Nonequilibrium terahertz phonons in spatially confined aluminum oxide crystalline materials studied by the Cr3+fluorescent phonon detector

Abstract

The propagation of nonequilibrium terahertz acoustic phonons in dense (alpha) -Al<SUB>2</SUB>O<SUB>3</SUB> ceramics with grain size approximately 10 micrometer is studied. Phonons are generated by the heat pulse technique and optical detection of 0.87 THz (29 cm<SUP>-1</SUP>) phonons via the Cr<SUP>3+</SUP> impurity ions fluorescence is used. The diffusive propagation of phonons from heater to optical detector is observed. Experimental results show that the mean free path of phonons is about 150 micrometers and thus is significantly larger than the grain size. These results are compared with computer calculations (by M. B. Melnikov) considering refraction and reflection of plane acoustic waves at the interfaces of disoriented grains in acoustic mismatch approximation. The dynamics of terahertz phonons generated at LHT by laser excitation in sol-gel technology grown monolithic transparent highly porous (gamma) - Al<SUB>2</SUB>O<SUB>3</SUB> samples was studied using the phonon-induced anti-Stokes FLN spectra of Cr<SUP>3+</SUP> impurity ions (<SUP>2</SUP>E - <SUP>4</SUP>A<SUB>2</SUB> lines). The generated phonon distribution besides long-living (approximately tens of ms) thermalized Planckian distribution contains essentially nonequilibrium vibrational excitations peak around (omega) approximately equals 0.6 THz (20 cm<SUP>-1</SUP>) with high occupation numbers n<SUB>(omega</SUB> ) and decay time approximately 1 ms. Discussed are the possible nature of these vibrations with frequencies (omega) which are consistent with acoustic size-resonance modes of (gamma) - Al<SUB>2</SUB>O<SUB>3</SUB> nanometer scale spherical microcrystals which form the material and the possible role of these vibrations in spectral dynamics of optically generated phonons.