the purpose of improving noise barrier effectiveness

The Type II Luke sequence diffusers have been known to redirect sound energy and act as a beam steerer at certain frequencies. If the incident sound from the top-edge of a noise barrier can be redirected away from the shadow zone by installing such a device, the insertion loss of the noise barrier can be significantly improved. Numerical analysis revealed that the use of multiple periods of Luke sequence diffusers as reflection phase gratings has potential to improve nose barrier effectiveness even more than using a single diffuser on the top-edge. Compared to a rigid T-barrier, improvement in overall A-weighted insertion loss of 8dBA was achieved by installing sound diffusers designed on the Type II luke sequence on the top-edge of the noise barrier.

A rigorous modal method of the grating with a piece-wise function and characteristics of reflection phase gratings

A rigorous and efficient grating-analysis method with a piece-wise function is presented and the total reflection phase grating is studied. Some results show that the rigorous theory method is several times faster and more accurate than the previous work. And the condition and physical mechanism of total reflection of phase gratings are also discussed too.

Robust Smartphone Assisted Biosensing Based on Asymmetric Nanofluidic Grating Interferometry.

Point-of-care systems enable fast therapy decisions on site without the need of any healthcare infrastructure. In addition to the sensitive detection, stable measurement by inexperienced persons outside of laboratory facilities is indispensable. A particular challenge in field applications is to reduce interference from environmental factors, such as temperature, to acceptable levels without sacrificing simplicity. Here, we present a smartphone-based point-of-care sensor. The method uses an optofluidic grating composed of alternating detection and reference channels arranged as a reflective phase grating. Biomolecules adsorbing to the detection channel alter the optical path length, while the parallel reference channels enable a direct common mode rejection within a single measurement. The optical setup is integrated in a compact design of a mobile readout device and the usability is ensured by a smartphone application. Our results show that different ambient temperatures do not have any influence on the signal. In a proof-of concept experiment we measured the accumulation of specific molecules in functionalized detection channels in real-time and without the need of any labeling. Therefore, the channel walls have been modified with biotin as capture molecules and the specific binding of streptavidin was detected. A mobile, reliable and robust point-of-care device has been realized by combining an inherently differential measurement concept with a smartphone-based, mobile readout device.

Miniaturized Optical Encoder with Micro Structured Encoder Disc

A novel optical incremental and absolute encoder based on an optical application-specific integrated circuit (opto-ASIC) and an encoder disc carrying micro manufactured structures is presented. The physical basis of the encoder is the diffraction of light using a reflective phase grating. The opto-ASIC contains a ring of photodiodes that represents the encryption of the encoder. It also includes the analog signal conditioning, the signal acquisition, and the control of a light source, as well as the digital position processing. The development and fabrication of the opto-ASIC is also described in this work. A laser diode was assembled in the center on top of the opto-ASIC, together with a micro manufactured polymer lens. The latter was fabricated using ultra-precision machining. The encoder disc was fabricated using micro injection molding and contains micro structures forming a blazed grating. This way, a 10-bit optical encoder with a form factor of only 1 cm3 was realized and tested successfully.

Efficiency of multi-beam Fourier phase gratings at 14 THz

We compare the results of simulated and measured power efficiency and far-field beam pattern, for two reflective Fourier phase gratings, designed to generate 2 × 2 and 2 × 4 beams respectively from a single-beam, coherent source at 1.4 THz. The designed surface structures were manufactured on aluminum plates by a computer numerical control (CNC) micro-milling machine. Despite small differences between the designed and fabricated gratings, we measured power efficiencies of both gratings to be around 70%, which is in a good agreement with the simulated values. We also find a good agreement between the simulated and measured diffracted beam size and spatial distribution. We demonstrate the application of both gratings as multiple beam local oscillators to simultaneously pump (or operate) a 4-pixel array of superconducting heterodyne mixers.

Spectral Properties of the Reflection Holograms Formed in Liquid Crystal Composites

The effect of the parameters of a registration system, detuning of the Bragg angle, and special features of the structure of the direct and inclined holographic reflection phase gratings formed in a liquid crystal composite on the spectral characteristics is investigated theoretically and experimentally. A comparative analysis of the calculated and experimental data is carried out. The spectral characteristics of the reflection holograms are modeled in the framework of coupled wave theory.

