Bragg Diffraction Condition Research Articles

Study on mechanism of sidewall roughness scattering in slot optical waveguides by FDTD simulation

The slot optical waveguide is different from the conventional solid-core optical waveguide in both structure and features and has many unique and important applications. However, serious sidewall roughness scattering (SRS) loss limits the performance of the devices based on it. So far, the mechanism of the SRS in the slot waveguide is still not clear, and it seems not reasonable to analyze it directly by the Bragg diffraction condition. In this paper, through finite-difference time-domain simulation, we demonstrate that the regimes of the SRS and backscattering in the slot waveguide are the same as those of the solid-core waveguide, and the conditions of the diffraction radiation caused by the outer and the inner sidewalls are the same. The rationality of these conclusions are explained by taking the slot waveguide as a whole. The scattering regimes revealed and verified in this paper will benefit the suppression of the SRS loss and the utilization of the slot waveguide.

2D scanning system of the acousto-optical deflector with high diffraction efficiency

The paper considers advantages and special features of an acousto-optic method for the development of a functional device for spatial and temporal control of laser light and reports results of investigations to create a two-directional acousto-optic laser scanner with diffraction efficiency as high as 80 percent or better. An acousto-optic deflector on a paratellurite crystal operating under the Bragg diffraction condition with a lithium niobate piezoelectric acoustic transducer was developed and tested experimentally. The results of this work can serve as a basis for the development of prototype functional acousto-optic devices for spatial and temporal control of laser light and their application to create laser television images.

Резонансное отражение света оптической решеткой экситонов, сформированной 100 квантовыми ямами InGaN

Optical properties of a structure with a periodic system of 100 InGaN quantum wells (QWs) separated by non-tunneling GaN barriers have been investigated at room temperature. The structure periodicity corresponded to the Bragg diffraction condition at the frequency of the QW excitons. Numerical modeling using transfer matrices gave a quantitatively accurate fit of the experimental results. The model included the resonance response of A, B, and C excitons in QWs and an optical absorption tail in the barriers and buffer layer. We have determined the radiative and non-radiative broadening of the excitons in the InGaN QWs.

Fabrication tolerance analysis of grating couplers between optical fibers and silicon waveguide

The grating structure and its performance for coupling between fiber and silicon-based waveguide are studied and analyzed. The coupling mechanism is illustrated through the grating Bragg diffraction condition, and the effect of physical parameters and alignment parameters of the grating coupler on the coupling efficiency is analyzed by FDTD (Finite-difference time-domain). The optimum design values of the period, duty cycle, etching depth and alignment parameters of the grating coupler are obtained, and at the same time, the deviation analysis on the optimal size is carried out. We present high-efficiency grating couplers fabricated both in EBL (Electron Beam Lithography) and Deep Silicon Etching through process optimization on device dimension, which can be useful for grating coupler fabrication.

Two beam toy model for dislocation contrast in ECCI

Dislocation contrast in the SEM, as observed though electron channelling contrast imaging (ECCI), is commonly treated analogously to the contrast in the TEM. This perception is based on early studies done for dislocations parallel with the surface where the surface relaxation is negligible. However, for threading dislocations (TD) that interact with the surface (normal or inclined), as is the case for nitrides materials, g b type invisibility criteria are no longer fully applicable to ECCI, especially in forward geometry [1]. Dislocations change locally the lattice curvature and Bragg diffraction conditions in the crystal, affecting the form and diffracting behaviour of the electron wavefunction in that region. More explicitly, Howie and Whelan [2] had shown that dislocation contrast is the result of interband transitions between Bloch waves states which, in turn, are caused by the change in the displacement field, u(r), around the dislocation or local “strain”. Dynamical models have been used successfully to both predict and characterise dislocations in ECCI [3]. Nevertheless, the behaviour of dislocation contrast in ECCI in particular and diffraction contrast in the SEM in general remains somewhat opaque. In the work we investigate the behaviour of contrast causing strain as a means of insight into this problem.

Generation of an optical vortex array in the course of acousto-optic diffraction.

Generation of an array of optical vortices (OVs) with fractional charges under the condition of acousto-optic Bragg diffraction is revealed experimentally. The OVs emerge in a wide diffracted optical beam due to reflection from a thick acousto-optic grating that contains bifurcated fringes of acoustic waves (AWs). Changes in the AW frequency lead to deflection of the wide optical beam containing the OV array. The OVs preserve their spatial positions only in the narrow AW frequency range, while larger changes in the frequency impose their movement out from the beam aperture, with appearance of the other OVs in the other places of the beam cross section.

