Oblique Incidence Geometry Research Articles

Second-order optical differentiation of a 3D light beam at oblique incidence using a multilayer metal-dielectric structure

We investigate the optical implementation of a second-order differentiation operation using a metal-dielectric layered structure in the oblique incidence geometry. It is shown that the transformation of the profile of a three-dimensional light beam occurring upon its reflection from a layered structure can be described using the theory of linear systems. The transfer function of the corresponding linear system is obtained, and it is shown that if a layered structure has a reflection zero of the second order with respect to the spatial frequency for one of the polarizations, the transformation performed by the structure corresponds to the weighted sum of the second derivatives of the incident beam profile with respect to the spatial coordinates. Using the presented theoretical description, layered metal-dielectric structures for computing the second derivative with respect to one of the spatial coordinates and for computing the Laplace operator of the profile of a three-dimensional linearly polarized light beam are calculated. The presented numerical simulation results demonstrate high-quality computation of these operators.

Optical computation of the Laplace operator at oblique incidence using a multilayer metal-dielectric structure.

We investigate the possibility of the optical computation of the Laplace operator in the oblique incidence geometry using a layered structure consisting of a set of homogeneous thin films. For this, we develop a general description of the diffraction of a three-dimensional linearly polarized optical beam by a layered structure at oblique incidence. Using this description, we derive the transfer function of a multilayer structure consisting of two three-layer metal-dielectric-metal structures and possessing a second-order reflection zero with respect to the tangential component of the wave vector of the incident wave. We show that under a certain condition, this transfer function can coincide up to a constant multiplier with the transfer function of a linear system performing the computation of the Laplace operator. Using rigorous numerical simulations based on the enhanced transmittance matrix approach, we demonstrate that the considered metal-dielectric structure can optically compute the Laplacian of the incident Gaussian beam with the normalized root-mean-square error of the order of 1%. We also show that this structure can be effectively utilized for optical edge detection of the incident signal.

Integrated diffraction gratings on the Bloch surface wave platform supporting bound states in the continuum

Abstract We propose and theoretically and numerically investigate integrated diffraction gratings for the Bloch surface wave (BSW) platform, which have subwavelength or near-subwavelength period. We demonstrate that, in the oblique incidence geometry of a transverse-electric polarized BSW and with a properly chosen band gap configuration of the photonic crystal supporting the surface waves, the proposed structures operate in the scattering-free regime, when the energy of the incident BSW is divided between the reflected and transmitted BSWs with the same polarization corresponding to the propagating diffraction orders of the grating, and not scattered away from the propagation surface. In this regime, the studied integrated gratings support high-Q resonances and bound states in the continuum not only in the subwavelength case when only the specular (zeroth) diffraction orders propagate, but also in the case when non-evanescent zeroth and −1st diffraction orders satisfy the so-called Littrow mounting condition. The proposed integrated gratings on the BSW platform can be used as efficient narrowband spatial or spectral filters operating in reflection, or as BSW beam splitters or deflectors operating in transmission. The obtained results may find application in two-dimensional photonic circuits for steering the BSW propagation.

Controlling acoustic waves using magneto-elastic Fano resonances

We propose and analyze theoretically a class of energy-efficient magnetoelastic devices for analog signal processing. The signals are carried by transverse acoustic waves while the bias magnetic field controls their scattering from a magnetoelastic slab. By tuning the bias field, one can alter the resonant frequency at which the propagating acoustic waves hybridize with the magnetic modes, and thereby control transmission and reflection coefficients of the acoustic waves. The scattering coefficients exhibit Breit-Wigner/Fano resonant behavior akin to inelastic scattering in atomic and nuclear physics. Employing oblique incidence geometry, one can effectively enhance the strength of magnetoelastic coupling, and thus countermand the magnetic losses due to the Gilbert damping. We apply our theory to discuss potential benefits and issues in realistic systems and suggest routes to enhance the performance of the proposed devices.

Thermal radiation in one-dimensional photonic quasicrystals with graphene

In this work we investigate the thermal power spectra of the electromagnetic radiation through one-dimensional stacks of dielectric layers, with graphene at their interfaces, arranged according to a quasiperiodic structure obeying the Fibonacci (FB), Thue-Morse (TM) and double-period (DP) sequences. The thermal radiation power spectra are determined by means of a theoretical model based on a transfer matrix formalism for both normal and oblique incidence geometries, considering the Kirchhoff's law of thermal radiation. A systematic study of the consequences of the graphene layers in the thermal emittance spectra is presented and discussed. We studied also the radiation spectra considering the case where the chemical potential is changed in order to tune the omnidirectional photonic band gap.

