OPTICAL-GENERATION OF NONEQUILIBRIUM TERAHERTZ RESONANT VIBRATIONAL EXCITATIONS IN HIGHLY POROUS ALUMINUM-OXIDE

The dynamics of terahertz phonons generated at liquid helium temperatu re by laser excitation in sol-gel technology grown transparent highly porous crystalline gamma-Al2O3 samples, was studied through the phonon -induced anti-Stokes FLN spectra of Cr3+ impurity ions ((2)E-(4)A(2) l ines). The generated phonon spectrum contains essentially a nonequilib rium narrow peak around omega approximate to 0.6 THz (20 cm(-1)) with high occupation numbers (n) over bar(omega) and decay time similar to 1 ms besides and a slowly cooling (similar to tens of ms) thermalized Planckian distribution. The frequency (20 cm(-1)) of these long-living nonthermalized vibrations is consistent with the lowest acoustic, siz e-resonant surface mode of gamma-Al2O3 spherical nanoparticles with 80 Angstrom diameter, which form the network of the sample. Discussed ar e the nature of the long lifetime and the possible role of these surfa ce modes in the spectral dynamics of optically generated phonons.