Borrmann effect in photonic crystals.

We present the experimental observation of the optical Borrmann effect in 1D porous silica-based photonic crystals (PhC) composed of several hundreds of dielectric layers with different porosity. Two mechanisms of the effect are demonstrated, which involve the optical losses associated with light scattering and absorption in nanoporous layers. Absorption is introduced by a small amount of silicon in partially annealed porous silicon PhC. We show that the PhC transmittance increases substantially under the Bragg diffraction condition, which is a manifestation of the Borrmann effect. The observed effects along with their polarization sensitivity are confirmed by numerical calculations.

Systematic study of the hybrid plasmonic-photonic band structure underlying lasing action of diffractive plasmon particle lattices

We study lasing in distributed feedback lasers made from square lattices of silver particles in a dye-doped waveguide. We present a systematic analysis and experimental study of the band structure underlying the lasing process as a function of the detuning between the particle plasmon resonance and the lattice Bragg diffraction condition. To this end, as gain medium we use either a polymer doped with Rh6G only, or polymer doped with a pair of dyes (Rh6G and Rh700) that act as a F\orster energy transfer (FRET) pair. This allows for gain, respectively, at 590 or 700 nm when pumped at 532 nm, compatible with the achievable size tunability of silver particles embedded in the polymer. By polarization-resolved spectroscopic Fourier microscopy, we are able to observe the plasmonic/photonic band structure of the array, unraveling both the stop gap width, as well as the loss properties of the four involved bands at fixed lattice Bragg diffraction condition and as a function of detuning of the plasmon resonance. To explain the measurements we derive an analytical model that sheds insights on the lasing process in plasmonic lattices, highlighting the interaction between two competing resonant processes, one localized at the particle level around the plasmon resonance, and one distributed across the lattice. Both are shown to contribute to the lasing threshold and the overall emission properties of the array.

Optimal Conditions for Defect Analysis Using Electron Channeling Contrast Imaging

The crystal defect analyses have been commonly conducted using transmission electron microscopy (TEM). There is no doubt that TEM is one of the best analyzing tool for examining crystal defects but limitations are bound to follow due to the shape of specimen; the thin foil (Williams & Carter, 2009). Recently, scanning electron microscopy (SEM) based technique for defect analysis, which is called electron channeling contrast imaging (ECCI), has got popularity. As a non-destructive and diffraction-based method, ECCI could take some advantages over analysis using TEM (Carnevale et al., 2015). In the first, the specimen preparation is relatively simple and only requires one side of a flat surface. It could exclude the concerns about whether defects to analyze are caused by the process of the specimen preparation or not. In addition, inspecting large area of flat surface could ensure the representativeness of the specimen. It is important to meet the exact Bragg-diffraction condition in order to distinguish dislocation lines from the background matrix (Simkin & Crimp, 1999). In the SEM, there are two approaches to achieve this condition. One is selected area channeling pattern (SACP), and the other is electron backscatter diffraction (EBSD). SACP could provide the rocking beam to induce electron channeling patterns (ECPs) in a small grain. With tilting the specimen, ECPs of the specific hkl-plane could move to meet the optic axis, i.e., the exact Bragg-diffraction condition. Despite its great angular accuracy, SACP has the poor resolution (Lloyd, 1987). Therefore it is almost impossible to conduct SACP at a few or sub-micrometer sized grain. The latter, EBSD, could overcome this situation. But it has its own limitation; poor angular accuracy. So we have to use both techniques to determine the Bragg-condition complementarily.

Toward efficient light diffraction and intensity variations by using wide bandwidth surface acoustic wave

We have developed acoustic-optic (AO) based display units for implementing a handheld hologram display by modulating light deflection through wide bandwidth surface acoustic wave (SAW). The developed AO device consists of a metal layer, a ZnS waveguide layer, SAW inter digital transducers (IDTs), and a screen for display. When RF power with a particular resonant frequency was applied to IDTs, SAW was radiated and interfered with confined beam propagating along ZnS waveguide layer. The AO interacted beam was deflected laterally toward a certain direction depending on Bragg diffraction condition, exited out of the waveguide layer and then directed to the viewing screen placed at a certain distance from the device to form a single pixel. The deflected angles was adjusted by modulating the center frequency of the SAW IDT (SAW grating), the RF power of SAW, and the angles between propagating light beam path along waveguide and radiating SAW. The diffraction efficiency was also characterized in terms of waveguide thickness, SAW RF input power, and aperture length. Coupling of mode (COM) modeling was fulfilled to find optimal device parameters prior to fabrication. All the parameters affecting the deflection angle and efficiency to form a pixel for a three-dimensional (3D) hologram image were characterized and then discussed.

