Multiwave Interaction of Light in a Bismuth Silicon Oxide Crystal in an Applied Meander Electric Field

An analysis is made of photorefractive nonlinearity resulting from the interaction between the charge gratings formed in a bismuth silicate crystal in an applied meander electric field under illumination by a strong reference and two signal light beams. It is shown that the nonlinear cross interaction between the primary holographic gratings recorded in the crystal should be taken into account where the intensities of the reference and signal light beams differ from one another in magnitude (less than 300 times).__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 24–29, February, 2005.     

On-demand superradiant conversion of atomic spin gratings into single photons with high efficiency

We create quantized spin gratings by single-photon detection and convert them on demand into photons with retrieval efficiencies exceeding 40% (80%) for single (a few) quanta. We show that the collective conversion process, proceeding via superradiant emission into a moderate-finesse optical resonator, requires phase matching. The storage time of 3 micros in the cold-atom sample, as well as the peak retrieval efficiency, are likely limited by Doppler decoherence of the entangled state.     

高质量全息光栅的制作

全息光栅由感光版记录高斯光束干涉条纹制得 ,干涉条纹的不等宽性降低了光栅品质。制取高质量全息光栅的关键是要用均匀平面波作光源。在分析干涉区内光强分布的基础上 ,给出了制作等周期正弦全息光栅的两种方法 :限制曝光区大小的方法和高斯光束强度均匀化的方法。     

Photofabrication of periodic microstructures in azodye-doped polymers by interference of laser beams

Volume holographic gratings and two-dimensional periodic microstructures in azodye-doped polymethylmethacrylate were fabricated, respectively, by interference of two coherent beams of a femtosecond laser and by interference of three coherent beams of a nanosecond laser. The dependence of the first-order Bragg diffraction efficiency and the photoinduced refractive-index modulation of the gratings on the intensity of the writing light was investigated. The measurements of the absorption spectra before and after irradiation with the writing light suggest that the photoinduced gratings were refractive-index-modulated gratings, which arose from a photoinduced decomposition reaction of the azodye molecules through multiphoton absorption. In the experiments involving the interference of three beams, the period of the two-dimensional periodic microstructures was changed by adjusting the angle between the three writing beams. Received: 10 July 2002 / Revised version: 5 September 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. Fax: +81-774/955206, E-mail: jhsi@photon.jst.go.jp     

Relaxation of the optical density of glass modulated with bichromatic radiation

Exposure of oxide glass to two coherent light beams with different frequencies ω and 2 ω leads to the buildup of spatial index gratings in the glass. The kinetics of the writing and relaxation of the gratings is investigated. Illumination with one beam or a background illumination of the sample accelerates the relaxation process; this acceleration is due to the appreciable photoconductivity of the glass. The diffraction is found to reach a residual self-maintaining level when a grating is illuminated continuously with a beam at the fundamental frequency. The results are analyzed on the basis of a model that includes the coherent photogalvanic current, the accumulation of a grating of nonuniform charges under the action of this current, and the back-effect of the static field, via electrooptic effects, on the propagation of the beams. It is shown that an optical instability of the photoinduced index gratings exists in the glass. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 3, 166–170 (10 February 1996)     

Sorting and detecting orbital angular momentum states by using a Dove prism embedded Mach-Zehnder interferometer and amplitude gratings

An experimental system consisting of a Dove prism embedded Mach-Zehnder (M-Z) interferometer and two gratings was built to sort and detect orbital angular momentum (OAM) states of light beams. The helical beams with different OAM states were generated by propagating the fundamental-mode Gaussian beam through a spatial light modulator. The Dove prism embedded M-Z interferometer was used to sort helical beams with odd or even OAM number and two binary amplitude gratings were used to measure the OAM states. Sorting of coaxially-superposed coherent and incoherent helical beams with odd and even OAM states was realized in the experiment.     

