Coherent Phonon Spectroscopy
Ultrafast optical excitation of solids suddenly turns on forces that can excite the coherent couple electron and lattice oscillations. This coherent response provides information on the electron-phonon interacation in a solid under highly nonequilibrium conditions. The coherent phonons oscillate approximately at frequencies of the solid under equilibrium conditions. In the presence of nonequilibrium electron-hole plasma, however, the oscillation frequencies vary with time as a spectator of the carrier dynamics. We follow the coherent response of semiconductors and semimetals by measuring the transient reflectivity change upon interand optical excitation. In transparent materials, such as diamond, the excitation light itself excites coherent lattice response thorugh the stimulated Raman effect.