Femtosecond optical generation and detection of coherent acoustic phonons in GaN single crystals

Abstract

We report our experimental and theoretical studies on the time-resolved generation and detection of coherent acoustic phonons (CAPs) in very high quality bulk GaN single crystals, performed using a femtosecond, two-color, all-optical pump-probe technique. A train of ultraviolet laser pulses with energy above the GaN energy gap induced the transient electronic stress at the GaN surface, responsible for the CAP generation. Subsequent CAP oscillations, propagating without any measurable intrinsic attenuation, were observed by scanning the transient differential reflectivity signal $(\ensuremath{\Delta}R∕R)$ of the near-infrared, far-below-band-gap probe beam. The $\ensuremath{\Delta}R∕R$ CAP oscillation amplitude was of the order of ${10}^{\ensuremath{-}5}--{10}^{\ensuremath{-}6}$ and was dependent only on the pump-photon absorption coefficient spectral characteristics. The CAP oscillation frequency was dispersionless (proportional to the probe-beam wave vector) with the slope corresponding to $8002\ifmmode\pm\else\textpm\fi{}22\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}$---the speed of sound in GaN---while the CAP signal phase was constant within the entire range of our experiments. The above experimental results are in excellent agreement with our theoretical modeling and the published literature data.