Multiple Diffracted Beams Research Articles

Common-path digital holographic microscopy based on a volume holographic grating for quantitative phase imaging.

Digital holographic microscopy (DHM) is a powerful quantitative phase imaging (QPI) technique that is capable of recording sample's phase information to enhance image contrast. In off-axis DHM, high-quality QPI images can be generated within a single recorded hologram, and the system stability can be enhanced by common-path configuration. Diffraction gratings are widely used components in common-path DHM systems; however, the presence of multiple diffraction beams leads to system power loss. Here, we propose and demonstrate implementation of a volume holographic grating (VHG) in common-path DHM, which provides single diffraction order. VHG in common-path DHM (i.e., VHG-DHM) helps in improving signal-to-noise ratio as compared to the conventional DHM. In addition, VHG, with inherently high angular selectivity, reduces image noise caused by stray light. With a simple fabrication process, it is convenient to utilize VHG to control the beam separation angle of DHM. Further, by using Bragg-matched wavelength degeneracy to avoid potential cell damaging effect in blue light, the VHG is designed for recording at a maximum sensitive wavelength of ∼488 nm, while our VHG-DHM is operated at the longer wavelength of red 632.8 nm for cell observation. Experimental results, measured by the VHG-DHM, show the measurement of target thickness ranging from 100 nm to 350 nm. In addition, stability of the system is quantitatively measured. High-contrast QPI images of human lung cancer cells are demonstrated.

Extreme Diffraction Management in Phase‐Corrected Gradient Metasurface by Fourier Harmonic Component Engineering

AbstractBeam diffraction management with on‐demand efficiency over compact devices is important in various applications, such as communications, spectroscopy, wireless power transfer, and others. Recently, the in‐depth study of metasurfaces, such as phase gradient metasurfaces (PGMs) or metagratings (MGs) made of discrete elements, has promoted an ultrathin platform to manipulate diffractions. However, most studies only focus on symmetrical diffraction orders or different propagating diffraction orders with equally distributed energy. It is difficult to efficiently excite beams with arbitrary energy distribution by phase‐only metasurfaces due to the complex optimization procedure. Here, to address these challenges, Fourier harmonic component engineering is proposed to allocate the energy between multiple diffraction beams. By introducing phase‐corrected gradient (PCG) on the metasurface platform, lossless transformation from the incidence to far‐field patterns can be obtained. A variety of diffraction situations are considered (symmetric and asymmetric, with equal or arbitrary energy ratio), where the simulated and measured far‐field patterns are in excellent agreement with the theoretical predictions and the achieved diffraction efficiency is up to 98.3%. The proposed method paves the way for multichannel wireless communication applications and can be readily extended to other frequency regions.

Spin-dependent diffraction of evanescent waves by subwavelength gratings.

We present a way to observe the spin-to-orbital conversion phenomenon. A spinning evanescent wave can be asymmetrically transformed into propagation waves through one certain diffraction order by a periodical subwavelength grating. By detecting diffraction field distribution behind the grating, we observed spin-dependent diffraction patterns. Furthermore, replacing the periodical grating by a Fibonacci grating, we can simultaneously observe multiple order diffractions of a spin evanescent wave. In this case, the multiple diffraction beams can interfere with each other behind the quasi-periodical grating to form asymmetric interference patterns. Our work provides another way toward the realization of spin-to-orbital conversion of light.

Polarization-Selective Liquid-Crystal Device and Its Application to Recordable Integrated Optical Unit

A polarization-selective multiple-beam liquid crystal grating (MBLCG) was designed. It can be applied to an integrated optical unit (IOU) for a recordable optical pickup head in small form factor with the functions of recording data using a single beam and retrieving data using multiple beams, respectively. This device is constructed using the technologies of liquid crystal light modulator for switching diffraction effects of a single transmissive beam and multiple diffracted beams. The proposed IOU composes of a laser diode, a photodetector, a MBLCG and a holographic optical element (HOE). The optical axes of the MBLCG and HOE are mutually perpendicular so that the round-trip light efficiency of the IOU can be maximized. Experimental results show that the MBLCG is capable of generating seven diffracted beams with equal light intensities and polarization-selective corresponding to an incident beam of the ordinary or extraordinary state.

