Beam Shift Research Articles

Bistable reflection and beam shifts with excitation of surface plasmons in a saturable absorbing medium.

We investigate the nonlinear reflection of a light beam from a Kretschmann configuration with saturable absorbing medium. The absorption of medium has direct influence on the intrinsic loss of the system, thus affecting the reflectivity and the phase variation when the surface plasmons are resonantly excited. As the incident power changes, the reflectivity can be switched between high and low values and exhibits absorptive optical bistability as a result of the inherent positive feedback by the intensity-dependent saturation effect. The Goos-Hänchen and the Imbert-Fedorov shifts of the reflected beam have the same bistable behavior as the reflectance. The effects of the thickness of metal film and the linear absorption coefficient on the hysteresis loop are analyzed in detail by considering the system losses and the saturated absorption. The bistable reflection and beam shifts may have applications in all-optical devices, such as optical switching.

Vectorial spin Hall effect of light upon tight focusing.

The spin Hall effect of light is a manifestation of angular momentum conservation in the process of spin-orbit interaction of light. This optical Hall effect is exhibited in tight focusing of a circularly polarized asymmetric input beam as a shift of the center of gravity of the focal spot in the transverse plane, perpendicular to the direction/axis of symmetry breaking. It is commonly established that the direction of this shift depends on the sign of the spin. Here we show, for the first time, to the best of our knowledge, both analytically and by numerical simulation, that different Cartesian components of an asymmetric circularly polarized focused beam shift in opposite directions by different amounts. Moreover, these shifts depend on the type and degree of the asymmetry and thus can be tuned/controlled. We show how these field components' shifts are related to spin and orbital angular momentum shifts. These findings shed new light on the spin optical Hall effect, facilitate new/simpler ways to measure it, and may broaden the gamut of its applications in manipulation and trapping of particles by light and precision metrology.

Autofocusing self-imaging: symmetric Pearcey Talbot-like effect.

The Talbot-like effect of symmetric Pearcey beams (SPBs) is presented numerically and experimentally in the free space. Owing to the Talbot-like effect, the SPBs have the property of periodic, multiple autofocusing and self-healing. Meanwhile, the focusing positions and focusing times of SPBs are controlled by the beam shift factor and the distribution factors. Furthermore, the beam shift factor can also affect the Talbot-like effect and the Talbot period. It is believed that the results can diversify the application of the Talbot effect.

Applications of Machine Learning to Improve the Clinical Viability of Compton Camera Based in vivo Range Verification in Proton Radiotherapy.

We studied the application of a deep, fully connected Neural Network (NN) to process prompt gamma (PG) data measured by a Compton camera (CC) during the delivery of clinical proton radiotherapy beams. The network identifies 1) recorded “bad” PG events arising from background noise during the measurement, and 2) the correct ordering of PG interactions in the CC to help improve the fidelity of “good” data used for image reconstruction. PG emission from a tissue-equivalent target during irradiation with a 150 MeV proton beam delivered at clinical dose rates was measured with a prototype CC. Images were reconstructed from both the raw measured data and the measured data that was further processed with a neural network (NN) trained to identify “good” and “bad” PG events and predict the ordering of individual interactions within the good PG events. We determine if NN processing of the CC data could improve the reconstructed PG images to a level in which they could provide clinically useful information about the in vivo range and range shifts of the proton beams delivered at full clinical dose rates. Results showed that a deep, fully connected NN improved the achievable contrast to noise ratio (CNR) in our images by more than a factor of 8x. This allowed the path, range, and lateral width of the clinical proton beam within a tissue equivalent target to easily be identified from the PG images, even at the highest dose rates of a 150 MeV proton beam used for clinical treatments. On average, shifts in the beam range as small as 3 mm could be identified. However, when limited by the amount of PG data measured with our prototype CC during the delivery of a single proton pencil beam (~1 × 109 protons), the uncertainty in the reconstructed PG images limited the identification of range shift to ~5 mm. Substantial improvements in CC images were obtained during clinical beam delivery through NN pre-processing of the measured PG data. We believe this shows the potential of NNs to help improve and push CC-based PG imaging toward eventual clinical application for proton RT treatment delivery verification.

