Spatial Periodic Modulation Research Articles

Использование модуляционной акустооптической спектрометрии для определения местоположения спектральных особенностей в перекрывающихся спектрах

The spectrometer is not able to accurately determine the spectral position of the smaller maximum if it is located against the background of a powerful broad peak. The solution of the problem is known it is enough to differentiate the existing dependence, and the position of the smaller peak is determined with sufficient accuracy. Based on a quasi-collinear acousto-optic (AO) cell, a prototype spectrometer was created, which makes it possible to record both the spectrum of the optical signal and its derivative in real time. In the course of the work, a more detailed research of the work of the created model was carried out: the value of the phase shift was changed programmatically from zero to 360°, and the spatial modulation period - from zero to the length of the AO cell L. A neon lamp and all the data presented were used as a radiation source, were obtained for the same emission line. The maximum signal for the derivative corresponds to the phase values of 90° and 270°. In another series of experiments, the work of the model was investigated with a change in the modulation period (the phase shift is fixed, ψ=π/2). The maximum signal for the derivative corresponds to the values of the modulation period duration equal to L/2, half the length of the acousto-optic cell. The graphs of the results of "physical" and "mathematical" differentiation do not coincide. We assume that the differences are related to the shape of the instrumental function of the acousto-optical spectrometer. However, at the tops of the spectral peaks, in the regions of intersection of derivatives with a zero line, the results coincide, which makes it possible to use acousto-optic differentiation to reveal the "fine structure" of optical spectra, and in real time. Thus, as a result of the work carried out: a method was proposed for the precise determination of the position of spectral maxima in complex overlapping spectra in real time; an instrumental implementation of the proposed method was created, and it was shown that for the visible range (532 nm) the accuracy of determining the spectral position of the maxima is 0.2 nm.

Space-Time-Modulated Metasurfaces with Spatial Discretization: Free-Space N -Path Systems

This work theoretically and experimentally studies metasurfaces with spatially-discrete, traveling-wave modulation (SD-TWM). A representative metasurface is considered consisting of columns of time-modulated subwavelength unit cells, referred to as stixels. SD-TWM is achieved by enforcing a time delay between temporal waveforms applied to adjacent columns. In contrast to the continuous traveling-wave modulation commonly assumed in studies of space-time metasurfaces, here the modulation is spatially discretized. In order to account for the discretized spatial modulation, a modified Floquet analysis is introduced based on a new boundary condition that has been derived for SD-TWM structures. The modified Floquet analysis separates the scattered field into its macroscopic and microscopic variations. The reported theoretical and experimental results reveal that the electromagnetic behavior of an SD-TWM metasurface can be categorized into three regimes. For electrically-large spatial modulation periods, the microscopic field variation across each stixel can be neglected. In this regime, the space-time metasurface allows simultaneous frequency translation and angular deflection. When the spatial modulation period on the metasurface is electrically small, the microscopic variation results in new metasurface capabilities such as subharmonic mixing. When the spatial modulation period of the metasurface is wavelength-scale, the metasurface allows both subharmonic mixing and angular deflection to be achieved simultaneously. To verify our analysis, a dual-polarized, spatio-temporally modulated metasurface, is developed and measured at X-band frequencies.

Holographic reflective Bragg gratings of gain and their role in the operation of high-power pulsed lasers

The influence of reflective Bragg gratings that are formed in an active medium due to the spatial periodic modulation of gain on the spectral properties of lasing are investigated by an example of a pulsed Nd : YAG laser. It is shown that, at a certain choice of the formation regime of reflective gratings of gain, the lasing bandwidth can be reduced by a factor of 3 – 5, with a corresponding increase in the pulse energy and significant improvement of spatial beam quality.

X-ray interferometry without analyzer for breast CT application: asimulation study.

