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 \ensuremath{\tau}=8 ns, coherence time ${\mathrm{\ensuremath{\tau}}}_{\mathit{c}}$=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 ${\mathit{T}}_{\mathit{p}}$). By varying the time delay ${\mathrm{\ensuremath{\tau}}}_{\mathit{d}}$ between the excitation beams, the influence of the temporal laser coherence is analyzed. The signal for incoherent excitation (${\mathrm{\ensuremath{\tau}}}_{\mathit{c}}$\ensuremath{\ll}\ensuremath{\Vert}${\mathrm{\ensuremath{\tau}}}_{\mathit{d}}$\ensuremath{\Vert}\ensuremath{\ll}\ensuremath{\tau}) is nonvanishing and depends only weakly on ${\mathrm{\ensuremath{\tau}}}_{\mathit{d}}$. It is reduced by about a factor ${\mathrm{\ensuremath{\tau}}}_{\mathit{c}}$/\ensuremath{\tau} compared to coherent excitation, if ${\mathit{T}}_{\mathit{p}}$\ensuremath{\gg}\ensuremath{\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.