Ultrafast Dynamics of Coherent Phonons in P-type Si Driven by Sub-10-fs Laser Pulses


Keiko Kato, Atsushi Ishizawa, Katsuya Oguri, Hideki Gotoh, and Hidetoshi Nakano
Optical Science Laboratory

@Coherent phonons are the in-phase lattice vibrations driven by the ultrashort laser pulses whose duration is shorter than the oscillatory period. In contrast to frequency-domain approaches, such as Raman spectroscopy or infrared-active spectroscopy, the observation of coherent phonons provides time-domain information about the lattice vibrations.
@For Si, which is the fundamental semiconductor, the Raman spectrum is modified by p-type doping, reflecting the asymmetric shape of the valence band in k-space [1]. To study the electron-phonon dynamics dominated by the asymmetry in the carrier distribution, we have observed coherent phonons in p-type Si [2].
We performed time-resolved reflectivity measurements with a sub-10-fs Ti:sapphire oscillator at a center wavelength of 780 nm. Samples were non-doped Si and p-type Si. The carrier concentration in the p-type Si was 3~1019-1.5~1020 /cm3.
@The time-resolved reflectivity changes for non-doped Si and the p-type Si are shown in Figs. 1(a) and (b), respectively. The oscillatory signal, originating from coherent phonons in Si, is observed after an overlap between the pump and probe pulses at t=0. In the p-type Si, the decay from the photo-generated carriers, which distribute anisotropically in k-space (i.e., anisotropic state-filling), is observed as shown by the dotted line in Fig. 1(b). In the p-type Si, the Fermi level is lowered in the valence band with the non-parabolic structure, which causes the anisotropic hole distribution [2]. When the oscillatory signal is fitted with cos(0t + ), where 0 and correspond to the frequency and initial phase of the coherent phonons, respectively, the initial phase in the p-type Si is shifted to the cosine phase with the anisotropy in the hole distribution.
@This research was supported by KAKENHI.

[1] F. Cerdeira, T. A. Fjeldly, and M. Cardona, Phys. Rev. B 8 (1973) 4734.
[2] K. Kato, A. Ishizawa, K. Oguri, K. Tateno, T. Tawara, H. Gotoh, and H. Nakano, Jpn. J. Appl. Phys. 48 (2009) 100205.

Fig. 1. Time-resolved reflectivity for (a) non-doped Si and (b) the p-type Si. Insets show enlarged graphs around
t=0. Black dots are the experimental results. The gray line is the fitting result, the black solid line is the
component of coherent phonons, and the black dotted line is the decay of anisotropic state-filling.