Dynamic two-dimensional refractive index modulation for high performance acousto-optic deflector.

The performance of an acousto-optic deflector is studied for two-dimensional refractive index that varies as periodic and sinc functions in the transverse and longitudinal directions, respectively, with respect to the direction of light propagation. Phased array piezoelectric transducers can be operated at different phase shifts to produce a two-dimensionally inhomogeneous domain of phase grating in the acousto-optic media. Also this domain can be steered at different angles by selecting the phase shift appropriately. This mechanism of dynamically tilting the refractive index-modulated domain enables adjusting the incident angle of light on the phase grating plane without moving the light source. So the Bragg angle of incidence can be always achieved at any acoustic frequency, and consequently, the deflector can operate under the Bragg diffraction condition at the optimum diffraction efficiency. Analytic solutions are obtained for the Bragg diffraction of plane waves based on the second order coupled mode theory, and the diffraction efficiency is found to be unity for optimal index modulations at certain acoustic parameters.

Statistics of Randomized Plasmonic Lattice Lasers

We study lasing in randomized lattices of silver particles in a dye-doped waveguide. We set out to answer a basic question, triggered by earlier observations of distributed feedback lasing in 2D periodic plasmonic particle lattices: how much order do you need to obtain lasing? We start from a diffractive 2D square lattice of silver nanoparticles with a pitch that satisfies the second-order Bragg diffraction condition at the emission wavelength of the dye. By randomly removing particles and by displacing particles we increase disorder. We observe that lasing at the second-order Bragg diffraction condition is very robust, with lasing even persisting when 99% of particles are removed. Above a certain amount of disorder new features appear in the spectrum as well as in the Fourier image that are due to random lasing. We classify Fourier space output on the basis of structure factor calculations. In addition we apply speckle intensity statistics analysis to real-space fluorescence images and introduce a new me...

Atomic Surface Structure of CH3-Ge(111) Characterized by Helium Atom Diffraction and Density Functional Theory

The atomic-scale surface structure of methyl-terminated germanium (111) has been characterized by using a combination of helium atom scattering and density functional theory. High-resolution helium diffraction patterns taken along both the ⟨121⟩ and the ⟨011⟩ azimuthal directions reveal a hexagonal packing arrangement with a 4.00 ± 0.02 A lattice constant, indicating a commensurate (1 × 1) methyl termination of the primitive Ge(111) surface. Taking advantage of Bragg and anti-Bragg diffraction conditions, a step height of 3.28 ± 0.02 A at the surface has been extracted using variable de Broglie wavelength specular scattering; this measurement agrees well with bulk values from CH3-Ge(111) electronic structure calculations reported herein. Density functional theory showed that methyl termination of the Ge(111) surface induces a mild inward relaxation of 1.66% and 0.60% from bulk values for the first and second Ge–Ge bilayer spacings, respectively. The DFT-calculated rotational activation barrier of a sin...

Directional luminescence control of InGaN/GaN heterostructures using quantum structure lattice arrays

The spontaneous surface luminescence properties of InGaN/GaN quantum structure lattice (QSL) are reported. The QSL consists of a two-dimensional array of InGaN/GaN quantum boxes (QBs) arranged in a rectangular pattern of 200 nm periodicity. The measured angular dependent photoluminescence (PL) spectra show a strong dependence on the in-plane Bragg diffractions between QBs. The maximum PL intensity of the InGaN/GaN QSL array that fulfill the Bragg condition points in the normal direction of the sample surface with a narrow radiation angle of ∼ ±12°. In addition, a small side lobe is also shown at ±40°. For the QSL sample that does not fulfill the Bragg diffraction condition, the radiation pattern shows a conventional cosine distribution. The finite-difference time-domain numerical analysis confirms that the lowest order and higher order Bragg diffractions between QBs determine the main and the small side lobe of the radiation pattern measured in QSLs, respectively.

Study of heterostructures according to single-crystal X-ray diffractometry

Opportunities of the use of an XMD-300 diffractometer in three mapping schemes have been considered, namely, grazing primary beam, grazing diffracted beam, and the θ-2θ scheme, for the study of crystal perfection of semiconductor heterostructures (silicon-on-sapphire, silicon-on-insulator, ion-doped silica layers, and AlGaN/GaN/Si structures). It has been shown that the measurements with the use of three schemes in scattered radiation and in the exact performance of Bragg diffraction condition enable one to obtain a fringe pattern of diffraction simultaneously from crystal lattices of several layers of the heterostructure and interference peaks of maximum intensity for each individual layer.

