Diffracted Beam Profile Research Articles

Optical vortex beam controlling based on fork grating stored in a dye-doped liquid crystal cell.

In this paper, we investigate the generation and controlling of the optical vortex beam using a dye-doped liquid crystal (DDLC) cell. The spatial distribution of the quasi-sinusoidal orientation of the liquid crystal molecules creates a quasi-sinusoidal phase grating (PG) in the DDLC cell. Depending on the incident light pattern, Trans to Cis photoisomerization of the dye molecules affects the orientation of the liquid crystal molecules. To do so, an amplitude fork grating (FG) is used as a mask, and its pattern is stored in the cell by a pattern printing method as the PG. One of the particular features of the stored grating in the cell is its capability in the diffraction efficiency controlled by the applied electric field. The results show, based on the central defect in the FG pattern, the diffracted probe beam in different orders is optical vortices. As a new technique, this type of stored pattern acts like an amplitude grating but according to the results, its structure is in fact a PG. This technique leads to the vortex beam switching capability by applying an electric field to the cell. The results show that by applying 22V, all the diffraction orders vanish. Meanwhile, the vortex beams reappear by removing the applied voltage. The diffraction efficiency of the vortex beams as well as its generation dependency on the polarization of the incident beam studied. The maximum efficiency of the first diffraction order for linear polarized incident beam was obtained at 0V, about 8%. Based on the presented theory, a simulation has been done which shows the Cis form of the dye molecules has been able to change the angle of LC molecules on average about 12.7°. The study of diffracted beam profiles proves that they are electrically controllable vortex beams.

Laser processing of sapphire and fabrication of diffractive optical elements.

Sapphire is a high-temperature optical material with a melting temperature of over 2000ºC, high strength, good thermal conductivity, and low thermal expansion, and it exhibits chemical inertness and is optically transparent over a wavelength range of 0.15 µm to 5.5 µm. We report the detailed results of the laser processing of sapphire, fabrication of diffraction gratings, and the photon sieve, which have applications in the infrared spectrum using a nanosecond pulsed laser of 355 nm wavelength. Laser removal rates and surface roughness were studied. The fabricated gratings and photon sieve were characterized in terms of diffracted beam profile, optical diffraction efficiency, surface morphology, and depth profile.

Design of optical meta-structures with applications to beam engineering using deep learning

Nanophotonics is a rapidly emerging field in which complex on-chip components are required to manipulate light waves. The design space of on-chip nanophotonic components, such as an optical meta surface which uses sub-wavelength meta-atoms, is often a high dimensional one. As such conventional optimization methods fail to capture the global optimum within the feasible search space. In this manuscript, we explore a Machine Learning (ML)-based method for the inverse design of the meta-optical structure. We present a data-driven approach for modeling a grating meta-structure which performs photonic beam engineering. On-chip planar photonic waveguide-based beam engineering offers the potential to efficiently manipulate photons to create excitation beams (Gaussian, focused and collimated) for lab-on-chip applications of Infrared, Raman and fluorescence spectroscopic analysis. Inverse modeling predicts meta surface design parameters based on a desired electromagnetic field outcome. Starting with the desired diffraction beam profile, we apply an inverse model to evaluate the optimal design parameters of the meta surface. Parameters such as the repetition period (in 2D axis), height and size of scatterers are calculated using a feedforward deep neural network (DNN) and convolutional neural network (CNN) architecture. A qualitative analysis of the trained neural network, working in tandem with the forward model, predicts the diffraction profile with a correlation coefficient as high as 0.996. The developed model allows us to rapidly estimate the desired design parameters, in contrast to conventional (gradient descent based or genetic optimization) time-intensive optimization approaches.

Optical-vortex diagnostics via Fraunhofer slit diffraction with controllable wavefront curvature.

Far-field slit diffraction of circular optical-vortex (OV) beams is efficient for measurement of the topological charge (TC) magnitude but does not reveal its sign. We show that this is because in the common diffraction schemes the diffraction plane coincides with the incident OV waist plane. Based on the examples of Laguerre-Gaussian incident beams containing a spherical wavefront component, we demonstrate that the far-field diffracted beam profile possesses an asymmetry depending on the incident wavefront curvature and the TC sign. This finding enables simple and efficient ways for the simultaneous diagnostics of the TC magnitude and sign, which can be useful in many OV applications, including OV-assisted metrology and information processing.

High-power, low-lateral divergence broad area quantum cascade lasers with a tilted front facet

We introduce a simple technique to improve the beam quality of broad area quantum cascade lasers. Moderately tilted front facets of the laser provide suppression of higher order lateral waveguide modes. A device with a width of 60 μm and a front facet angle of 17° shows a nearly diffraction limited beam profile. In addition, the peak output power and the slope efficiency of the device are increased since most of the light inside the cavity is emitted through the tilted front facet by an asymmetric light intensity distribution along the cavity.

Theoretical Study of Optical Field Distribution of TeO2 Modulator Diffraction

In this paper, attention is focused on the analysis of the modulation transfer function form and its dependence on the normalized distance along the crystal (ξ) and peak phase delay (α). The numerical calculations show that the diffracted beam profile has very complicated waveform structure depending on both acoustic and optical parameters.

