Optical generation of nonequilibrium terahertz resonant vibrational excitations in highly porous aluminium oxide

The dynamics of terahertz phonons generated at liquid helium temperature 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) lines). The generated phonon spectrum contains essentially a nonequilibrium 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, size-resonant surface mode of gamma-Al2O3 spherical nanoparticles with 80 Angstrom diameter, which form the network of the sample. Discussed are the nature of the long lifetime and the possible role of these surface modes in the spectral dynamics of optically generated phonons.