Raman-Nath Diffraction Regime Research Articles

Generalized Space-Time-Periodic Diffraction Gratings: Theory and Applications

This paper studies the theory and applications of the diffraction of electromagnetic waves by space-time periodic (STP) diffraction gratings. We show that, in contrast with conventional spatially periodic grating, a STP diffraction grating produces spatial diffraction orders, each of which is formed by an infinite set of temporal diffraction orders. Such spatiotemporally periodic gratings are endowed with enhanced functionalities and exotic characteristics, such as asymmetric diffraction pattern, nonreciprocal and asymmetric transmission and reflection, and an enhanced diffraction efficiency. The theory of the wave diffraction by STP gratings is formulated through satisfying the conservation of both momentum and energy, and rigorous Floquet mode analysis. Furthermore, the theoretical analysis of the structure is supported by time and frequency domain FDTD numerical simulations for both transmissive and reflective STP diffraction gratings. Additionally, we provide the conditions for Bragg and Raman-Nath diffraction regimes for STP gratings. Finally, as a particular example of a practical application of the STP diffraction gratings to communication systems, we propose an original multiple access communication system featuring full-duplex operation. STP diffraction gratings are expected to find exotic practical applications in communication systems, especially for the realization of enhanced-efficiency or full-duplex beam coders, nonreciprocal beam splitters, nonreciprocal and enhanced-resolution holograms, and illusion cloaks.

Shear-enhanced diffraction from thin permanent gratings in dye-doped nematic liquid crystal cells

Permanent gratings are recorded in planar-aligned dye-doped nematic liquid crystal cells under visible light illumination. By increasing the irradiation intensity and exposure time, several diffraction orders of the recorded gratings are obtained in the Raman-Nath diffraction regime. By applying a dynamic transverse shear on one of the confining plates of the cell, an enhancement of the diffraction efficiency is achieved, which follows the period of the grating. By microscope inspection under static displacement of the upper plate, surface gratings formed by the dye adsorption are revealed in both the front and rear windows of the cell, indicating that the diffraction amplification originates from a coherent superposition of the diffracted orders when the gratings are displaced half a period. The effect provides self-matched amplification of diffraction with a simple cell, a single photo-inscription stage, and elementary displacement steps.

Free space diffraction on active gratings with balanced phase and gain/loss modulations

A theoretical analysis of asymmetrical diffraction in Raman-Nath, intermediate and Bragg diffraction regimes is presented. The asymmetry is achieved by combining matched periodic modulations of the phase and of the loss/gain of the material, which enables the breakdown of optical symmetry and redirects all resulting optical energy in only positive or only negative diffraction orders, depending on the quarter period shift directions between the phase and the loss/gain modulations. Analytic expressions for the amplitudes of the diffraction orders are derived based on rigorous multimode coupled mode equations in slowly varying amplitude approximation.

PHOTOREFRACTIVE AND PHOTOCONDUCTIVE FEATURES OF THE NANOSTRUCTURED MATERIALS

Photoconductive and photorefractive characteristics of fullerene- and nanotubes-doped organic thin films based on conjugated organics, such as polyimide, polyaniline, pyridine, etc. have been studied. In addition, the liquid crystal mesophase with nanoobjects has been investigated. The increase of the charge carrier mobility of nanosensitized organics has been established. The nonlinear refraction and cubic nonlinearity have been investigated at wavelength of 532 nm via four-wave mixing technique using Raman-Nath diffraction regime. The thin holographic grating has been written at the spatial frequencies placed in the range of 90-150 mm-1. The energy density has been chosen in the range of 0.1-0.9 J × cm-2. The correlation between photoconductive and nonlinear optical parameters has been revealed. The nanostructured materials can be proposed for different area of nano- and microelectronic applications.

Light diffraction by two parallel superposed ultrasonic waves of the frequency ratio 1:2

The paper constitutes a concise study of light diffraction by two superposed ultrasonic waves of the frequency ratio 1:2, beyond the Raman–Nath diffraction regime. Relevant numerical simulations of light intensity distribution within separate diffraction orders are shown based on the Nth order approximation method (NOA). A comparison is given between generalized theoretical predictions ( N = 7) and simplified formulas of Mertens ( N = 2) [R. Mertens, Diffraction of light by two parallel superposed supersonic waves, one being the nth harmonic of the other: A critical study of the methods leading to approximate solutions in finite form, Proc. Indian Acad. Sci. 55 (1962) 63–98.]. Moreover, the results of the performed experiment are presented which indicate limited applicability range of Mertens’ pioneer description of 1962.