Enhancing image contrast using coherent states and photon number resolving detectors

We experimentally map the transverse profile of diffractionlimited beams using photon-number-resolving detectors.We observe strong compression of diffracted beam profiles for high detected photon number. This effect leads to higher contrast than a conventional irradiance profile between two Airy disk-beams separated by the Rayleigh criterion.

Control of duty ratio in waveguide gratings using a near-field holographic lithography system with a variable aperture

A waveguide grating coupler has been developed for optical coupling between a planar waveguide and an optical fiber by some researchers. In a waveguide grating coupler, the shape of diffracted output beam profiles and their focusing performance are dependent on the variations of duty ratios. Therefore, a control of duty ratio in waveguide grating is important for more efficient optical coupling. However, the patterning of a non-uniform duty ratio in waveguide grating is generally very difficult to realize. In our experiments, we propose a new simple fabrication method, which is a variable aperture system incorporating to near-field holographic (NFH) lithography system, to realize a non-uniform duty ratio in waveguide gratings.

Mapping local strain in thin film/substrate systems using x-ray microdiffraction topography

The authors report experimental data and modeling results for reflection microbeam x-ray topographs from a Si substrate strained by an overlying pseudomorphic SiGe film edge. The diffracted x-ray intensity from the Si substrate is strongly asymmetric as a function of distance from the film edge. A model of the diffracted intensity based on the classical Ewald–von Laue dynamical diffraction theory for an antisymmetric strain distribution indicates that the asymmetry in the diffracted beam profile is only due to the scattering process; individual intensity maxima in the intensity profile cannot be uniquely ascribed to individual features in the local strain distribution.

Beam-width-dependent filtering properties of strong volume holographic gratings

The finite dimension of the incident beam used to read out volume holographic gratings has interesting effects on their filtering properties. As the readout beam gets narrower, there is more deviation from the ideal response predicted for monochromatic plane waves. In this paper we experimentally explore beam-width-dependent phenomena such as wavelength selectivities, angular selectivities, and diffracted beam profiles. Volume gratings in both reflection and transmission geometries are investigated near 1550 nm. Numerical simulations utilizing the technique of Fourier decomposition provide a satisfactory explanation and confirm that the spread of spatial harmonics is the main contributing factor.

Three-dimensional converging–diverging Gaussian beam diffraction by a volume grating

The diffraction characteristics of a volume grating (VG) illuminated by a three-dimensional (3-D) converging-diverging Gaussian beam at conical incidence are investigated by applying 3-D finite-beam (FB) rigorous coupled-wave analysis (RCWA) based on the conventional 3-D RCWA in conjunction with two-dimensional plane-wave decomposition. The Gaussian beam is assumed to have an arbitrary incidence angle, an arbitrary azimuthal angle, and any linear polarization. The two cases with linear polarizations of the central beam of the Gaussian (E perpendicular K and H perpendicular K) are investigated. The diffraction efficiencies and the diffracted beam profiles for both unslanted VGs and slanted VGs (designed for substrate-mode optical interconnects) are presented. In general, the diffraction efficiencies of a converging-diverging spherical Gaussian beam diffracted by both unslanted VGs and slanted VGs increase and approach the central-beam results as the refractive-index modulation increases.

Polarization of holographic grating diffraction. I. General theory.

The full polarization property of volume holographic grating diffraction is investigated theoretically. With a simple volume grating model, the diffracted fields and Mueller matrices are first derived from Maxwell's equations by using the Green's function algorithms. The formalism is derived for the general case that the diffraction beam and the grating wave vector are not in the plane of incidence, where s waves and p waves are not decoupled. The derived photon-momentum relations determine the Bragg angle selectivity. The parameters of diffraction strength related to the hologram-writing process and material are defined and are not necessarily small in general. The diffracted-beam profiles are analytically calculated by using the known grating shape function. This theory has provided a fundamental understanding of the polarization phenomena of a real holographic diffraction grating device. The derived algorithm would provide a simulation-analysis tool for the engineering design of real holographic beam combiner/splitter devices.

Symmetric profile beams from waveguides with asymmetric grating couplers

An accurate method to analyze integrated optics waveguides with a grating is used to design diffractive gratings. A novel approach to produce symmetric diffracted beams is proposed. In this approach, we vary the grating duty cycle along the propagation direction. We demonstrate that this method results in symmetrical diffracted beam profiles.

Absorption effects in photorefractive volume-holographic memory systems I Beam depletion

We consider how absorption-induced beam depletion alters the index-modulation profile, the diffracted beam profile, and the Bragg selectivity of volume holograms. We derive theoretical equations that consider beam depletion (in the absence of two-beam coupling) and compare these with experimental observations. We also derive a smart scheduling technique whereby such absorption effects can be compensated for during the scheduled recording of multiple holograms. In our smart scheduling approach, during recording the profile of the input signal beam is modified such that during recall its output profile is uniform.

