Dynamic studies on laser-induced gratings in azobenzene-doped polymer film

Studies on the dynamic behaviours of laser-induced gratings through two-photon absorption and single-photon absorption were carried out in an azobenzene-doped PMMA film. The experimental results exposed the structures of energy levels and the relaxation between them, in which different lifetimes of the triplet states of trans and cis isomers of azobenzene molecule were recognized. The temporal characteristics of the diffracted signal from the induced-gratings showed also the dynamic processes of the photoisomerization reaction between two isomers.     

时间延迟激光感生双光栅的理论及实验研究

从理论上分析了激光相位及振幅的随机涨落对由热效应引起的时间延迟激光感生双光栅(TDLIDG)的影响.结果表明,四波混频信号调制的振幅随两束泵光相对延迟的增加而衰变.实验结果与理论预测基本一致.我们还证实用TDLIDG测量两束光的波长差可以达到与激光线宽同一量级的精度.     

Reflection-type photorefractive gratings in tin hypotiodiphosphate

Reflection photorefractive gratings recorded by nearly counterpropagating light waves in the near infrared are studied in tin hypothiodiphosphate. The ratios are established for certain electrooptic tensor components responsible for reflection grating recording, and the Debye screening length is evaluated. Reflection holograms of binary objects are recorded. Received: 26 June 2001 / Revised version: 3 September 2001 / Published online: 29 November 2001     

Frequency degenerate oscillation in semilinear photorefractive oscillator with reflection gratings

The oscillation intensity of a semilinear photorefractive oscillator with reflection gratings is calculated for different cavity losses, different pump intensity ratios and different crystal orientations. Distinct from an oscillator with transmission gratings, this oscillator always has a soft excitation in the vicinity of the threshold. The theoretical results are in reasonable agreement with that of experimental studies with BaTiO₃:Co as a photorefractive crystal.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Ultrafast optical switching by instantaneous laser-induced grating formation and self-diffraction in barium fluoride

Received: 13 January 1999     

Phase matched vectorial three-wave mixing in isotropic Kerr media

A theoretical analysis of vectorial three-wave mixing in isotropic Kerr media is presented. The analysis based on the coupled-wave equations describes the amplification of a weak wave by coupling with a strong pump wave phase matched by optically induced change of average refractive index. The phase matching condition for interacting waves with different polarizations is calculated, allowing a large amplification of a weak wave by three-wave mixing in thick Kerr media. The main conclusions of our theoretical analysis are supported by the results of experiments in CS₂ liquid medium.     

Sound-velocity measurements in gases by laser-induced electrostrictive gratings

Laser-induced electrostrictive gratings have been applied to measure the adiabatic sound velocity in various gases on a single-pulse basis. The gratings are generated by the interference of two parallel polarized, crossed excitation beams arising from a pulsed Nd: YAG laser at 532.1 nm, and are detected by diffracting a probe beam originating from a cw Ar⁺ laser operating single-line at 514.5 nm. Measurements were performed in the overlap volume of unfocused and focused excitation beams. Using unfocused beams, the sound velocities in various gases at pressures of 5 bar were measured with a statistical error for single-pulse measurements of about 0.3%. With focused beams, the accuracy of the measurements is reduced because of the propagation of the sound waves out of the smaller overlap volume of 0.2 mm diameter and 4 mm length. Measurements with focused beams were performed in air and CO₂, with an error of about 1%.     

Propagation of Gaussian beams in negative-index metamaterials with cubic nonlinearity

Propagation of Gaussian beams in the negative-index metamaterials (NIMs) with cubic nonlinearities is investigated, both theoretically and numerically. The role of the status of the incident Gaussian beam, which is scaled by a converging parameter in this paper, in beam self-focusing and self-defocusing in NIMs is specially identified. The expressions for beam self-focusing and self-defocusing for different converging parameter cases, and the dependence of the critical power and the focus location of self-focusing in NIMs on the converging parameter are obtained. It is found that it is the divergent rather than convergent incident beams which are self-focused more quickly in NIMs with defocusing nonlinearities, in sharp contrast with the propagation property of Gaussian beams in conventional Kerr media, in which beam self-focusing only occurs in the media with focusing nonlinearities and a convergent incident beam self-focuses more quickly than a divergent one. By adjusting the converging parameter of incident Gaussian beam or the controllable magnetic permeability of NIM, or both, one can manipulate the beam self-focusing in NIMs at will.     