Tunable giant two-dimensional optical beam shift from a tilted linear polarizer

We demonstrate an intriguing giant optical beam shift in a tilted polarizer system analogous to the Imbert-Fedorov shift in partial reflection around the Brewster's angle of incidence. We explain this giant shift using a generalized theoretical treatment for beam shift in a tilted uniaxial anisotropic material incorporating the three-dimensional orientation of its optic axis in the laboratory frame. A regulated control and tunability of both the magnitude and direction of the shifts are demonstrated further by wisely tuning the input polarization and the corresponding orientation angles. These findings open up the possibility of precise micron-level beam steering using a simple linear polarizer.

Spatial shifts of reflected beams from surface polaritons in antiferromagnets

Goos–Hänchen (GH) and Imbert–Fedorov (IF) shifts of reflected beams and the effects of two Dyakonov surface magnon polaritons (DSMPs) on these shifts are studied in the Otto configuration with an antiferromagnetic substrate. A modified dispersion equation of the DSMPs is proposed to recognize GH and IF shifts related to the DSMPs. Reflected beam shift spectra are simulated for a F e F 2 substrate. Shift features around the two DSMPs are observed by frequency scanning with a fixed incident angle. The GH-shift amplitude corresponding to either DSMP is situated between the immediately adjacent maximum and minimum of a GH-shift curve. The shift-amplitude maximum of an IF-shift curve corresponds to the relevant DSMP accurately. GH and IF shifts also are given by frequency scanning along dispersion curves.

Investigation of the Shadow Effect in Focused Ion Beam Induced Deposition.

Due to the precursor gas flow in the focused ion beam induced deposition process, a shadow effect appears behind the shading structures. This article carries out experiments with phenanthrene as the precursor gas and establishes a numerical model to define the shadow area and estimate the intensity of the shadow effect, considering the morphology of shading structure, the beam shift, and the nozzle parameters. Within the shadow area, the precursor molecule adsorption contribution is estimated by calculating the fraction of precursor gas flow in a specific direction. Finally, the number of precursor molecules within the beam impact area influenced by the shadow effect is obtained, emphasizing the important role of gas surface diffusion. The adsorption contribution within the shadow area differs a lot while deposited structures are similar in height. The error between the simulation and the experimental results is about 5%, verifying the accuracy of the proposed model.

Electron beam offset welding to ameliorate metallurgical compatibility and mechanical performance of refractory metal/Ni-base superalloy dissimilar alloys: Nb/GH3128

Electron beam welding of Nb/GH3128 dissimilar alloys was conducted to obtain crack-free joints. The beam offset process contributed to the improvement of microstructure, and segregation behavior of alloying elements, as well as the elevation of mechanical properties of joints. Cracked joints were obtained as electron beam shift to GH3128 superalloy with 0.5 mm. Brittle Ni6Nb7/Laves reaction layer combined with a great number of brittle Laves phase and γ/Laves eutectic in fusion zone acted as the intrinsic factors for cracking. Relative low segregation propensity of alloying elements helped to the promotion of generation of γ/Laves eutectic. As the beam offset value increased from 0.5 mm to 0.9 mm, the tendency of alloying elements to segregate was intensified, promoting the Laves phase and γ/Laves eutectic to be replaced by γ phase, and the hardness of the joints was gradually decreased from 594 HV to 340 HV correspondingly. High hardness is detected in the reaction layer between Nb and the fusion zone, which triggered the brittle fracture of all joints. The joint obtained with a beam offset value of 0.8 mm possess the highest tensile strength 163 MPa, 65% of Nb base metal.