Grazing angle Mach-Zehnder interferometer using reflective phase gratings and a polychromatic, un-collimated light source

Normal incidence Talbot-Lau interferometers in x-ray applications have the drawbacks of low fringe visibility with polychromatic sources when the wave propagation distance is increased to achieve higher phase sensitivity, and when fabrication limits the attainable grating density. In contrast, reflective gratings illuminated at grazing angles have dramatically higher effective densities than their physical values. However, new designs are needed for far field interferometers using grazing angle geometry with incoherent light sources. We show that, with the appropriate design and choice of reflective phase gratings, there exist pairs of interfering pathways of exactly equal lengths independent of the incoming beam's incidence angle and wavelength. With a visible light grazing angle Mach-Zehnder interferometer, we show the conditions for achieving near ideal fringe visibility and demonstrate both absolute and differential phase-contrast imaging. We also describe the design parameters of an x-ray interferometer and key factors for its implementation.

Transmission and reflection phase gratings formed in azo-dye-doped chiral nematic liquid crystals

Transmission and reflection gratings were simultaneously formed in azo-dye-doped chiral nematic liquid crystals (N∗LCs) with planar alignment. The formation process is based on a phototuning of the Bragg reflection band of the N∗LC. The helical pitch of the photoreactive N∗LC was spatially controlled with intensity variation of interference light. The resultant periodic structure showed both transmissive and reflective diffractions due to the spatially modulated light intensities. The observed dependence of diffraction efficiencies on the polarization states of the probe beam was well explained by considering a spatial modulation of the helical pitch.

Sequences from number theory for physics, signal processing, and art

This work will discuss numerical sequences of potential interest to physicists, engineers, and artists. Foremost among these are the maximum-length sequences, based on the prime number 2 and larger primes, which have found wide use in physics, including acoustics, and engineering: (1) the measurement of impulse responses in concert halls, radar echoes from planets (to check the general theory of relativity) and travel times in the deep-ocean sound channel (to monitor water temperature and global warming); (2) the spatial diffusion of sound waves, coherent (laser) light, and electromagnetic waves; (3) algebraic error-correcting codes (simplex and hamming codes); (4) minimizing peak factors for radar signals, synthetic speech, and computer music; and (5) the formation of x-ray images with 2-D masks (in x-ray astronomy). Other sequences to be described include quadratic-residue sequences for the construction of wideband diffusing reflection phase gratings in one and two dimensions; the MorseThue, Fibonacci, and rabbit sequences and their musical potential; and certain self-similar sequences from number theory that engender attractive visual patterns, rhythms, and melodies.

Application of number theory sequences in architectural acoustics

One of the challenges in architectural acoustics is to develop contemporary diffusing surfaces that complement contemporary architecture in the way that statuary, columns, and relief ornamentation complemented classic architecture. Number theory sequences invented by Schroeder [J. Acoust. Soc. Am. 65, 958–963 (1979)] and further developed by D’Antonio et al. [Appl. Acoust. 60, 143–165 (2000)] have been used for two decades to design reflection phase gratings, which provide useful sound diffusion in architectural acoustics. The use of modulation techniques, to enhance performance, and the development of proof-of-performance techniques, to evaluate their performance, will be described. The development of flat and curve optimized binary amplitude gratings will also be presented. The current state-of-the-art utilizing boundary-element multidimensional optimization to design 1- and 2-D diffusors, which incorporate aesthetic constraints, will offer a view of future design possibilities. Photos of installations using these acoustic tools will also be presented to illustrate the usefulness of these devices and their impact on architectural acoustics.