Use of the application of heterodyne laser interferometer in power ultrasonics

We are particularly interested in the measurement of nonlinear acoustic waves in air, within the range of the Raman-Nath diffraction regime. We used a heterodyne laser interferometric system with a Bragg cell frequency of 80 MHz. A harmonic power acoustic wave with a frequency in the range of 20 kHz was generated, but higher harmonics appear when nonlinear propagation occurs in restricted volumes or waveguides. This topic raises a number of interesting questions. The most complex problem arises when processing an extremely wide-band output signal from a photo-detector, where the signal contains a huge number of high amplitude harmonics in a measured acoustic signal. The wide-band output signal from nonlinear wave measurement was simulated by means of similar wide-band output signals obtained by measuring known dynamic mechanical displacements. Because the frequency band of common analog circuits used for signal processing is too narrow, the output signal from the interferometer was digitalized and processed. This paper discusses the results of these tests and provides estimates of the frequency restrictions of signal processing.

Diffraction of light by two ultrasonic beams of frequency ratio 1:2, beyond the Raman–Nath diffraction regime

Light diffraction by two ultrasonic waves of frequency ratio 1:2 is examined beyond the Raman–Nath diffraction regime. Measurements were taken of ±1 order diffracted light intensities for some Raman–Nath parameter values, as a function of the phase shift between the sound beams. The experimental results obtained are discussed and compared with relevant theoretical predictions. Within the experimental conditions, a quite good quantitative agreement between the experimental data and the appropriate theoretical expectations is observed.

Diffraction of light by two parallel adjacent ultrasonic beams of the frequency ratio 1:1, beyond the Raman–Nath diffraction regime

Light diffraction by two adjacent ultrasonic waves having the same frequencies is examined experimentally beyond the Raman-Nath diffraction regime. Measurements were taken of the 0 and 61 order diffracted light intensities for a few (settled) Raman-Nath parameter val- ues, as a function of the phase shift between the sound beams. The experimental results obtained are discussed and compared with rel- evant, generalized, theoretically numerical predictions, as well as with the ones obtained by means of the 0 and 61 order approximation only. In both cases, however, within all the experimental conditions consid- ered, a quite good quantitative agreement between the experimental data and the appropriate theoretical predictions is observed. © 1999 So- ciety of Photo-Optical Instrumentation Engineers. (S0091-3286(99)00607-8)

Near field of light diffracted by an ultrasonic wave, beyond the Raman–Nath diffraction regime

The near field of ultrasonically diffracted light is examined for a wide range of Raman-Nath (v) and Klein-Cook (Q) parameter values up to the region of total light rediffraction phenomenon occurrence to the zeroeth diffraction order. The behavior of the secondary interference plane position is studied, both theoretically and experimentally, for vari- ous Q and v values. In addition, the relationship is shown between dif- fracted light intensity modulation minima localization shift and rediffrac- tion phenomenon. A comparison is made of the results of the general Fresnel region theory, including the influence of complex light amplitudes over the diffracted light intensity distribution, with those obtained from Cook's simplified near-field description. The same, limited validity range of the latter theory is revealed. For all the experimental conditions con- sidered, quite good agreement is observed between experimental data obtained and the appropriate theoretical predictions. © 1999 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(99)00507-3)

Optical amplification with high gain in hybrid-polymer–liquid-crystal structures

Efficient coherent light amplification with a very high net exponential gain Γ≈2600 cm−1 has been observed in sandwich-type hybrid-polymer–liquid-crystal structures. The amplification is a result of an energy exchange between two beams in a two-beam coupling experiment. The reported gain coefficient has been measured in a tilted sample configuration within a Raman–Nath diffraction regime at electric fields of the order of 1 V/μm.

Diffraction of light from phase gratings at large modulation depths:?transient-grating experiments in liquids at high laser powers

Generation of transient gratings in weakly absorbing liquids with a high-power laser gives rise to changes in the fluid's index of refraction that are sufficiently large to produce both multiple diffraction of a probe laser beam and a time response anomaly. The coupled-wave approach to the solution of the volume diffraction problem is shown to predict the existence of high-order diffraction of the probe beam and the time dependence of the diffraction intensity of each order. In addition, criteria for the Raman-Nath and Bragg diffraction regimes are derived from the first-order, coupled-wave equations.

Distinctive features of Raman-Nath scattering of light by sound in easy-plane antiferromagnets

This paper discusses the acoustic diffraction of light in the presence of an exchange-enhanced photoelastic interaction of aniferromagnetic origin resulting from acoustic modulation of the dielectric permittivity due to oscillations in the antiferromagnetism vector L. In the “easy-plane” type of antiferromagnet these oscillations arise from antiferromagnetoelastic interactions, and can be so large that the photoelastic interaction corresponding to them can be comparable in value to (or even exceed) the interaction in well-known nonmagnetic crystals actually used in acoustooptic devices. The advantage of antiferromagnets lies in the fact that both the diffraction angle and the amplitude of the diffracted light can depend on the magnitude and direction of a magnetic field in these materals. Here the Raman-Nath diffraction regime is discussed, which is probably more favorable from an experimental point of view for the antiferromagnets in question. It is shown that for these materials, the usual mechanism of photoelastic interaction associated with acoustic modulation of the index of refraction is accompanied by an additional mechanism arising from modulation of the polarization of the optical modes. Qualitative estimates are given for FeBO3.