Schlieren Imaging by the Interference of Two Beams in Raman-Nath Diffraction

Visualization of ultrasonic wavefronts at 2.0–2.5 MHz is demonstrated by using the interference of two diffracted beams. For relatively narrow ultrasonic beams with sufficiently high intensity, light is scattered as the Raman-Nath diffraction to generate multiple diffracted beams. Two beams of neighboring diffraction orders are detected, the result of which exhibits interference fringes in the images. It is proven, both theoretically and experimentally, that these fringe patterns are identical to the wavefront patterns of ultrasound. Bright images with wavelength-filtered xenon stroboscopic illumination were obtained for ultrasonic beams of several mm-beamwidths.

Phase-Type Gratings Formed by Photochemical Phase Transition of Polymer Azobenzene Liquid Crystal. 2. Rapid Switching of Diffraction Beams in Thin Films

Dynamic holographic gratings were investigated with thin films of polymer azobenzene liquid crystals by periodic induction of photochemical nematic-to-isotropic phase transition. On irradiation of writing beams (488 nm), multiple diffraction beams of a reading beam (633 nm) were immediately observed. The first-order diffraction efficiency reached to a maximum value within several tens of milliseconds: the rise time was approximately 50 ms for PM6AB2 films and 30 ms for PA6ABCN films. Quite large modulation in refractive index (∼0.08) was obtained in both polymer films. Studies on effects of temperature and light intensity on the grating formation suggested that photochemical reactions of azobenzene moieties followed by photochemical phase transition in bright fringes of the interference pattern would be responsible for the formation of holographic gratings. By turning on and off the writing beams, the diffraction beams could be switched dynamically without significant fatigue.

Holographic Grating Formed by Photochemical Phase Transition of Polymer Azobenzene Liquid Crystal

ABSTRACTWe explored the formation of holographic gratings by means of a polymer azobenzene liquid crystal (PALC). When two writing beams (argon-ion laser, 488 nm, unpolarized) were interfered on the surface of the PALC film (thickness, 500 ∼ 600 nm), multiple diffraction beams were observed immediately, resulting from grating formation. Such generation of the diffraction beams was assumed to be due to a periodic induction of photochemical phase transition in the bright fringes of the interference pattern. The grating is namely made up of a periodic arrangement of nematic (N, dark fringes) and isotropic (I, bright fringes) phases. Observation of recorded interference patterns under a cross-polarized optical microscope supported our speculation for the structure of the grating. On the basis of these results, we succeeded in recording an image hologram of a photomask as an object.

Generation of multiple-pitch gratings in WDM devices by using a single computer-generated phase mask

Use of a single computer-generated phase mask (CGPM) for monolithic fabrication of submicronmeter multiple-pitch gratings is proposed. The CGPM is a computer-generated Fourier hologram generating multiple diffracted beams with equal intensity from a single incident beam. The submicronmeter grating periods differing by as little as a few angstroms are estimated with basis of the multiple diffraction angles. As a result of preliminary experiment, an array of nine gratings has been achieved by using a CGPM with 2-μm minimum feature size. Limit of the CGPM period due to increase in wavelength spacing of WDM devices is also discussed.

Energy-dispersive diffraction with synchrotron radiation: Optimization of the technique for dynamic studies of transformations

Abstract Energy-dispersive powder diffraction can be optimized to suit particular studies and carry out measurements delivering the required information with an ease that cannot be matched by other techniques. Starting from the basic theory, factors such as resolution, scattering angle, beam geometry, sample geometry and electronics are considered in the context of the optimization and performance of the technique for dynamic diffraction studies. The examples chosen to illustrate the discussion are drawn from a wide range of studies which includes the hydration of high-alumina cements, the high-temperature synthesis of zirconia and the high-temperature crystallization of metallic glasses. These cover the use of reflection and transmission geometries, single and multiple diffracted beams, thin and thick samples and various scattering angles and energies.

Multiple reflection high-energy electron diffraction beam intensity measurement system

A video-based analysis system for reflection high-energy electron diffraction (RHEED) is described which simultaneously measures the intensities and profiles of multiple diffraction beams. This system is used to record real-time RHEED intensity oscillations for layer-by-layer epitaxial growth. Fast Fourier transform analysis of the oscillation data is used to directly determine the growth rate and to accurately obtain phase information about the oscillations. This system is demonstrated and compared to other methods of recording RHEED oscillation data.