Electrostrictive generation of nonresonant gratings in the gas phase by multimode lasers.

Abstract

Nonresonant gratings have been generated in gases using as excitation beams the second-harmonic output of a multimode Nd:YAG (yttrium aluminum garnet) laser (pulse duration {tau}=8 ns, coherence time {tau}{sub {ital c}}=20 ps). The grating reflectivity is measured by scattering a probe beam off the grating. It shows the damped oscillation of a standing acoustic wave (period {ital T}{sub {ital p}}). By varying the time delay {tau}{sub {ital d}} between the excitation beams, the influence of the temporal laser coherence is analyzed. The signal for incoherent excitation ({tau}{sub {ital c}}{much_lt}{vert_bar}{tau}{sub {ital d}}{vert_bar}{much_lt}{tau}) is nonvanishing and depends only weakly on {tau}{sub {ital d}}. It is reduced by about a factor {tau}{sub {ital c}}/{tau} compared to coherent excitation, if {ital T}{sub {ital p}}{much_gt}{tau}. The measurements are interpreted within a calculation of electrostrictively generated gratings from beams having Gaussian temporal pulse shape, Gaussian statistics for the intensity, and a Gaussian frequency spectrum. Furthermore, the measurement of acoustic quantities (sound velocity and attenuation) with the described setup is discussed.