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

• Published in: Journal of luminescence 1995-12, Vol.66, p.349-357
• Main Authors: Feofilov, S.P., Kaplyanskii, A.A., Zakharchenya, R.I.
• Format: Article
• Language: English
• ISSN: 0022-2313; 1872-7883
• DOI: 10.1016/0022-2313(95)00168-9

The dynamics of terahertz phonons generated at liquid helium temperature by laser excitation in sol-gel technology grown transparent highly porous crystalline γ-Al 2O 3 samples, was studied through the phonon-induced anti-Stokes FLN spectra of Cr 3+ impurity ions ( 2E- 4A 2 lines). The generated phonon spectrum contains essentially a nonequilibrium narrow peak around ω ∼ 0.6 THz (20 cm −1) with high occupation numbers n ̄ ω and decay time ~ 1 ms besides and a slowly cooling (∼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 γ-Al 2O 3 spherical nanoparticles with 80 Å 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.