.Purpose: We investigate an analyzer-less x-ray interferometer with a spatially modulated phase grating (MPG) that can deliver three modalities (attenuation image, phase image, and scatter images) in breast computed tomography (BCT). The system can provide three x-ray modalities while preserving the dose to the object and can achieve attenuation image sensitivity similar to that of a standard absorption-only BCT. The MPG system works with a source, a source-grating, a single phase grating, and a detector. No analyzer is necessary. Thus, there is an approximately 2x improvement in fluence at the detector for our system compared with the same source–detector distance Talbot–Lau x-ray interferometry (TLXI) because the TLXI has an analyzer after the object, which is not required for the MPG.Approach: We investigate the MPG BCT system in simulations and find a clinically feasible system geometry. First, the mechanism of MPG interferometry is conceptually shown via Sommerfeld–Rayleigh diffraction integral simulations. Next, we investigate source coherence requirements, fringe visibility, and phase sensitivity dependence on different system parameters and find clinically feasible system geometry.Results: The phase sensitivity of MPG interferometry is proportional to object–detector distance and inversely proportional to a period of broad fringes at the detector, which is determined by the grating spatial modulation period. In our simulations, the MPG interferometry can achieve about 27% fringe visibility with clinically realistic BCT geometry of a total source–detector distance of 950 mm and source–object distance of 500 mm.Conclusions: We simulated a promising analyzer-less x-ray interferometer, with a spatially sinusoidal MPG. Our system is expected to deliver the attenuation, phase and scatter image in a single acquisition without dose or fluence detriment, compared with conventional BCT.

Spatial frequency controlled by a phase carrier in spatial periodic modulation

An approach to controlling the spatial frequency of spatial periodic modulation (SPM) on focal plane has been presented. This approach is based on the application of phase carrier in the near-field beam. The phase amplitude of phase carrier is able to modulate the intensity of the spatial frequency of SPM from the maximum to 0. In addition, the period T of phase carrier determines the position of the strongest spatial frequency of amplitude SPM. When the period is increased, the position of the strongest spatial frequency is closer to the center. In order to avoid introducing new modulation to the output near-field beam, it is necessary to choose appropriate value of T to ensure that the strongest spatial frequency is beyond the cutoff frequency of pinhole. The feasibility of this method is verified by theoretical analysis and experimental results. This method provides a different perspective to solve the problems that spatial filter encounter, such as the pinhole closure and the damage to the material of pinhole filter periphery, in high power laser systems, as well as improving the quality of output near-field beam.

用于光束周期性调制补偿的相位载波技术研究

用于光束周期性调制补偿的相位载波技术研究

Dual-mode tunable terahertz generation in lithium niobate driven by spatially shaped femtosecond laser.

A new approach for dual-mode (namely broadband mode and narrowband mode) terahertz (THz) pulses generation in a single lithium niobate (LN) crystal excited by spatially shaped tilted-pulse-front femtosecond (fs) laser pulse was proposed and experimentally demonstrated. The two THz emission modes are generated simultaneously while spatially separated. Both central frequency and bandwidth of narrowband THz emission is controllable by in situ tuning the spatial modulation period and beam size of the fs-laser, and the broadband (0.1-1.5 THz) THz emission keeps almost unchanged while tuning the narrowband emission. Further optimization achieves the narrowband THz emission with energy spectral density up to 0.27 μJ/THz and with bandwidth narrowly down to 23 GHz. Such dual-mode THz source is useful for nonlinear THz optics, such as selected resonant THz excitation with broadband THz probe spectroscopy of crystalline matters.

Compensation for the spatial periodic modulation of the near-field beam with an improved iterative weight-based method

An improved iterative weight-based method is studied to compensate for the spatial periodic modulation (SPM) of the near-field beam. Based on the beam angular spectrum transmit formula (ASTF), the required phase that compensates for the intensity distribution of the incident SPM beam is iterated by this algorithm, which can contribute to improve the uniformity and quality of the output near-field beam. The experimental results show that the similarity value ε is improved from 0.9878 to 0.9947 and is getting closer to 1. The modulation degree M of the output near-field beam is decreased from 1.3631 to 1.3401 and the contrast degree C is decreased from 0.3018 to 0.2635 by using the liquid-crystal spatial light modulator (SLM). This indicates that the experiment verifies the feasibility of this iterative method to compensate for the SPM of the near-field beam.

