Shank, Yen, and Hirlimann Respond

Abstract

Shank, Yen, and Hirlimann Respond:Combescot and Bok 1 have attempted to improve on the rough estimate of electron-hole plasma density given in our recent Letter 2 concerning the dynamics of the reflectivity of silicon excited with femtosecond optical pulses. Their approach is to determine a more precise effective mass in order to prescribe more accurately the density dependence of the plasma frequency and the dielectric constant. At the very high plasma density and plasma temperature encountered in this experiment this is a very complex process.Combescot and Bok use the theory of the dielectric constant and plasma frequency derived by Combescot and NoziSres 3 for germanium. Application of this theory to the case of highly excited Si is fraught with difficulty. First, while this theory may be entirely adequate for Ge at low intensities, the two-band model is inadequate to describe Si at even modest densities. The splitoff band in Si is within kT of the band edge, thus requiring a reformulation of the problem with a three-band model including the split-off band. In addition the equation for the reduced mass given by Combescot and Bok is an approximation in the high-frequency limit which does not satisfy the experimental conditions. According to Combescot and NoziSres 3 the reduced mass for holes is given by in the high-frequency approximation where (v»A = € h {m H /m L )

Coherent phonons in condensed media

“…Most of them are based on time-resolved second harmonic generation (SHG), which vanishes when the crystal loses its symmetry. It has been reported, that this process occurs on time scales which are shorter than the typical energy transfer times from the electronic system to the lattice 103,104,105]. Theoretically it has been predicted that a non-thermal phase transition may occur when more than 9% of all valence electrons are excited into antibonding conduction band states 106, 1 0 7 ].…”

Laser-induced structural relaxation and crystallization phenomena in the picosecond time scale in GeSbO thin films

“…The pump pulse is focused at the sample site to a size of Ϸ300 m while the probe beam is focused to a size about eight times smaller. Since the effect of a single pump pulse may induce a permanent change in the state of relaxation of the surface, the reconstruction of the time evolution of the reflectivity was performed in single exposure steps, 7 unlike the normal situation in experiments aimed at studying carrier relaxation dynamics. 16 For a given delay ( ϭt), and a prefixed value of the pulse energy ͑E͒, the reflectivity of the surface before irradiation ͓R( ϭϪϱ)͔, the instantaneous reflectivity value ͓R( ϭt)͔ and the reflectivity after irradiation ͓R( ϭϱ)͔ are measured exposing a region of the surface to the pump pulse only once.…”

“…Ultrafast melting processes in Si and GaAs have been thoroughly studied 6,7 and subnanosecond crystallization phenomena have been reported in nobel metals undergoing rapid solidification under ps laser pulses. 8 To the best of our knowledge, no attempt has been made to resolve structural relaxation processes in the ps time scale.…”

Optical Second Harmonic Generation for Surface Studies

“…From simple symmetry argument, it is easily seen that the SH signal as a function of sample rotation about the surface normal should reflect the structural symmetry of the surface layer [7,11]. SHANK et al [19] have actually used the technique to study laser melting. More recently, HEINZ and LOY [20] have succeeded in using SHG to monitor the transformation of the freshly cleaved 2 x 1 Si(100) surface to the annealed 7 x 7 surface.…”