Nanoscale Strain Mapping in SIMOX 3-D Sculpted Silicon Waveguides Using Tip-Enhanced Raman Spectroscopy

Strain in silicon plays a significant role in exploring electro-optical material, boosting transistor performance, tuning birefringence of optical silicon waveguides, and so on. In this paper, we measured, for the first time, the nanoscale strain in the SIMOX 3-D sculpted silicon waveguides using tip-enhanced Raman spectroscopy (TERS). A model, which relates the observed Raman peak shifts to the localized stresses for our TERS experiments, was presented. The tip-induced electric-field enhancements, tip-induced depolarization of incident light, and oblique incidence geometry of the TERS system were included in the model. Both polarizations of incident light and Raman scattered light are selected appropriately with the guidance of the model for obtaining enough electric-field enhancements and accurately calculate the stresses. A 2-D stress map inside the silicon waveguide with spatial resolution of about 20 nm was obtained, and corresponding strains were calculated based on Hooke's law. We observed that the stresses are compressive, and the strains show inhomogeneity. The origins of strain and the strain-induced second-order optical susceptibility inside the SIMOX 3-D sculpted buried silicon waveguides are discussed and analyzed. The mapping of 2-D nanoscale strain and the analysis of second-order optical susceptibility in this work suggest that the SIMOX 3-D sculpted strained silicon might be a potential metamaterial for electro-optical modulation and optical signal processing. Moreover, the presented TERS model could be a valuable tool for probing strain in strained silicon devices.

Spin‐Structured Multilayers: A New Class of Materials for Precision Spintronics

Magnetoelectronic multilayer devices are widely used in today's information and sensor technology. Their functionality, however, is limited by the inherent properties of magnetic exchange or dipolar coupling which constrain possible spin configurations to collinear or perpendicular alignments of adjacent layers. Here, a deposition procedure is introduced that allows for a new class of layered materials in which complex spin structures can be accurately designed to result in a multitude of new and precisely adjustable spintronic and magnetoresistive properties. The magnetization direction and coercivity of each individual layer are determined by the deposition process in oblique incidence geometry and can be completely decoupled from neighboring layers. This applies for layers of any ferromagnetic material down to layer thicknesses of a few nm and lateral dimensions of a few 100 nm, enabling the design of efficient and compact magnetoelectronic devices, encompassing precision magnetoresistive sensors as well as layer systems with multiple addressable remanent states for magnetic memory applications.

Performance of low-cost X-ray area detectors with consumer digital cameras

We constructed X-ray detectors using consumer-grade digital cameras coupled to commercial X-ray phosphors. Several detector configurations were tested against the Varian PaxScan 3024M (Varian 3024M) digital flat panel detector. These include consumer cameras (Nikon D800, Nikon D700, and Nikon D3X) coupled to a green emission phosphor in a back-lit, normal incidence geometry, and in a front-lit, oblique incidence geometry. We used the photon transfer method to evaluate detector sensitivity and dark noise, and the edge test method to evaluate their spatial resolution. The essential specifications provided by our evaluation include discrete charge events captured per mm2 per unit exposure surface dose, dark noise in equivalents of charge events per pixel, and spatial resolution in terms of the full width at half maximum (FWHM) of the detector`s line spread function (LSF). Measurements were performed using a tungsten anode X-ray tube at 50 kVp. The results show that the home-built detectors provide better sensitivity and lower noise than the commercial flat panel detector, and some have better spatial resolution. The trade-off is substantially smaller imaging areas. Given their much lower costs, these home-built detectors are attractive options for prototype development of low-dose imaging applications.

Theory of oblique and grazing incidence Talbot‑Lau interferometers and demonstration in a compact source x‑ray reflective interferometer

With the advent of Talbot-Lau interferometers for x-ray phase-contrast imaging, oblique and grazing incidence configurations are now used in the pursuit of sub-micron grating periods and high sensitivity. Here we address the question whether interferometers having oblique incident beams behave in the same way as the well-understood normal incidence ones, particularly when the grating planes are non-parallel. We derive the normal incidence equivalence of oblique incidence geometries from wave propagation modeling. Based on the theory, we propose a practical method to correct for non-parallelism of the grating planes, and demonstrate its effectiveness with a polychromatic hard x-ray reflective interferometer.