Polarization features of the 2D contouring of an optical image under double Bragg diffraction conditions

The polarization features concerning the formation of a 2D image contour in the first diffraction order, which arise from the double Bragg diffraction occurring in the uniaxial gyrotropic crystal, are investigated. It is demonstrated that changes in sound power make it possible to create 2D image contours in different polarization states. Experiments based on the double diffraction in the TeO2 crystal have confirmed the basic theoretical assumptions.

Control of spontaneous emission in InGaAs/GaAs quantum structure lattices

We report the control of spontaneous emission wavelength of quantum wells (QWs) using an artificial design of semiconductor lattice, i.e., quantum structure lattice (QSL). Two-dimension square quantum box arrays fabricated in strained InGaAs/GaAs QW samples are used as active lattice units in QSL for this study. The photoluminescence peak wavelength of the QSL can be adjusted to blue-shift, red-shift, or unchanged from that of the as-grown QW by fitting the in-plane Bragg diffraction condition of the QSL. The wavelength-shift has values of multiple phonon energies indicating that the phonon-assisted recombination process may play a significant role in the observed spontaneous emission.

A novel self-seeding scheme for hard X-ray FELs

Typical SASE XFEL pulses exhibit poor longitudinal coherence, a characteristic inherited from the start-up from shot noise. Self-seeding schemes are an answer to the call for improved longitudinal coherence. If applied to already working or designed XFELs, these schemes are subject to constraints, including minimal change to the baseline design, and possibility to recover the baseline mode of operation. In this work we propose a novel single-bunch self-seeding method, based on a particular kind of monochromator that satisfies these constraints. We will limit ourselves to the analysis of the simplest possible configuration, consisting of an input undulator and an output undulator, separated by our novel monochromatization stage. Such a stage consists of a weak few-meter long chicane, acting as a tunable delay stage, washing out the electron beam microbunching, and creating a transverse offset for the monochromator. In essence, the monochromator consists of a single crystal in Bragg-transmission geometry, which operates as a bandstop filter for the transmitted X-ray SASE radiation pulse. When the incident angle and the spectral contents of the incoming beam satisfy the Bragg diffraction condition, the temporal waveform of the transmitted radiation pulse shows a long monochromatic tail, whose duration is inversely proportional to the bandwidth of the absorption line in the transmittance spectrum. The magnetic chicane is tuned to shift the electron bunch on top of the monochromatic wake created by the bandstop filter thus selecting (temporal windowing) a part of the wake. By this, the electron bunch is seeded with a radiation pulse characterized by a bandwidth much narrower than the natural FEL bandwidth. The output power from our setup can be further increased by tapering the magnetic field of the undulator, yielding a tremendous increase in peak brightness with respect to the baseline mode of operation. In this work we present a feasibility study and exemplifications for the LCLS, and we discuss advantages of our method compared to other self-seeding techniques.

Diffraction of entangled photon pairs by ultrasonic waves

In this paper, we have presented and established a new theoretical formulation of photon optics based on photon path and Feynman path integral idea. We have used Feynman path integral approach to discuss Fraunhofer, Fresnel diffraction of single photon and entangled photon pairs by ultrasonic wave and obtained the following results: i) quantum state and probability distribution of single photon and entangled photon pairs by Fraunhofer and Fresnel ultrasonic diffraction, ii) oblique incidence Raman-Nath and Bragg diffraction conditions, iii) total correlation state and its probability distribution. Our calculation results are in agreement with the experiment results. Comparing one-photon and two-photon diffraction effects by ultrasonic waves, we have found that two-photon diffraction by ultrasonic waves is also a sub-wavelength diffraction.

Light scattering into silicon-on-insulator waveguide modes by random and periodic gold nanodot arrays

Experimental characterization and finite-element numerical simulations of the electromagnetic interaction between random or periodic Au nanodot arrays patterned atop a silicon-on-insulator (SOI) photodetector and incident electromagnetic plane waves have been performed at wavelengths of 400–1100nm. The presence of the Au nanodots is found to lead to increased electromagnetic field amplitude within the semiconductor and, consequently, increased photocurrent response for both cases. Random arrays tend to exhibit broad increases in photocurrent over wavelength, whereas periodic arrays demonstrate sharp resonance peaks in the photocurrent absorption spectrum. Such features are due to the coupling of normally incident light into waveguide modes that satisfy the Bragg diffraction condition. Analysis of the dispersion relation of the waveguide modes allows for accurate prediction of the resonance peaks in the photocurrent absorption spectrum of the SOI photodetectors patterned with periodic nanodot arrays.