Transient radiation from a ring resonant medium excited by an ultrashort superluminal pulse

We report some specific features of transient radiation from a periodic spatially modulated one-dimensional medium with a resonant response upon excitation by an ultrashort pulse. The case of ring geometry (with particle density distributed along the ring according to the harmonic law) is considered. It is shown that the spectrum of scattered radiation contains (under both linear and nonlinear interaction), along with the frequency of intrinsic resonance of the medium, a new frequency, which depends on the pulse velocity and the spatial modulation period. The case of superluminal motion of excitation, when the Cherenkov effect manifests itself, is also analysed.

Spatially periodic modulation of optical conductivity in doped graphene by two-dimensional diffraction grating

We show theoretically that the optical conductivity of doped graphene can exhibit a spatially periodic modulation by a diffraction grating. Doped graphene placed above the grating exhibits a periodic electrostatic potential distribution, resulting in a periodic charge-density redistribution inside the graphene. The optical conductivity of doped graphene depends linearly on the Fermi energy, which is proportional to the square root of the charge density. Therefore, the optical conductivity exhibits a spatial-periodic modulation. The periodicity implies a band structure formation of the graphene plasmon polariton. Band engineering of the graphene plasmon polariton is also discussed.

Beating pattern in quantum magnetotransport coefficients of spin–orbit coupled Dirac fermions in gated silicene

We report a theoretical study of magnetotransport coefficients of spin–orbit coupled gated silicene in the presence and absence of spatial periodic modulation. The combined effect of spin–orbit coupling and perpendicular electric field manifests through the formation of a regular beating pattern in Weiss and SdH oscillations. Analytical results, in addition to the numerical results, of the beating pattern formation are provided. The analytical results yield a beating condition which will be useful to determine the spin–orbit coupling constant by simply counting the number of oscillation between any two successive nodes. Moreover, the numerical results of modulation effect on collisional and Hall conductivities are presented.

The parameters that play role in numerical parameter variation in two-center holographic recording in photorefractive lithium niobate (LiNbO3 ) and strontium barium niobate (Sr x Ba1 − x Nb2 O6 )

We have conducted a numerical study of the dependence of nonvolatile data storage on the holographic parameter variation for a two-center holographic recording (TCHR) process. Parameters of the photorefractive crystals and the experimental conditions such as the electron mobility in the conduction band, the level concentrations, the electron-recombination coefficients, the bulk photovoltaic coefficient, the spatial period of modulation, the dielectric constant, and the modulation depth are studied numerically. The results of the numerical calculations showed that the significant parameters are the electron mobility in the conduction band, the electron-recombination coefficient, and the concentrations of the dopant levels (levels created due to the dopants) in the TCHR process. It seems that the electron-recombination coefficient for deep levels plays a more important role than the other parameters.

Evolution of low-frequency noise passing through a spatial filter in a high power laser system

The theoretical model of spatial noise passing through a spatial filter is established in high power laser system under the small signal approximation. The transmission characteristic for a noise signal passing through spatial filters with different magnifications is analyzed by numerical simulation, according to the actual structure of the high power laser system. The results show that the spatial modulation period of low-frequency noise getting through the pinhole will be proportional to the magnification of the spatial filter. When the magnification is less than 1, the safe low-frequency noise will be extruded into the high-frequency region, which is the fast increasing part, and finally develops into the most dangerous part which can damage the optical devices. The conclusion of this research improves the relay imaging theory of a spatial filter and provides an important theoretical basis for a general design of high power laser systems.