Analysis and observation of finite beam Bragg diffraction by a thick planar phase grating

A numerical-impulse-response technique for studying the propagation and diffraction of finite-width beams in planar phase gratings is described. It can account for both symmetric and asymmetric diffractions. The grating-length-to-beam-width ratio is shown to govern the extent of beam-profile distortion and selectivity sidelobe suppression. Trade-offs between diffraction efficiency and beam profile have also been demonstrated. Theoretical results have been verified by experimental observations in a planar waveguide geometry of diffracted beams that change from a single diffraction peak to multiple peaks as the grating-length-to-beam-width ratio increases.

In-Situ Characterization of Growth and Intermixing at a Heteroepitaxial Interface: Fe on Au(001)

ABSTRACTThe growth and chemical intermixing of submonolayer and a few monolayer thick Fe films on a Au(001) surface was studied by High Resolution Low Energy Electron Diffraction (HRLEED) technique. Through the analysis of the energy dependent angular profiles as a function of time, we obtained the distribution of islands and distribution of spacings during submonolayer growth. The interference of electron waves from different chemical elements in terraces at different heights in the surface contributes to the background intensity and broadening in the angular profiles of diffraction beams. A subsurface Fe capped by Au islands as a result of atomic place exchange was observed at the initial stage of monolayer growth. From the energy dependent angular profiles as a function of temperature, we determine the quantitative change of inhomogeneity length (∼20 Å) at the interface of ultrathin films at elevated temperatures due to intermixing.

Influence of grating shapes on beam profiles for right-angle guided-wave diffraction

We present simulation results on the influence of grating shapes on diffracted beam profiles for right-angle diffraction. The first-order two-dimensional coupled-mode equations are solved numerically by using the finite-difference method for As(2)S(3) waveguide surface-relief gratings on titanium-indiffused planar waveguides. The simulation results show that square or rectangular gratings result in a distorted profile for the diffracted beam and that truncated grating shapes significantly improve the output beam quality.

High resolution LEED studies of surface misorientation and incommensurate domain walls on reconstructed Pt(110) surface

Using the unique capability of the high-resolution LEED, we can precisely measure the Bragg peak positions and angular profiles of electron diffraction beams scattered from a crystal surface. The observations reported here relate to a reconstructed Pt(110)(1 × 2) surface at room temperature. Oscillations of the Bragg peak position and periodic variations of the FWHM of both the superlattice and integral beams along the reciprocal rods over a wide range of energies have been studied. The oscillation of the peak position of the superlattice beam results from phase shifts of electrons scattering from incommensurate (1 × 2) reconstruction domain walls between adjacent terraces. On the other hand, the oscillation of the integral-order beam is due to a surface misorientation. Furthermore, the periodic variation of the FWHM of the superlattice beam is dominated by the degree of surface misorientations. However, the periodic variation of the FWHM associated with the integral beam results mainly from the existence of steps. Comparing the measurements with an existing model we have estimated the degree of the surface misorientation, the distribution of the incommensurate domain walls, and the lateral incommensurability on the Pt(110)( l × 2) surface.

High resolution LEED studies of surface misorientation and incommensurate domain walls on reconstructed Pt(110) surface

Using the unique capability of the high-resolution LEED, we can precisely measure the Bragg peak positions and angular profiles of electron diffraction beams scattered from a crystal surface. The observations reported here relate to a reconstructed Pt(110)(1 × 2) surface at room temperature. Oscillations of the Bragg peak position and periodic variations of the FWHM of both the superlattice and integral beams along the reciprocal rods over a wide range of energies have been studied. The oscillation of the peak position of the superlattice beam results from phase shifts of electrons scattering from incommensurate (1 × 2) reconstruction domain walls between adjacent terraces. On the other hand, the oscillation of the integral-order beam is due to a surface misorientation. Furthermore, the periodic variation of the FWHM of the superlattice beam is dominated by the degree of surface misorientations. However, the periodic variation of the FWHM associated with the integral beam results mainly from the existence of steps. Comparing the measurements with an existing model we have estimated the degree of the surface misorientation, the distribution of the incommensurate domain walls, and the lateral incommensurability on the Pt(110)(l × 2) surface.

Formation of Turing structures in catalytic surface reactions: The facetting of Pt(110) in CO+O2

The Pt(110) surface facets during the catalytic oxidation of CO, if reaction conditions are adjusted such that the CO‐induced 1×1⇄1×2 phase transition can take place simultaneously. A detailed low‐energy electron diffraction beam profile analysis revealed that regularly spaced (430) and (340) facets are formed with a lateral periodicity of ∼70 lattice units along the [11̄0] direction. This result, together with the observation that the facetted surface is only stable under reaction conditions, indicates a dissipative structure of the Turing type. Such structures, which are stationary but exhibit a periodic variation of the concentration variables in space, have so far almost exclusively been discussed theoretically. The interpretation of the facetted surface as a Turing structure could be confirmed by a Monte Carlo simulation based on the Langmuir Hinshelwood mechanism of catalytic CO oxidation and the CO‐induced 1×1⇄1×2 phase transition.