Ultrafast dynamics of low energy elementary excitations in semiconductors.

Abstract

The theory of impulsive stimulated Raman scattering (ISRS) is extended to include the generation and detection of coherent phonons in the opaque regime. A quantum mechanical calculation yields an expression for the frequency-dependent Raman susceptibility appropriate for impulsive excitation. Expressions for the time-dependent phonon amplitude and relative change in transmissivity Delta<italic>T</italic>/<italic>T</italic> and reflectivity Delta R/R are derived for two-band processes and important differences between stimulated and spontaneous Raman scattering are discussed. Experiments in which impulsively-stimulated coherent phonons were resonantly excited are presented. Continuously tunable pulses produced by an optical parametric amplifier were used in a pump-probe geometry. Changes in the reflectivity induced by the <italic>A</italic>_1g phonon in the vicinity of the E'2 gap at 1.95 eV in the semimetal Sb and changes in the transmissivity induced by the A'1 phonon in the vicinity of the fundamental gap at 2.1 eV in the semiconductor GaSe were measured. Calculations of Delta R/R and Delta<italic>T</italic>/<italic>T</italic> based on the theory of ISRS compare well with the experimental results. An investigation of coherent polaritons in the optically isotropic semiconductor ZnSe at 10 K is presented. The polaritons were generated with a single ultrafast pulse and their dispersion was measured for wave vectors in the range 1370 cm<super>-1</super> to 2780 cm<super>-</super>1 by varying the central pulse energy in the range 1.97 eV to 2.3 eV. Phase matching was achieved due to the large dispersion in the index of refraction. The observed monotonic increase in decay rate as the wave vector decreases is attributed to inhomogeneous broadening and propagation effects. Observations of several inverted peaks and plateaus in the transmissivity as a function of delay in the semiconductor AS₂Se₃ are discussed. Interactions between the incident probe pulse and co- and counter-propagating internally-reflected pump pulses account for the inverted peaks and plateaus, respectively. Based on phase matching considerations and the relative strength of various nonlinear processes, it is determined that four-wave mixing, two-photon absorption (TPA) and photon-photon scattering (PPS) contribute to the inverted peaks and that TPA and PPS are responsible for the plateaus. A calculation of the change in transmissivity associated with the plateaus agrees well with the data.