What are we observing by the detection frequency resolved measurement of coherent phonons?

The coherent phonons are detected through the oscillation of transient reflection or transmission of the probe pulse as a function of the delay τ after the pump pulse. Usually, only the integrated intensity of the reflected or transmitted probe pulse is measured. Recently, Mizoguchi et al. performed systematic measurements of the frequency-resolved reflection spectra for CdTe crystal. In their experiment, the reflection change induced by the coherent phonon generated by a femto second pulse was measured as a function of the delay τ, the central frequency Ω₁ of the probe pulse, and the detection frequency Ω₂ as ΔR(τ, Ω₁, Ω₂)/R⁰ (R⁰ is the unperturbed reflection). This revealed Ω₁ dependent peculiar behaviors.The features of the experimental data, and the theoretical results as well, are summarized as follows. (1) The maximum energy of the oscillation in ΔR (τ,Ω₁,Ω₂ )/R⁰ is at Stokes and anti-Stokes side of Ω₁ with about one or two phonon energy. (2) The oscillation is out-of-phase between the Stokes side and the anti-Stokes side in the case of transparent region excitation, while it is in-phase in the opaque region. (3) The exciton peak at the band edge gives rise to a strong singular structure independently of the phonon structure. All of these features are well reproduced by the theory. This indicates the importance of the detection frequency resolved measurement, and the possibility to extend the coherent phonon spectroscopy as a tool to detect the detailed electron-phonon interactions in solids.