Diffractive Beam-Shaping Element for Generating Multiple Structured Beams with Distinct Spatial Structures

This paper presents a novel approach for generating structured light using a diffractive beam-shaping element that can generate multiple beams with distinct spatial structures in the focal plane. The element is constructed by restructuring a spiral zone plate with fork gratings, providing a tunable and controllable means for generating structured light. The element can create linear and 2D arrays of various structured beams, including vortices with adjustable charge and vorticity, petallike beams, and ring-lattice structures. To control the generation of these beams, it introduces several parameters of the fork gratings, enabling tailoring of the beam properties to specific applications. The proposed approach involves significant mathematical analysis, including the derivation of complex equations and expressions to understand the behavior of the proposed element and its impact on the generated beams. It conducts simulations and experiments to validate the mathematical analysis and demonstrate the feasibility of the proposed approach.     

偶氮苯聚合物薄膜光致六角对称微结构

利用前向箭并四波混频光路,在偶氮苯聚合物薄膜表面制作出六角对称的晶体微结构,观察到多个相位共轭波,当三束入射光都处在p偏振态时,在样品表面可得到一维光栅,当三束入射光中有一束在p偏振态,另两束在s偏振态时,在样品表面上得到的是六角对称微结构,这一结果表明,光场强度周期性变化及光场偏振态对晶格微结构的形成非常重要。     

高质量全息光栅实验系统研究

基于全息光栅在信息光学及光谱仪中的重要地位,对全息光栅进行了理论分析,提出了一种新的制作高质量全息光栅的实验系统.该实验系统的优点在于:将激光器发出的高斯光束改造成为均匀平面光波,以此平面光波作为光源可以制作出高质量的全息光栅.     

Theory of light and atom scattering in the Bose-Einstein condensate of a dilute gas

A semiclassical theory of superradiant light scattering from a Bose-Einstein condensate of a dilute gas is developed without recourse to the mean field approximation. The dynamics and spectrum of superradiant field, as well as the kinetics of formation of coherent atomic states with various translational momenta are calculated. The results are qualitatively consistent with experimental data for atoms scattered in the backward direction relative to that of the exciting laser beam propagation.     

Superradiant light scattering from thermal atomic vapors

Superradiant light scattering from noncondensed, thermal atomic vapors was experimentally studied. We found that superradiant gain is independent of quantum degeneracy and determined only by the shape of the atomic cloud and a contained number of atoms. Superradiant pump-probe spectroscopy was also developed to measure the atomic correlation function, revealing the Doppler-width-limited coherence time of the thermal gas and sudden buildup of long-lived coherence below the transition temperature.     

Small-angle scattering during formation of periodic structures in light-sensitive films subjected to laser radiation

Patterns of small-angle scattering that appear in thin AgCl films containing granular silver upon the formation of spontaneous periodic structures by p-polarized and circularly polarized laser beams (He-Ne laser, P=8 mW, λ = 632.8 nm) are studied. It is found that, at angles of incidence exceeding 4° regardless of the type of polarization, the diffraction of the waveguide modes excited by the dominant C gratings from neighboring microgratings with the vectors K ≠ K c dominantly contributes to the small-angle scattering. In the case of circularly polarized light, the pattern of small-angle scattering becomes more complicated as a result of the formation of S _? gratings and related secondary regular gratings.     

Measuring Stokes parameters by means of a polarization grating

Ordinary gratings act on the amplitude and (or) the phase of a wave front. Polarization gratings produce instead a periodic modulation of the state of polarization. A simple grating of the latter type is constituted by a linear polarizer whose orientation varies periodically along a line. It is shown that, for a generic polarization state of the incident field, such a grating gives rise to first-order diffracted beams with counterrotating circular polarizations. It is also shown that such a grating can be used for measuring the Stokes parameters of a light beam in an achromatic manner. Several extensions are briefly discussed.     

Noise gratings in silver halide volume holograms

Diffraction efficiencies of 70% have been reported for planar gratings in silver halide emulsion. The main obstacle in increasing the efficiency is the granular structure of the recording material which causes scattering of the input light both at recording and at reconstruction. This study concentrates on the effect of noise gratings recorded when the incident beam interferes with its own scattered radiation. The experimental evaluation is divided into two parts. Firstly, one single beam is used for recording a holographic plate, and the transmitted intensity of the reconstructing beam illuminating the developed hologram is measured as a function of incident angle and wavelength. Dips in the measured intensity are associated with the reconstruction of a strong scattered beam. Secondly, a main grating is recorded by two incident beams, which also interfere with the field scattered by the grains and give rise to noise gratings. When a beam is incident upon the developed hologram at the same angle and wavelength as one of the recording beams then besides the desired beam the scattered radiation is also reconstructed. This may reduce the otherwise available efficiency by several percent.     