Theoretical and experimental studies on laser-induced transient gratings in laser dyes

Laser-induced transient grating technique has been used to measure the diffraction efficiency (η) and calculate the third-order nonlinear susceptibility (χ⁽³⁾) of some laser dyes. Theoretical simulations have been carried out on η and χ⁽³⁾ as a function of wavelength covering the spectral range corresponding to the first excited singlet state of the dyes. Theoretically simulated values have been found in agreement to those observed experimentally. The decay profiles for these dyes have been measured by using diffraction of a delayed probe laser pulse to estimate the relaxation times in the excited state.     

Temperature measurements in gases using laser-induced electrostrictive gratings

Received: 1 December 1997 / Revised version: 27 January 1998     

Gas-phase diagnostics by laser-induced gratings II. Experiments

In this article we review the results achieved in the past ten years at the Paul Scherrer Institute on the topic of diagnostics in the gas phase by laser-induced gratings (LIGs). The technique has been applied for thermometry in air and in flames at different pressures, for flow velocimetry, for concentration measurements, and for imaging purposes. The influence of collisional energy-transfer and relaxation processes in molecules on the temporal evolution of the LIG signals has also been investigated. It has been demonstrated that, for molecules with a low fluorescence quantum yield, excitation of laser-induced thermal gratings can be used as a sensitive spectroscopic tool. For the quantitative interpretation of the experiments shown in this work, the findings presented in the companion paper [1] have been used.     

Photorefractive optical properties of volume phase gratings induced in sillenite crystals, when the grating vector lies on the (111) plane

We study the diffraction efficiency of volume phase gratings in Bi₁₂GeO₂₀, when the grating vector, lying on a (111) plane, is at an angle with respect to the crystallographic direction. An external bias field parallel to the direction is applied during recording, thus, the recording conditions depend on the grating vector orientation. The basic parameters determining the diffraction efficiency are the grating vector orientation, the rotatory power and the field-induced linear birefringence (primary and secondary). Analytic expressions for the diffraction efficiency have been obtained by taking into account all the above-mentioned parameters, provided that linearly polarized light is incident on the crystal. In this configuration, the influence of the secondary electro-optic effect (inverse piezoelectric and photoelastic effects) to the diffraction efficiency is actually stronger than the influence of the primary effect. Experimental results are given. Received: 3 January 2000 / Revised version: 6 June 2000 / Published online: 13 September 2000     

Ultra fast third-order non-linear response of amino-triazole donor-acceptor derivatives by optical Kerr effect

We report the study of the temporal dependence of the non-linear optical response of novel organic materials in solution. The experimental results of the optical Kerr gate using 70 fs pulses show a quasi-instantaneous response for three derivatives of an amino-triazole donor-acceptor system. The non-linearity of the compounds is identified as arising from the electronic contribution to the third-order non-linear susceptibility. The non-linear parameters of each sample were obtained using the optical Kerr response of CS₂ as reference.     

Off-Bragg-angle light diffraction and structure of dynamic interband photorefractive gratings

Steady-state and time-resolved off-Bragg-angle diffraction experiments are used to determine the structure and the dynamics of photorefractive gratings induced by interband photoexcitation. In potassium niobate, we identify in such gratings basically a two-layer structure. Close to the surface, we find a space-charge electric field generated by a charge modulation stored directly in the bands. This grating component is typically 50 μm thick, the amplitude of the refractive index modulation is larger than 10^-4, and the response time is a few μs for resonant intensities of 100 mW cm^-2. This component is also robust under non-resonant illumination. Deeper in the crystal, a second holographic layer extends over a few hundreds of μm, its amplitude is smaller, and its slower response time is in the ms range. The mutual phase shift between the grating components is also determined. Received: 23 November 1998 / Revised version: 14 January 1999 / Published online: 12 April 1999     

Photorefractive four-wave mixing in a high-contrast regime

The effect of high contrast in photorefractive four-wave mixing is studied using a recently proposed empirical formula for the grating amplitude. An analytical solution to the coupled-wave equation is obtained and its properties are discussed in the case of a double-phase-conjugate mirror and semilinear phase-conjugate mirror.     

