Raman-Nath Diffraction Research Articles

Observation of Acoustic Diffusion Wave

The acoustic diffusion wave in a turbid medium containing randomly dispersed wave scatterers was observed for the first time. Ultrasonic wave at 15 MHz was modulated in amplitude and emitted into an aqueous suspension of polystyrene beads of 0.2 mm diameter. The ray trace of the wave takes a “random walk” in the multiple scattering, and the acoustic energy flows in a diffusional manner. The spatial distribution of the oscillating acoustic energy was observed with an optical probe based on the Raman-Nath diffraction. The wave number and the spatial damping of the diffusion waves were determined at the modulation frequency range of 4–20 kHz. The results were well described by a modified diffusion theory that included the effect of ultrasonic absorption. The diffusion constant D and the lifetime of the acoustic energy were determined, which led to the transport mean free path l* as D=l*ν/3, with ν being the ultrasonic velocity. The experiment was conducted at different volume fractions of the suspension, and l* and the lifetime were found to decrease with the number density of the scatterers. These results were discussed in correlation with a theoretical prediction of the geometrical mean free path and other factors. This study is to show the potential of using acoustic diffusion waves in the characterization of materials with strong ultrasonic attenuation due to scattering.

From monochromatic light diffraction to colour schlieren photography

The early experiments of Hiedemann on light diffraction led to an understanding of many experimental aspects of the subject, and defined many topics for further investigation. The theory of Raman and Nath broadened the understanding and defined many concepts that were not apparent to the experimenter. Although this theory is basically correct, it has been revised and improved over the years. The transition between Raman-Nath and Bragg diffraction was illuminated by the definitions of Klein, Cook and Mayer. Initially, the main focus of many investigations was on light diffraction by ultrasonic waves because there are many aspects of the subject. Later, it was realized that this interaction is an ideal way to study wave phenomena generally since optical diffraction does not disturb the ultrasonic wave. The unique perspective provided by acousto-optic interactions has made this a useful tool. By imaging ultrasonic waves one can make detailed studies of ultrasonic wave interactions and apply the results to many other types of waves. With acousto-optics one can study evanescent waves, ultrasonic finite amplitude effects, Bragg diffraction, Bragg imaging, etc. The application to other systems makes this a truly formidable study. The development of modern optics has made possible the description and understanding of wave interactions that heretofore had been mysterious. This paper begins with a description of early experiments. Later, it deals with finite-amplitude effects, parametric oscillations, Bragg diffraction and Bragg imaging. The study of evanescent waves by schlieren photography is a special case. Some topics have been studied by theory and experiment; some still await an acoustical theory although an optical theory exists, along with acoustical verification. Contributions of many citizens of the USA are considered. The paper ends with the application of the modern concepts of spatial filtering and information content to the analysis of colour schlieren photography.

Acoustooptical Spectral Processing of Radar Signals Taking into Account the Signal Spectra of Local Heterodyne and Optical Radiation

We describe the formation of a radio signal designed for acoustooptical processing based on the radar signal analyzed which is characterized by a spectrum, a heterodyne signal with a spectrum of known structure, and a mixer. The signal formed fits the working frequency of the acoustooptical cell. In the cell, the signal produces a traveling acoustic wave due to propagating inhomogeneities of the dielectric constant or refractive index. Within the framework of physical optics, we consider how the acoustooptical cell used for processing the signal works in the regime of the Raman–Nath diffraction. The cell is illuminated by quasimonochromatic radiation of known arbitrary spatial and spectrum structures. One works in the first and higher diffraction orders. Using an original method proposed earlier, we analyze the functioning of the acoustooptical cell in the Bragg diffraction regime at small radiation transformation coefficients. The description is done for a probing signal of known spatial and spectrum structures. The method of acoustooptical analysis of the spectrum structure of a radio signal or radar signal is considered under the decreased influence of the spectra of the probing optical radiation and heterodyne signal. The effect of the spectrum structure of the probing radiation used when processing the radio signal which propagates within the acoustooptical cell is studied in complete analogy with taking into account the exciting radiation spectrum in spectroscopy. For signals with a low level of noise, we use the method based on the Fourier transformation of the intensity distribution registered and on that of the intensity distribution of the initial probing radiation spectrum. Effects of the width and spectrum structure of the heterodyne can also be taken into account similar to the method of excluding the spread function used in investigating spectrograms. The processing uses Fourier transforms of the registered spectrum intensity distribution and of the known spectrum intensity distribution of the heterodyne signal.

Ultrafast unidirectional beam deflection using an electrooptic traveling phase grating with periodic domain inversion

We report a new electrooptic traveling-phase grating with periodic domain inversion and its application to ultrafast unidirectional beam deflection. The electrooptic traveling phase grating utilizes Raman-Nath diffraction to spatially separate input light to many diffracted beams on the far-field plane. Then, unidirectional optical beam deflection can be achieved by adjusting amplitudes and phases of the diffracted beams. In the experiment, ultrafast unidirectional deflection by the traveling phase grating is demonstrated at 16.25 GHz using a simple phase-only control.

Holographic recording in thin C 70-doped polymer organic films

The dynamic and static thin holographic gratings have been recorded in C 70-doped photosensitive polyimide 6B and 2-cyclooctylamino-5-nitropyridine. The drastic change in diffraction efficiency has been observed under the Raman–Nath diffraction condition using the second harmonic of pulse Nd laser. The threshold of the transition from phase grating to amplitude one has been determined. The results obtained have been explained by the absorption spectral peculiarities of the fullerene-doped films and by their thermal properties.