Anomalous patterned scattering spectra of one-dimensional porous silicon photonic crystals

Far-field secondary emission spectra of one-dimensional periodic photonic structures based on porous silicon show characteristic co-focal rings centered close to the structure plane normal. The rings appear when the frequency of picosecond excitation laser pulses is tuned to the edges of the fourth photonic band gap. They can be clearly distinguished from the typical reflected and transmitted light in the oblique incidence geometry. The rings number is dependent on the excitation frequency and the incidence angle. We explain these anomalous spectral features of porous silicon structures by the spectral filtering of light elastically scattered inside the photonic structure by the narrow photonic bands. The elastic scattering of light due to the photonic disorder in the structure causes the appearance of secondary waves propagating in any direction. But only those waves which fall into the allowed photonic bands penetrate through the whole structure and move through its front or back surfaces. The observed patterned secondary emission is an example of efficient photonic engineering by simple means of multilayer porous silicon structures.

Investigations of plasma jet interaction with ambient gases by multi-frame interferometric and X-ray pinhole camera systems

AbstractInteractions of laser driven plasma jets with He and Ar gas puffs was investigated experimentally by means of three-frame interferometric/shadowgraphic system and three-frame X-ray pinhole camera. A defocused iodine laser beam using the Prague Asterix Laser System (PALS) interacting with massive planar Cu targets generated high-speed well-collimated plasma jets. The PALS third harmonic (0.438 µm), with pulse duration of 250 ps (full width at half maximum), and energy of 100 J was employed in two irradiation geometries: with an incidence normal to the target surface and with an oblique one (30° with respect to the target normal), in order to minimize the heating of the ambient gas by the laser beam. The results of these interaction experiments, in particular, those obtained in case of the oblique incidence geometry, are presented and discussed. They show the effect of the double shock formation in ambient gases: starting by the ablative plasma action, followed by that of the jet.

Thermal radiation in quasiperiodic photonic crystals

In this work we investigated the thermal power spectrum of the electromagnetic radiation through one-dimensional stacks of alternating negative and positive refractive index layers, arranged as truncated quasiperiodic photonic structures obeying the Fibonacci (FB), Thue–Morse (TM), and double-period (DP) sequences. The thermal radiation power spectra are determined by means of a theoretical model based on a transfer matrix formalism for both normal and oblique incidence geometries, together with Kirchoff's second law. We studied the radiation spectra by considering the case where both refractive indices of layers A and B are assumed to be a constant, as well as a more realistic case which takes into account the frequency-dependent electric permittivity ε and magnetic permeability μ to characterize the negative refractive index n in layer B.

Surface roughness effects on the terahertz reflectance of pure explosive materials

We studied the effect of surface roughness on terahertz reflectance spectra of explosives at both quasinormal and oblique incidence by Fourier-transform spectroscopy. The optical constants of 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane and 1,3,5 trinitro-1,3,5 triazacyclohexane were determined from the reflectance spectra of pure optically flat pellets and clear spectral signatures were found in the 0.5–8THz range. In the case of a realistic surface roughness of several tens of microns, the observation of the spectral signatures at quasinormal incidence is hindered by the specular reflectance roll-off at terahertz frequencies. On increasing the angle of incidence, the roll-off occurred at higher frequency and the surface roughness was optically measured. Oblique incidence geometries can benefit the development of terahertz standoff detection devices.

Intense single attosecond pulses from surface harmonics using the polarization gating technique

Harmonics generated at solid surfaces interacting with relativistically strong laser pulses are a promising route towards intense attosecond pulses. In order to obtain single attosecond pulses one can use few-cycle laser pulses with carrier-envelope phase stabilization. However, it appears feasible to use longer pulses using polarization gating—the technique known for a long time from gas harmonics. In this paper, we investigate in detail a specific approach to this technique on the basis of one-dimensional-particle-in-cell (1D PIC) simulations, applied to surface harmonics. We show that under realistic conditions polarization gating results in significant temporal confinement of the harmonics emission allowing thus the generation of intense single attosecond pulses. We study the parameters needed for gating only one attosecond pulse and show that this technique is applicable to both normal and oblique incidence geometry.