Steady-state four-wave mixing in photorefractive quantum wells with femtosecond pulses

Photorefractive AlGaAs–GaAs multiple-quantum-well structures perform as ultrafast field autocorrelators during steady-state four-wave mixing by use of femtosecond pulses. The photorefractive thin films operate in the Raman–Nath diffraction regime with thin transmission gratings. The thin gratings permit degenerate and nondegenerate four-wave mixing without the need to satisfy phase-matching conditions. Nondegenerate mixing has several advantages over degenerate four-wave mixing. The autocorrelation is broadband and is viable for photon energies larger than the band gap of the photorefractive sample, and the diffraction efficiency is unaffected by the bandwidth of the femtosecond pulses. In addition, the readout laser can be a low-power low-cost cw diode laser.

Volume-diffraction analysis using bound-state perturbation theory

One-dimensional volume diffraction may be analyzed in a manner similar to the problem of an electron subject to a periodic potential. The quantity analogous to the diffraction Nath parameter ρ is then the ratio ΔW0/V, where V is the strength of the electron potential and ΔW0 the spacing between the free-electron-energy eigenstates. The crossover between the Bragg and Raman–Nath diffraction regimes is marked by a violation of the (1/ρ)≪1 condition, and corresponds to the requirement that (V/ΔW0)≪1 in order to preclude electron eigenstate mixing. Bound-state perturbation theory as developed for use in quantum mechanics may be applied to the diffraction problem to obtain a (1/ρ) expansion for the intensities of diffracted grating orders.

Optical modulation by a traveling surface acoustic wave and a holographic reference grating

The interaction of an optical beam with a traveling surface wave and a stationary holographic reference grating is studied theoretically and experimentally. The spatial phase modulation produced by the traveling wave results in an intensity modulation in the diffracted orders of the optical beam. The intensity modulation is at the fundamental frequency of the traveling wave, and its amplitude depends periodically on the separation distance between the plane of the traveling wave and the plane of the stationary holographic reference grating. Experiments were carried out with 87-MHz surface acoustic waves traveling on a LiNbO3 substrate separated from a holographic reference grating in film. The amplitudes of the detected modulation in the m = 0 and m = ±1 diffraction orders of a probing beam are in close agreement with a plane-wave analysis in the Raman–Nath diffraction regime.

Enhanced detection of acoustic waves using thick and thin reference gratings

Propagating acoustic waves typically produce small effective refractive-index variations and can thus be difficult to detect by simple optical probing. It has been shown that use of stationary reference gratings can greatly facilitate the detection of these waves. In this paper, the application of thick (Bragg diffraction regime) and thin (Raman–Nath diffraction regime) sinusoidal-phase reference gratings is considered for enhanced detection of acoustic waves, and their relative performance is compared. Simple, analytical expressions are obtained that describe the diffraction characteristics in each case. It is shown that thick reference gratings produce a purely sinusoidal temporal-intensity modulation, whereas thin gratings, in general, produce nonsinusoidal modulation.

Criterion for Bragg and Raman-Nath diffraction regimes

The idea is well entrenched in the literature that thin phase gratings (whether holographic or acoustically induced) should exhibit Raman-Nath behavior (and thus give several diffracted waves), and that thick phase gratings should show Bragg behavior (one diffracted beam and that only for Bragg angle incidence). The parameter Q of Klein and Cook, which is a normalized measure of grating thickness, has been extensively used as a criterion for deciding which regime will apply. It is perhaps not generally realized that Q is not a reliable parameter for this purpose but requires, as indeed Klein and Cook noted, a limitation on grating strength. This limitation is a matter of practical concern. For example, we have observed Raman-Nath behavior with Fe-doped LiNbO(3) even for very large values of Q. The purpose of the present paper is to note that a parameter rho (first defined by Nath) is an effective replacement for Q, since rho is reliable and Q is not. rho is defined as lambda(0)(2)/Lambda(2)n(0)n(1), where lambda(0) is the vacuum wavelength of the light, Lambda is the grating spacing, n(0) is the mean refractive index, and n(1) is the amplitude of the sinusoidal modulation of the refractive index. The grating thickness does not enter rho, so the terms thin and thick are, strictly speaking, irrelevant to the question of which regime is operative. However, thin enough gratings will tend to operate in the Raman-Nath regime because the index modulation must be large for a thin grating to produce appreciable diffraction.