Relaxation Time Of Grating Research Articles

Non-degenerate two-wave mixing in the subwavelength periodic motion regime

We experimentally study non-degenerate two-wave mixing where reflection from a moving mirror is used to create frequency shifts and show that both the subwavelength motion of the mirror and periodic reversal of the intensity grating direction significantly affect the frequency dependence of the two-wave mixing signals detected both in ruby and colloidal media. It is shown that the Doppler shifts are suppressed when the reflecting moving mirror excursions are small compared to a wavelength and that the periodic reversal of the mirror motion results in a response that does not produce the expected maximum at the grating relaxation time. An approximate theoretical model agrees well with the results of the experiments and elucidates the transition from the observed response to the previously described idealized Doppler shift models.

Observation of stimulated thermal scattering of light upon nonsteady-state interaction between a laser pulse and a medium

The induced thermal scattering of light has been experimentally investigated for short laser pulses of different durations. It is shown that the observed anomalously large anti-Stokes shift (Ω ≫ 1/(2πτ), where τ is the thermal grating relaxation time) in the reflected signal depends on the pump pulse duration.

Non-equilibrium transient thermal grating relaxation in metal

Thermal grating relaxation in a metal with predominantly electronic heat transport is analyzed within the framework of the two-temperature model. Analytical solutions are obtained in the one-dimensional case, whereas the two-dimensional model simulating laser-induced transient grating experiment requires a numerical step to obtain time-domain solutions. When the grating period becomes comparable to (λ/G)1/2, where λ is the thermal conductivity and G the electron-phonon coupling constant, local thermal equilibrium between electrons and the lattice is not established even on a time scale much longer than the electron-phonon relaxation time. As a result, the thermal grating relaxation time is longer than predicted by the classical thermal diffusion model. For metals with high electron mobility such as gold, the effect is significant for thermal grating periods on the order of or under a micron and needs to be taken into account in the analysis of the laser-induced transient grating measurements.

Model of temperature grating relaxation times in distributed feedback dye lasers

A theoretical model and its experimental realization for the temperature grating relaxation time constant and its impact on the temporal and the spectral profiles of a Q-switched and modelocked Nd:YAG laser pumped distributed feedback dye laser (DFDL) is reported. Boundary conditions for different types of excitation pulses have been established to predict the effect of temperature phase gratings on laser gain build-up and temporal elongation of the DFDL pulses. The proposed transient grating method is useful in measuring grating relaxation time constants for lasing dye solutions. The proposed mathematical model is demonstrated by measurement of the relaxation time constant of R6G in ethanol at 10-3M. The measured relaxation time constant of 16+/-0.2 ns is very close to the tabulated values determined using other techniques.

Transient grating in a ferrofluid under magnetic field: Effect of magnetic interactions on the diffusion coefficient of translation.

Diffusion processes in a magnetic colloid are studied by a forced Rayleigh scattering technique under an applied static magnetic field. A periodic spatial modulation of the particle concentration (transient grating) is induced in the colloid with three different field geometries, B being either parallel or perpendicular to the grating direction. The value of the translational diffusion coefficient of the particles is given by the transient grating relaxation time. It depends on the magnetic field strength and on the field geometry. A theoretical model based on a mean field approximation taking magnetic interactions of particles under a field into account is given which agrees with experimental results.

Stimulated scattering during periodic-pulse pumping

Stimulated scattering during periodic-pulse pumping with a spiking period comparable to the grating relaxation time is examined theoretically. The decrease in the stimulated-scatteringthreshold is derived as a function of the pulse length, the spiking interval, the relaxation time, and the difference between the pump and signal group velocities.

STUDY OF DEGENERATE FOUR-WAVE MIXING IN DYE-DISSOLVING LIQUID CRYSTAL IN ITS NEMATIC PHASE

Degenerate four-wave mixing effect was studied in dye-dissolving liquid crystal iu its nematic phase. Thermal grating was generated after resonant absorption of light by the dye. The anisotropy of signal and thermal grating relaxation time were also studied.