Specific features of holographic recording of diffraction gratings in thin films of fullerene-containing organic systems

Thin films based on fullerene-containing polyimides COANP and PNP were studied using methods of dynamic holography under conditions of Raman-Nath diffraction. A significant increase in the diffraction efficiency of the fullerene-containing samples was detected, and the threshold of the transition from phase grating to amplitude grating was determined. The qualitative explanation of the results is based on the spectral features and the thermal properties of the systems under study.

Phase-modulated beams technique for thin photorefractive films characterization

The phase-modulated beams technique is developed for nonlinear thin photorefractive films characterization. In the Raman–Nath diffraction approximation, the formulas are deduced, allowing us to measure the amplitude of phase grating recorded in a film and its nonlinear refractive index n2. The method is applied for studying Langmuir–Blodgett multilayer thin (∼0.6 μm) films of Bacteriorhodopsin at wavelength 633 nm.

Enhanced wave mixing in photorefractive rhodium-doped barium titanate crystals

We present an experimental study of the self-diffraction in Rh:BaTiO 3 and BaTiO 3 crystals for small angle between the interfering laser beams. Much larger diffraction efficiencies and beam amplifications are measured in the self-diffraction pattern of Rh:BaTiO 3 than in the corresponding pattern of undoped BaTiO 3 crystals. The enhancement increases with the Rh concentration in the barium titanate crystals. This effect cannot be explained by a simple Raman–Nath diffraction model, although it gives acceptable predictions for the undoped crystals, at small modulation index. In the same crystals, the results of self-diffraction in the Bragg regime remain in a good agreement with the existing theory.

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.

Raman-Nath diffraction caued at the focal point of focused ultrasound

Raman-Nath diffraction caued at the focal point of focused ultrasound

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)

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)

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.

Quantum Stochasticity in the Stokes Parameters of Light, Polarization, Switching and Procedure of Nondemolition Measurements of Distributed Feedback Systems

The analysis of formation of chaotic quantum states for two (or four) orthogonally polarized modes in a tunnelly-coupled optical fiber (TCOF) has been carried out. A special attention is devoted both to the structure of arising polarization instabilities and to the possibility to control of such a quantum state by procedure of quantum nondemolition measurements. New effect of polarization switching is obtained. A new method of generating correlated fluctuations in optical field, based on Raman-Nath diffraction by a fine grating, is discussed. A nonclassical effect is predicted: a suppression of quantum fluctuations below the level of full coherence in the sum of difference photon numbers for the transmitted and scattered waves. The physics of this phenomenon involves a correlation of quantum fluctuations in the intensities of two modes that are coupled nonlinearly. This correlation is most effective in the limit of parametric interaction between the waves in the medium.

Raman–Nath diffraction caused at the focal point of a concave type transducer

To estimate the intensity of sound pressure in the vicinity of the focal point of a concave type transducer, Raman–Nath diffraction caused at the focal point of a concave type transducer is theoretically and experimen- tally discussed. Sound field, near the focal point, including the acoustic nonlinearity, is theoretically calculated by using the KZK equation. Light intensity of zeroth order Raman–Nath diffraction is also calculated as a function of the sound pressure at the focal point. In the high intensive sound region, it is found that the acoustic nonlinearity makes the light intensity curve disappear at the local peaks. The He–Ne laser whose beam spot diameter is 1.2 mm is supplied as a light source. Light intensities of the zeroth-order Raman–Nath diffraction caused at the focal point are observed by changing the driving voltage of the transducer. The experimentally obtained graph shows the relatively same pattern compared with the theoretically calculated one. The sound pressure at the focal point is estimated by fitting those two results. It is confirmed that the sound pressure estimated by the optical method shows good agreements with the estimated one by the hydrophone measurements.

Orientational photorefractive effects observed in poly(vinyl alcohol)/liquid crystal composites

) as a photoconductive sensitizer under an applied dc field. Orientational photorefractivity was demonstrated by observing Raman–Nath diffraction beams with an external dc field. The photorefractive gratings were partially memorized even in the absence of the applied dc field.

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.

Frequency Characteristics of an Acoustooptic Modulator with Two Interdigital Transducers

An acoustooptic (AO) modulator with two interdigital transducers (IDTs) is investigated from the viewpoint of frequency characteristics of the modulation. The 1st IDT is used for driving an acoustic wave in a piezoelectric substrate and the 2nd IDT is used for controlling the reflection and the radiation of the acoustic wave propagating in the substrate. Light intensity modulation using Raman-Nath diffraction is controlled by changing the electrical terminal condition at the 2nd IDT. The modulation characteristics are found to be periodic functions of the frequency of acoustic wave, which suggests that the interference of acoustic waves in the substrate influences the modulation. In the present AO modulator, the interference of fields is utilized effectively and the electrical terminal condition with regard to not only the amplitude but also the phase can control the modulation.

Effect of Microparticles on Acousto-Optic Diffraction in Water

In Raman-Nath diffraction by microparticles in water, even-order diffraction is enhanced selectively by the suspension of microparticles. Intensities of diffracted beams increase markedly for particles larger than 10 µ m in diameter. By microscopic observation, particles 10–20 µ m in diameter align periodically at the nodes of an ultrasonic standing wave. On the other hand, particles 1–7 µ m in diameter distribute widely around the nodes. Since larger particles tend to remain at the nodes even after the ultrasonic wave is removed, even-order beams remain diffracted for several seconds.