Impulsive Softening of Coherent Phonons in Tellurium.

We investigate the dynamics of coherent optical phonons in tellurium after high-density excitation with femtosecond laser pulses. The data show a continuous redshift of the phonon frequency with increasing excitation density. Experiments with double-pulse excitation prove that the observed frequency shift is of purely electronic origin. We demonstrate that coherent phonons allow monitoring the pathway to nonthermal laser-induced melting of crystalline materials.