Superradiant rayleigh scattering and collective atomic recoil lasing in a ring cavity

Collective interaction of light with an atomic gas can give rise to superradiant instabilities. We experimentally study the sudden buildup of a reverse light field in a laser-driven high-finesse ring cavity filled with ultracold thermal or Bose-Einstein condensed atoms. While superradiant Rayleigh scattering from atomic clouds is normally observed only at very low temperatures (i.e., well below 1 microK), the presence of the ring cavity enhances cooperativity and allows for superradiance with thermal clouds as hot as several 10 microK. A characterization of the superradiance at various temperatures and cooperativity parameters allows us to link it to the collective atomic recoil laser.     

Opto-Mechanical Effects in Superradiant Light Scattering by Bose-Einstein Condensate in an Optical Cavity

We investigate the effects of a movable mirror (cantilever) of an optical cavity on the superradiant light scattering from a Bose-Einstein condensate (BEC) in an optical lattice. We show that the mirror motion has a dynamic dispersive effect on the cavity-pump detuning. Varying the intensity of the pump beam, one can switch between the pure superradiant regime and the Bragg scattering regime. The mechanical frequency of the mirror strongly influences the time interval between two Bragg peaks. We find that when the system is in the resolved side band regime for mirror cooling, the superradiant scattering is enhanced due to coherent energy transfer from the mechanical mirror mode to the cavity field mode.     

Opto-Mechanical Effects in Superradiant Light Scattering by Bose-Einstein Condensate in an Optical Cavity

We investigate the effects of a movable mirror (cantilever) of an optical cavity on the superradiant light scattering from a Bose-Einstein condensate (BEC) in an optical lattice. We show that the mirror motion has a dynamic dispersive effect on the cavity-pump detuning. Varying the intensity of the pump beam, one can switch between the pure superradiant regime and the Bragg scattering regime. The mechanical frequency of the mirror strongly influences the time interval between two Bragg peaks. We find that when the system is in the resolved side band regime for mirror cooling, the superradiant scattering is enhanced due to coherent energy transfer from the mechanical mirror mode to the cavity field mode.     

利用相位型衍射光栅生成能量按比例分布的多个螺旋光束的研究

研究了基于计算全息光栅的方法产生能量按比例分布的多个螺旋光束的技术.在计算全息光栅的过程中引入了Gerchberg-Saxton迭代算法,并在此基础上利用位相型空间光调制器产生了具有轨道角动量态的多种光束,实现了多个指定轨道角量子数的螺旋光光场分布.给出了计算机仿真结果和实验结果,表明利用Gerchberg-Saxton迭代算法生成的位相型光栅可以生成多个指定轨道角动量量子数的光束,研究结果对于利用轨道角动量进行信息传输有益.     

On the evolution of N-atom state prepared by absorption of a single photon

We study evolution of timed symmetric N-atom state prepared by conditional absorption of a single photon and exhibiting superradiant decay. We find analytical expression for the initial decay rate of the state valid for any size of spherical atomic cloud. We show that the timed symmetric state is only approximately an eigenstate of the system for a large atomic cloud even if virtual processes are neglected.     

Superradiant spin-flip radiative emission of a spin-polarized free-electron beam

Radiative emission from the magnetic moments of the spins of an electron beam has never been observed directly, because it is fundamentally much weaker than the electric charge emission. We show that the detectivity of spin-flip and combined spin-flip-cyclotron-resonance-emission radiation can be substantially enhanced by operating with ultrashort spin-polarized electron beam bunches under conditions of superradiant (coherent) emission. The proposed superradiant spin-flip radiative emission scheme can be used for noninvasive diagnostics of polarized electron or positron beams. Such beams are of relevance in important scattering experiments off nucleons in nuclear physics and off magnetic targets in condensed matter physics.