Ultrafast impulsive excitation of coherent longitudinal acoustic phonon oscillations in highly photoexcited InSb

We report on the direct time-resolved measurement of high-frequency coherent longitudinal acoustic (LA) phonon oscillations in InSb. Two LA-phonon modes are impulsively generated in a highly photoexcited InSb sample and detected through reflectivity modulation with a temporal resolution of 15 fs. The measured frequencies, νLA(L1)=4.34 THz and νLA(X3)=3.75 THz, are in excellent agreement with those obtained through time-independent techniques. A frequency redshift in the LA(L1)-phonon frequency, as the energy fluence is increased beyond 10% of the damage threshold fluence, is attributed to an anisotropic impulsive softening of the InSb lattice. copyright 2001 American Institute of Physics