Towards the interpretation of a shift of the central beam in nano-beam electron diffraction as a change in mean inner potential

The measurement of electric fields in scanning transmission electron microscopy (STEM) is a highly investigated field of research. The constant improvement of spatial resolution in STEM and the development of new hardware for the fast acquisition of diffraction patterns even paved the way for the measurement of atomic electric fields. Although the basic principle that an electric field leads to a tilt of the focussed electron probe that can be detected as a shift of the diffraction pattern in the back focal plane of the objective lens seems quite simple, many challenges arose in the measurement of fields in a quantitative way. In the present study we investigate whether a shift of the diffraction pattern that occurs at an interface between two materials can be related to the electric field which is caused by the difference of the mean inner potentials of the two materials. To this end, experiments and simulations are compared. It is demonstrated that the difference in mean inner potential has an influence on the observed effect, but a quantitative interpretation is difficult. The influence of image recording effects such as shot noise and the modulation transfer function are investigated as well as further effects such as e.g. sample tilt. In addition, the influence of the observed effect on a strain measurement is shown.

Sensitivity of Microwave Interferometer in the Limiter Shadow to filaments in ASDEX upgrade

AbstractMicrowave Interferometer in the Limiter Shadow (MILS) is a new diagnostic, installed on ASDEX Upgrade for electron density measurements in the far Scrape‐Off Layer (SOL). At the chosen frequency of 47 GHz, the region of measurements varies within several centimetres before and after the limiter, depending on the density. 200 kHz bandwidth allows resolving transient events such as edge localized modes filaments and turbulence filaments. The measured quantities, phase shift, and power decay of the microwave beam, which crosses the plasma, are directly connected to the density and do not depend on any other plasma quantity. In this work, we analyse the influence of a filamentary perturbation on MILS signals. Simple representation of a filament is adopted, with parameters relevant to experimental filament properties, reported for ASDEX Upgrade. Forward modelling is done in COMSOL software by using RAPLICASOL, to study the response of the MILS synthetic diagnostic to the presence of a filament. Qualitative and quantitative dependencies are obtained and the boundaries of MILS sensitivity to filaments, or to the density perturbation in far SOL in general, are outlined.

Electron beam splitting at topological insulator surface states and a proposal for electronic Goos-Hänchen shift measurement

The hexagonal warping effect on transport properties and Goos-H\"anchen (GH) lateral shift of electrons on the surface of a topological insulator with a potential barrier is investigated theoretically. Due to the warped Fermi surface for incident electron beams, we can expect two propagating transmitted beams corresponding to the occurrence of double refraction. The transmitted beams have spin orientations locked to their momenta so one of the spin directions rotates compared to the incident spin direction. Based on a low-energy Hamiltonian near the Dirac point and considering Gaussian beams, we derive expressions for calculating lateral shifts in the presence of warping effect. We study the dependence of transmission probabilities and GH shifts of transmitted beams on system parameters in detail by giving an explanation for the appearance of large peaks in the lateral shifts corresponding to their transmission peaks. It is shown that the separation between two transmitted beams through their different GH shifts can be as large as a few micrometers, which is large enough to be observed experimentally. Finally, we propose a method to measure the GH shift of electron beams based on the transverse magnetic focusing technique in which, by tuning an applied magnetic field, a detectable resonant path for electrons can be induced.

Beam Shift Modulation Based on Larmor Resonance in Graphene–VO2 Photonic Crystal

A new method to enhance the beam shift of Goos–Hänchen (GH) effect in a 1D photonic crystal based on Larmor resonance in terahertz (THz) region is proposed. Larmor resonance can enhance the surface plasmon resonance (SPR) effect, which enhances the GH effect. A 1D photonic crystal with a graphene–VO2 periodic structure is designed. As a result of different states at different temperatures, the SPR effect of graphene–VO2 periodic structure photonic crystals will change. In this structure, Larmor resonance caused by a strong magnetic field perpendicular to the incident surface will resonate with surface plasmons. It is found that the strength of the magnetic field required to enhance SPR is relative to the frequency of electromagnetic waves and the Fermi level. The enhancement of the GH effect by the Larmor resonance theory provides a new way for us to measure strong magnetic fields.