Optical properties of chiral photonic crystals with graded modulation parameters

The oblique transmission of light through a layer of a chiral photonic crystal with graded modulated parameters is considered. The problem is solved by the Ambartsumian’s layer addition method. Dependences of amplitude characteristics on the wavelength at different incident angels are presented for two cases: namely, for the case of a chiral photonic crystal with a linearly varied spatial period of modulation and in the case of a chiral photonic crystal with a linearly varied spatial depth of modulation. The case of a minimal influence of dielectric boundaries is considered. It is shown that in both cases a broadening of the photonic forbidden band gap takes place.

Theory of Tunneling Spectroscopy in the Larkin-Ovchinnikov State

We present a theory of tunneling spectroscopy for normal metal/Larkin-Ovchinnikov state junctions in which the spatial periodic modulation in the pair potential amplitude is taken into account. The tunneling spectra show the characteristic line shapes reflecting the minigap structures under the periodic pair potentials depending on the boundary condition of the pair potentials at the junction interface. These features are qualitatively different from the tunneling spectra of the Fulde-Ferrell state. We propose an experimental setup which identifies the superconducting state of CeCoIn5.

Stimulated Raman Scattering of a Modulated Laser Beam in Air:a Perturbation Approach

Stimulated Raman scattering (SRS) of a third-harmonic beam of ahigh-power laser in air is analysed by using a perturbation approach.It is found that intensity modulations of the third-harmonic beam,resulting from the surface ripple of nonlinear crystal, can besignificantly enhanced due to SRS, and the gain of instabilityincreases with the spatial period of modulation. Numerical simulationsbased on the nonlinear coupled-wave equations verify the validity ofthe perturbation approach.

Commensurate oscillations of the magnetoresistance of a two-dimensional electron gas in GaAs quantum wells with corrugated heteroboundaries

Oscillations of the magnetoresistance commensurate with the spatial modulation period of the growth surfaces were observed in selectively doped GaAs quantum wells with AlAs/GaAs superlattice barriers grown by molecular beam epitaxy. The experimental data obtained are explained by the lateral potential modulation of the two-dimensional electron gas in narrow GaAs quantum wells with corrugated heteroboundaries and agree with the two-dimensional distribution of the local capacitance in such structures.

The modulated magnetic structure of α-Fe2O3: Ga weak ferromagnet

The domain structure of a diluted weak α-Fe2O3: Ga ferromagnet is considered. Magnetic fields of specific amplitudes and orientations applied in the basal plane of the crystal are found to produce nonuniform magnetic states due to a periodic deviation of the antiferromagnetic vector from the easy axis of crystallographic anisotropy. A phase diagram of the modulated magnetic state with the triad axis is constructed, and a dependence of the spatial period of modulation on an external magnetic field is studied. A phenomenological model and the nature of the magnetic superstructure discovered are discussed.

Geometry Control of Photo-induced Microstructures in anAzobenzene Polymer Film

The mechanisms of photo-induced microstructures in an azobenzene polymerfilm are presented. They are based on the spatial periodic modulation ofoptical intensity and the photoisomerization of azobenzene moleculeswith the movement of main chains. Experiment and theory jointly pointout the possibility of photo-inducing desired spatial microstructures in anazobenzene organic polymer via adequate optical lattices and adequatelypolarized `writing' beams.

Solid-State Dye Laser with Photo-Induced Distributed Feedback

This paper describes the action of a solid-state dye laser utilizing a photo-induced distributed feedback (DFB) resonator. We solidified rhodamine B into a dried xerogel monolith using the sol–gel method, and used it as an active laser medium. The pump source was third-harmonic generation (THG) produced by a pulsed Nd:YAG laser. We divided the pump beam in two and irradiated the two beams on the active medium. The interference fringes simultaneously form both a spatial periodic modulation of optical gain and a refractive index in the active medium. This periodical structure functions as a DFB resonator. Without an external resonator, we achieved a laser oscillation with a narrowed spectrum. The full width at half maximum (FWHM) of the laser wavelength was approximately 1 nm. We tuned the center wavelength of the laser from 600 nm to 635 nm. In addition, we achieved a second-order DFB laser oscillation with second-harmonic generation (SHG) of a Nd:YAG laser.