Picosecond Laser Beam Research Articles

Exciton–exciton interaction under elliptically polarized light excitation

We illustrate how interesting it is to use an elliptically polarized laser beam for the investigation of the polarization properties of dense excitonic systems in semiconductor quantum wells. We investigate the spectral splitting of the exciton luminescence, observed in GaAs multiple-quantum-well structures at high exciton density, when the polarization of the resonant laser beam is progressively varied from circular to linear: this method leads to the accurate determination of the strengths of both repulsive and attractive parts of the interaction between the excitons. We investigate the stability of the polarized excitonic phase as a function of both the ellipticity and the intensity of the photogenerating picosecond laser beam and measure the longitudinal and the transverse spin relaxation times on the same excitonic phase: the observations illustrate the driving role of the exciton – exciton exchange interaction in the spin relaxation mechanism at high density.

Exciton-exciton interaction in quantum wells

Fast initial decays of both the luminescence intensity and the circular luminescence polarization at high exciton densities, under resonant excitation, are reported for the first time. These fast decays are simultaneously initiated by the increase of the ellipticity of the photogenerating picosecond laser beam. The theory of the mechanism illustrates the driving role of the interexciton exchange interaction.

Surface-plasmon-ion interaction in laser ablation of ions from a surface.

Experimental work by Shea and Compton suggests that ${\mathrm{Ag}}^{+}$ ions emitted from a roughened Ag surface irradiated by a nanosecond or picosecond laser beam may absorb the full energy of the Ag surface plasmon (SP). We have modeled this process as an inverse bremsstrahlung-type absorption of the SP quantum by an ${\mathrm{Ag}}^{+}$ ion which undergoes a collision with the surface. We estimate the absorption probability and find it to be consistent with the Shea-Compton results.

Picosecond Time-Resolved Impulsive Stimulated Scattering in Rochelle Salt

Longitudinal acoustic phonons were excited by crossing two picosecond pulse laser beams in Rocbelle salt. The time dependence of phonon oscillation was detected by the time-delayed third probing beam. From the recorded signals for three crossing angles, the phonon frequencies were deduced to be 1.57 GHz, 1.12 GHz and 506 MHz. The results indicate that the thermal absorption process (ISTS) is predominant for the present impulsive stimulated scattering.

Investigation of viscosity and heat conduction effects on the evolution of a transient picosecond photoacoustic grating

The production of acoustic waves by the absorption of radiation, known as the photoacoustic effect, is described by two coupled differential equations for pressure and temperature. Here, solutions to the coupled equations are given for sinusoidal deposition of heat in space as is typically generated using two picosecond laser beams to form a transient grating. The solutions to the coupled equations, along with the equation of state of the fluid, gives the pressure, temperature, and density of the fluid following rapid deposition of heat. The time dependence of the grating diffraction efficiency is readily calculated from the modulation of the density and temperature. For small values of the viscosity and heat conduction parameters, closed form expressions for the state variables are given which describe thermal and acoustic modes of wave motion in the fluid. The method is applied for computation of the state variables using a frequency-dependent viscosity. Experiments with ethylene glycol are reported where the viscosity and sound speed are measured as functions of temperature.

Transmission of a pulsed thin light beam through thick turbid media: experimental results

Experimental results of light pulse transmission through thick turbid media are presented. Measurements have been carried out on polystyrene latex spheres by using a picosecond thin laser beam and a streak camera system. The results show that the shape of the received pulse depends mostly on the transport mean free path and on the absorption coefficient of the medium, indicating that both the absorption coefficient and the asymmetry factor of the scattering function can be obtained from the pulse shape. The results also show that a detectable amount of received photons follows trajectories near the source receiver line even for large values of optical depth, indicating the potential of a time-gated scanning imaging system to detect absorbing structures inside thick turbid media.

Subpicosecond cooling of photoexcited hot carriers studied by one-beam excite-and-probe Raman scattering

A technique using a single picosecond laser beam to excite and probe photoexcited hot electron and hole plasma by inelastic light scattering is proposed. The cooling rate of the hot electrons is determined by varying the pulse width of the laser beam. The technique is illustrated by measuring the subpicosecond cooling of hot carriers in GaAs and InGaAs. The advantages and limitations of the technique are discussed.

Effect of surface recombination on the decay of excess minority carriers in semiconductors induced by finely focused picosecond pulsed laser beams

A study is carried out of the effect of surface recombination on the decay of the excess minority carrier population induced in a semiconductor sample by a very short pulse of a finely focused laser beam. An expression is obtained for this decay in a substantially simplified form, which can be readily computed numerically. Some sample calculations are presented, which show that, in agreement with previous experimental observations, this effect is important only for the initial part of the decay and only for large values of surface recombination velocity (>104 cm/sec) and absorption coefficient. If the initial fast portion of the decay is neglected, the minority carrier lifetime can be determined fairly accurately from the average slope of the remaining part of the decay, the resultant error being typically up to 15%.

Picosecond laser-induced change of the magnetic properties of amorphous GdFeBi-films

We have investigated the interaction of a picosecond laser beam with amorphous (Gd <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.26</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.74</inf> )O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">.96</inf> Bi <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.04</inf> layers. Several regions of reversible magnetic and irreversible magnetic and structural changes were observed in the laser heated area. Further the influence of film thickness and substrate material on the heat distribution in the magnetic layer was studied in terms of a simple model of heat conduction.

Spatial Structure of a Picosecond Laser Beam in a Non-linear Medium

The spatial structure of laser and SRS waves through a non-linear medium have been studied with a mode-locked laser. The results are compared with those previously reported by several authors and with results obtained in nanosecond excitation. It is to be noted that the SRS and self-focusing threshold values are generally different. The importance of the different phenomena involved in self-focusing are discussed and an explanation taking into account not just the Kerr constant, but the ratio of Raman gain to the Kerr constant, is developed. Thus the limit size of the filaments can be predicted. Good agreement between this theory and experiment is found.