Topological Imbert–Fedorov shifts in silicene

In this paper, we study the topological Imbert–Fedorov (IF) shifts of a light beam reflected from a silicene–substrate system in the presence of an externally applied electric field and circularly polarized beam. Silicene is a topological material and due to the strong spin–orbit interaction has a rich phase diagram. Using the famous Kubo formalism, we derive analytical expressions for the longitudinal and transverse Hall conductivities of silicene. By employing angular spectrum analysis, the analytical expression of the spatial IF shift is obtained. We show that electric field/circularly polarized light beam modulated positive and negative spatial beam shifts can be obtained by tuning the angle of incidence near the Brewster angle and the incident photonic energy in the terahertz regime. Furthermore, we also investigate the IF shifts in different topological phases of the silicene by driving the system through distinct topological quantum phase transitions (TQPTs), i.e., from topologically nontrivial to a semi-metallic state and further to a band insulating state by manipulating the interplay between the electric field and circularly polarized light beam. We believe these results are helpful for developing novel optoelectronic, spintronic, and valleytronic devices and may provide a possible technique for probing Berry curvature, topological Chern numbers, and TQPTs by direct optical measurement.

Photonic Laser Thruster: Optomechanical and Quantum Electronical Analyses

The present paper reports optomechanical and quantum electronical analyses of the mechanism of photonic laser thruster (PLT) in achieving spacecraft velocities on the order of by using macroscale laser-like optical resonators. One remarkable result is that the resonance condition of the PLT optical cavity imposes the magnitude of the dynamic frequency shift of the circulating laser beam wavefunction by the moving mirror to equal that of the single-reflection Doppler shift of a nonresonant laser beam. However, their mechanisms are proven completely different. The optomechanical dynamics of PLT was analytically investigated and the chief results are as follows: 1) the maximum achievable PLT spacecraft velocity with existing laser technologies is estimated to be ; 2) for interplanetary propulsion applications the optimum range of the intrinsic PLT cavity power loss factors from absorption, scattering, and diffraction is ; and 3) the efficiency of transforming the laser energy to the spacecraft kinetic energy increases over 40% at velocities greater than . The present results validate the theoretical feasibility of PLT to enable unprecedentedly fast interplanetary transportation systems, such as the Photonic Railways. The required engineering and manufacturing capabilities for implementing the interplanetary PLT are discussed.

Goos-Hänchen shift at Brillouin light scattering by a magnetostatic wave in the Damon-Eshbach configuration [Invited

The lateral shift of an optical beam undergoing Brillouin light scattering by a spin wave propagating along the interface between magnetic and dielectric media (Damon-Eshbach configuration) in the total internal reflection geometry is studied theoretically. Linear and quadratic magneto-optic terms in polarization are taken into account. It is shown that the lateral shift depends on the polarization (s- or p-) state of the scattered electromagnetic wave as well as on the frequency of the spin wave.

Goos–Hänchen and Imbert–Fedorov shifts in tiltedWeyl semimetals

We establish the beam models of Goos–Hänchen (GH) and Imbert–Fedorov (IF) effects in tilted Weyl semimetals (WSMs), and systematically study the influences of Weyl cone tilting and chemical potential on the GH and IF shifts at a certain photon energy 1.96 eV. It is found that the GH and IF shifts in tilted type-I and type-II WSMs are both almost symmetric about the Weyl cone tilting. Meanwhile, the GH and IF shifts in type-I WSMs almost do not change with the tilt degree of Weyl cones, while those in type-II WSMs are extremely dependent on tilt degree. These trends are mainly due to the nearly symmetric distribution of WSMs conductivities, where the conductivities keep stable in type-I WSMs and gradually decrease with tilt degree in type-II WSMs. By adjusting the chemical potential, the boundary between type-I and type-II WSMs widens, and the dependence of the beam shifts on the tilt degree can be manipulated. Furthermore, by extending the relevant discussions to a wider frequency band, the peak fluctuation of GH shifts and the decrease of IF shifts occur gradually as the frequency increases, and the performance of beam shifts at photon energy 1.96 eV is equally suitable for other photon frequencies. The above findings provide a new reference for revisiting the beam shifts in tilted WSMs and determining the types of WSMs.