Hot topics in architectural acoustics: New materials

While both new metrics for measuring sound and analytical software programs for predicting sound performance have proven to be powerful tools of architectural acoustics, new acoustical materials have allowed us to further engineer these sensitive soundscapes. These developments embrace several areas of contemporary acoustical materials. HVAC liners have seen the addition of fiber-free alternatives to traditional linings. New solutions include coated polyimides and melamine foams. In electroacoustics, materials that have improved performance are now replacing some old standbys. PEN materials such as Kaladex are finding more acceptance for use in tweeter domes and microphone elements. Liquid crystal polymer fibers, such as Vectran, which can be used instead of aramid and carbon fibers, offer similar performance improvement for wet lay-up direct radiator transducer cones. Absorptive panels have moved beyond fabric wraps. They are now paintable and can mimic typically reflective treatments. Finally, diffusers have shown advances in both acoustical performance and geometries that better blend with aesthetic goals. These new surfaces consist of a wide range of surface type including modulated reflection phase gratings, binary amplitude gratings, and most recently boundary-element optimized surfaces of constant or variable impedance. Each of these exciting developments are reviewed in this presentation.

Theory and simulation of viscous damped reflection phase gratings

Micromachined reflection phase gratings operate via the electrostatic deflection of reflective microbeams. We develop a lumped parameter model for predicting the transient motion of these microbeams, taking into account electrostatic and tensile forces and viscous air damping via squeeze-film theory. We use this model to determine the time required for a deflected microbeam to return to its undeflected state. Our analysis shows that this re-set time is approximately one order of magnitude greater than the activation time.

Threshold for pattern formation in a medium with a local photorefractive response

Threshold equations for the instability of counterpropagating waves against spontaneous initiation of small-angle light patterns are analyzed for a medium with a local photorefractive response and various boundary conditions. These equations take into account all actual contributions to the reflection phase gratings. The form of the threshold equation is directly related to the spatial symmetry of the problem. We study the dependence of the threshold thickness of the crystal and the angle of light emission on experimentally controllable parameters and discuss the prospects for experimental studies of pattern formation in LiNbO3 crystals.

Multiple-level phase gratings fabricated using focused ion-beam milling and electron-beam lithography

The fabrication of eight-level reflective phase gratings in Si by electron-beam lithography and reactive ion etching, and focused ion-beam milling, has been investigated. Electron-beam lithography and reactive ion etching were used to fabricate gratings with 0.5-μm feature sizes. Alignment of successive levels was held to 50 nm by careful control of proximity effects. Focused ion-beam milling was used to fabricate continuously blazed surface reliefs. The effects of binary and continuously blazed surface reliefs on the first-order diffraction efficiency were determined. Eight-level binary reliefs produced efficiencies of 75% and were limited by phase errors while continuously blazed reliefs produced efficiencies of 90%.

Shared aperture for two beams of different wavelength using reflective phase gratings and the Talbot effect

A shared aperture system for two laser beams with different wavelengths and composed entirely of reflective phase gratings is described. Beams that share an aperture are collinear, and they have the same transverse phase profile across the aperture as their respective sources. Using the Talbot effect that is observed in Fresnel diffraction from periodic objects, we preserve the phase of the beams, and we maximize the efficiency of the system. An experimental Talbot shared aperture system using He-Ne and He-Cd beams has an efficiency of 88.1% for the He-Ne beam and 70.3% for the He-Cd beam. These measured efficiencies agree well with theoretical predictions and computer simulations.

Surface characterisation via optical diffraction

Through the consideration of a surface as a reflective phase grating, this paper establishes the various relationships between the statistical parameters of the surface topology and the far-field diffraction pattern. These relationships have been incorporated into a new measuring instrument designed primarily for the measurement of flats and cylinders, the Talyfine. The performance of this instrument is compared against an established stylus-based surface texture measuring instrument, the Talystep.

The unreasonable effectiveness of number theory in acoustics

Some of the problems that have occupied Vern O. Knudsen during his long career in acoustics have found surprising answers through applications of number theory, a field usually considered rather abstract even within mathematics. Among these unexpected applications are the distribution of normal modes in rooms, the design of surfaces (“reflection phase gratings”) that optimally diffuse sound, and the construction of new music scales. Another application is the selection of phase angles to minimize the peak factor in waveforms for speech synthesis and sonar. Finally, precision measurements of reverberation and absorption of sound in gases, pioneered by Knudsen fifty years ago [V. O. Knudsen, J. Acoust. Soc. Am. 6, 199–204 (1935)], can be made more robust by using “pings” shaped according to the theory of finite number fields.