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

We report on the direct time-resolved measurement of high-frequency coheren t longitudinal acoustic (LA) phonon oscillations in InSb. Two LA-phonon mod es are impulsively generated in a highly photoexcited InSb sample and detec ted through reflectivity modulation with a temporal resolution of 15 fs. Th e measured frequencies, nu (LA)(L-1)=4.34 THz and nu (LA)(X-3)=3.75 THz, ar e in excellent agreement with those obtained through time-independent techn iques. A frequency redshift in the LA(L-1)-phonon frequency, as the energy fluence is increased beyond 10% of the damage threshold fluence, is attribu ted to an anisotropic impulsive softening of the InSb lattice. (C) 2001 Ame rican Institute of Physics.