Mechanisms for Ultrafast Generation of Coherent Phonons, Polaritons and Spin Excitations
Roberto Merlin
FOCUS Center and Department of Physics, University of Michigan
Recent work on the generation of coherent low-lying excitations by ultrafast laser pulses will be reviewed, emphasizing the microscopic mechanisms of light-matter interaction. The topics covered include phonon polaritons [J. K. Wahlstrand and R. Merlin, Phys. Rev. B 68, 054301 (2003)], long-lived phonons in ZnO [C. Aku-Leh et al., Phys. Rev.B 71, 205211 (2005)], squeezed magnons [J. Zhao, A. V. Bragas, D. J. Lockwood and R. Merlin, Phys. Rev. Lett. 93, 107203 (2004)], and spin- and charge-density fluctuations in GaAs quantum wells [J. M. Bao et al., Phys. Rev. Lett. 92, 236601 (2004)]. In addition, unpublished results on surface-avoiding phonons in GaAs-AlAs superlattices [M. Trigo et al., unpublished] and magnons in ferromagnetic Ga1-xMnxAs [D. M. Wang et al., unpublished] will be discussed.
It is now widely accepted that stimulated Raman scattering (SRS) is the main mechanism responsible for the coherent coupling. Results will be presented showing that SRS is described by two separate tensors, one of which accounts for the excitation-induced modulation of the susceptibility, and the other one for the dependence of the amplitude of the oscillation on the light intensity [T. E. Stevens, J. Kuhl and R. Merlin, Phys. Rev. B 65, 144304 (2002)]. These tensors have the same real component, associated with impulsive coherent generation, but different imaginary parts. If the imaginary term dominates, that is, for strongly absorbing substances, the mechanism for two-band processes becomes displacive in nature, as in the DECP (displacive excitation of coherent phonons) model. It will be argued that DECP is not a separate mechanism, but a particular case of SRS.
In the final part of the talk, an attempt will be made to identify emerging areas of research on coherent excitations and coherent control, relevant to condensed matter systems, that could benefit from ultrafast x-ray diffraction studies.