Thermal radiation in quasiperiodic photonic crystals with negative refractiveindex

In this work we investigated the thermal power spectrum of the electromagnetic radiationthrough one-dimensional stacks of alternating negative and positive refractive index layers,arranged as truncated quasiperiodic photonic structures obeying the Fibonacci (FB),Thue–Morse (TM), and double-period (DP) sequences. The thermal radiation power spectraare determined by means of a theoretical model based on a transfer matrix formalism forboth normal and oblique incidence geometries, together with Kirchoff’s second law. Westudied the radiation spectra by considering the case where both refractive indices of layersA and B are assumed to be a constant, as well as a more realistic case whichtakes into account the frequency-dependent electric permittivityϵ and magneticpermeability μ to characterize the negative refractive indexn inlayer B.

Alignment and switching behaviors of liquid crystal on a-SiOx thin films deposited by a filtered cathodic arc process

A new method is described for the preparation of silicon oxide layers, which produces vertical alignment of liquid crystal with controlled pretilt angles, by a reactive filtered cathodic arc deposition under oblique incidence geometry. The pretilt angle is dependent on the angle of deposition, but is not simply caused by the surface roughness. The achievable pretilt angle by this method is ∼5.5°, which allows uniform switching behavior under an applied electric field.

Bi 3 Fe 5 O 12 ∕ Gd 3 Ga 5 O 12 ] m magneto-optical photonic crystals

A series of one-dimensional heteroepitaxial all-garnet magneto-optical photonic crystals has been synthesized and optically characterized. They are composed of alternating magneto-optically active Bi3Fe5O12 (BIG) and Gd3Ga5O12 (GGG) quarter-wave length layers pulsed laser deposited on to the GGG(001) substrate. Photonic crystals, as designed, exhibit stop band structure and the band gap with the transmittance central peak caused by the light localization in the half-wave length BIG cavity. Compared to previously reported [Bi3Fe5O12∕Y3Fe5O12]m photonic crystal [Appl. Phys. Lett. 84, 1438 (2004)], the replacement of optically dense Y3Fe5O12 garnet by transparent Gd3Ga5O12 enables significant enhancement of light rejection within the stop band. Photonic crystals spectra experience “blue” shift in oblique incidence geometry thus demonstrating feasibility to use this effect for tunable magneto-optical filters.

Direct measurement of ultrafast surface displacement in laser picosecond acoustics

Picosecond acoustic pulses generated in solids with ultrashort optical pulses can be detected through the reflectance or phase changes of optical probe pulses. The detection relies on acousto-optic coupling resulting from a combination of the photoelastic effect and transient surface displacements. The acoustic strain pulse shape gives access to the ultrafast dynamics of the electrons that influence the generation process. This pulse shape is directly related to the contribution to the probe light modulation resulting from surface displacement. Here we present a method that allows the discrimination between the photoelastic and surface displacement contributions using a novel oblique optical incidence geometry, allowing the strain pulse shape to be directly obtained in a single measurement.

Oblique-incidence illumination and collection for depth-selective fluorescence spectroscopy

Optimization of device-tissue interface parameters may lead to an improvement in the efficacy of fluorescence spectroscopy for minimally invasive disease detection. Although illumination-collection geometry has been shown to have a strong influence on the spatial origin of detected fluorescence, devices that deliver and/or collect light at oblique incidence are not well understood. Simulations are performed using a Monte Carlo model of light propagation in homogeneous tissue to characterize general trends in the intensity and spatial origin of fluorescence detected by angled geometries. Specifically, the influence of illumination angle, collection angle, and illumination-collection spot separation distance are investigated for low and high attenuation tissue cases. Results indicate that oblique-incidence geometries have the potential to enhance the selective interrogation of superficial or subsurface fluorophores at user-selectable depths up to about 0.5 mm. Detected fluorescence intensity is shown to increase significantly with illumination and collection angle. Improved selectivity and signal intensity over normal-incidence geometries result from the overlap of illumination and collection cones within the tissue. Cases involving highly attenuating tissue produce a moderate reduction in the depth of signal origin. While Monte Carlo modeling indicates that oblique-incidence designs can facilitate depth-selective fluorescence spectroscopy, optimization of device performance will require application-specific consideration of optical and biological parameters.

On the determination of molecular orientation from polarized imaging in second-harmonic microscopy

The polarized imaging of a monolayer film in second-harmonic microscopy in oblique incidence geometry has been analyzed theoretically. It has been demonstrated that both azimuthal and tilt angles of molecules in the film can be determined from measurements of relative second harmonic intensities at different orientations of a polarizer or an analyzer.