Real-time diffraction gratings in DDI-methanol solutions

Nondegenerate four-wave mixing has been performed in DDI-methanol solutions using ruby laser as writing beams. The reading beam was either a pulsed one at 538.8 nm or a continuous one at 632.8 nm. Contributions from both saturation and thermal gratings were separated temporally and evaluated.     

Gas phase diagnostics by laser-induced gratings I. theory

Electrostriction and collisional thermalization of absorbed laser energy are the two dominant mechanisms leading to the formation of laser-induced gratings (LIGs) in the gas phase. In this article the results of the theoretical investigations that have been achieved in the past ten years at the Paul Scherrer Institute on this issue are summarized and yield a comprehensive understanding of the underlying physical concepts. Furthermore, a study of the influence of various parameters, such as the alignment and the spatial intensity profile of the beams on the generated electrostrictive and thermal signal is presented for the first time to the authors’ knowledge. The variations of the refractive index responsible for the appearance of laser-induced gratings have been theoretically described by solving the linearized hydrodynamic equations. The contributions from electrostriction, as well as from instantaneous and slow relaxation of the absorbed radiation energy into heat is obtained. These expressions are employed for analysis of experimental data presented in the companion paper [1] which is devoted to the application of the technique for diagnostic purposes in the gas phase. Much effort has been undertaken in order to allow a straightforward physical interpretation of the experimental findings of the expressions presented here.     

Comparative analysis of multi-wave mixing and measurements of the higher-order nonlinearities in resonant media

The features of degenerate multi-wave mixing in resonant media (dye solutions) have been studied theoretically and experimentally. It has been demonstrated that thermal nonlinearity due to the induced absorption from the excited level contributes significantly to the efficiency of four-wave mixing, but results in lower efficiency of higher-order interactions. The measurement results obtained for the energy efficiency of four-, six- and eight-wave mixing enable calculations of the third-, fifth- and seventh-order nonlinear optical susceptibilities, respectively. Experimentally, the method proposed for measurements of the higher-order nonlinearities has been realized with the use of the multi-wave mixing at second harmonic λ = 532 nm of monopulse YAG:Nd³⁺ laser radiation in a Rhodamine 6G dye solution. The ratios ∣χ⁽⁵⁾∣/∣χ⁽³⁾∣ and ∣χ⁽⁷⁾∣/∣χ⁽⁵⁾∣ are determined to be of the order of 10⁻⁵ cm³/erg.     

χ(3) scattering noises in nonlinear optical phase conjugation

Phase conjugation by Four-Wave Mixing (FWM) is treated quantum mechanically as a multiphoton scattering process. The spontaneous scattering of two counter-propagating photons gives rise to a background noise, whereas the stimulated scattering in the presence of the probe beam generates the phase conjugate signal. In addition, the stimulated scattering also contributes to a background noise which has never been discussed. Compared with Rayleigh scattering noise, the ⁽³⁾ scattering noises are relatively small for intensity up to 10⁸ W/cm². For higher intensities, the ⁽³⁾ scattering noises, in conjunction with the Rayleigh scattering noise, will affect the spatial fidelity of the optical phase conjugation.     

Solitary Wave Evolution of Optical Planar Vortices in Self-Defocusing Photorefractive Media

Solitary wave evolution of optical planar vortices in isotropie self-defoeusing photorefractive media is investigated in detail. We demonstrate that the formation of a planar vortex soliton intensively depends on the diameter and maximum intensity of the input vortex Seam. The exact solutions of planar vortex solitons are obtained due to the Petviashvili iteration method. It is found that, with the increasing soliton maximum intensity, the soliton core will be gradually diminished to a minimum value.