Reflection and transmission of an Airy beam in a dielectric slab

The reflection and transmission of a finite-power Airy beam incident on a dielectric slab are investigated by an analytical method. Based on the plane-wave angular spectrum expansion and Fresnel approximation, the analytical expressions of the reflected field, internal field as well as transmitted field in each region are obtained. Through numerical simulations, the intensity distributions of the incident beam, reflected beam, internal beam as well as transmitted beam are presented at oblique incidence. Besides, we also compare the intensity distributions of the geometrical-optics beam field, the first order beam mode field and the actual beam field, which indicates that the contribution of each order beam mode field to the actual beam field is related to the refractive index of the dielectric slab. Meanwhile, the reflection characteristics of the Airy beams in the special cases of Brewster incidence and total reflection are investigated. Finally, the effects of the optical thickness and refractive index of the dielectric slab on the peak intensity distributions and beam shifts of the reflected and transmitted beams are also discussed in detail. The analytical and numerical results will be useful to analyze the propagation dynamics of Airy beam in the dielectric slab and provide some theoretical supports to the design of optical film.

Electrically tunable Goos-Hänchen shift in two-dimensional quantum materials

We theoretically investigate the tunable Goos-Hänchen (GH) shifts in silicene subjected to an external electric field and circularly polarized light. The prominent feature of these 2D quantum materials is the tunable bandgap that can be tuned by an external electric field or by irradiating circular polarized light beam. Using angular spectrum analysis, we obtain the analytical expressions for the spin and valley polarized spatial and angular GH shifts. We find that tuneable giant spatial and angular GH shifts exhibit extreme values near Brewster’s angles and away from the optical transition frequencies in the silicene. We demonstrate that both positive and negative giant GH shifts can be achieved in the graphene family by tuning the electric field and circularly polarized light in distinct topological regimes. Due to the topological properties of these materials, the GH shift is sensitive to the coupled spin and valley indices of the Dirac fermions as well as to the number of closed gaps. We further demonstrated that topology and spin-orbit interactions play a crucial role in beam shifts and topological quantum phase transitions of the silicene can be comprehensively and efficiently probed through GH shift at the nanoscale.

Вращение плоскости поляризации света в жидких кристаллах с сенсибилизаторами на основе WS-=SUB=-2-=/SUB=--нанотрубок

The effect of WS2 nanotubes on the rotation of the polarization plane of p-type laser radiation in the nematic liquid-crystal composites based on 4'-pentyl-4-biphenylcarbonitrile (5CB) has been considered. The correlation between the concentration of the injected sensitizer and the phase shift of the optical beam has been demonstrated. The prospects of these materials in display technologies, laser technology and quantum cryptography has been shown.

Rotation of the light polarization plane in Liquid Crystals with sensitizers based on WS-=SUB=-2-=/SUB=- nanotubes

The effect of WS2 nanotubes on the rotation of the polarization plane of p-type laser radiation in the nematic liquid-crystal composites based on 4'-pentyl-4-biphenylcarbonitrile (5CB) has been considered. The correlation between the concentration of the injected sensitizer and the phase shift of the optical beam has been demonstrated. The prospects of these materials in display technologies, laser technology and quantum cryptography has been shown. Keywords: liquid crystals, sensitizing, WS2 nanotubes, rotation of the light plane of polarization.