  We investigate the spectral profiles of time signals attributed to coherent
phonon generation in an undoped Si crystal. Here, the retarded
longitudinal-optical (LO) phonon Green function relevant to the temporal
variance of induced charge density of ionic cores is calculated by employing
the polaronic quasiparticle model developed by the authors [Y. Watanabe et al.,
Phys. Rev. B 95, 014301 (2017); ibid., 96, 125204 (2017)]. The spectral
asymmetry is revealed in the frequency domain of the signals under the
condition that an LO phonon mode stays almost energetically resonant with a
plasmon mode in the early time region; this lasts for approximately 100 fs
immediately after the irradiation of an ultrashort pump-laser pulse. It is
understood that based on the adiabatic picture in time, this asymmetry is
caused by the Rosen-Zener coupling between both modes. The associated
experimental results are obtained by measuring time-dependent electro-optic
reflectivity signals, and it is proved that these are in harmony with the
calculated ones. The spectra become more symmetric, as the photoexcited carrier
density further changes from that meeting the above condition to higher and
lower sides of carrier densities. Moreover, the effect of optical nutation of
carrier density on the CP signals is addressed, and the present results are
compared with the asymmetry caused by transient Fano resonance, and the
spectral profiles observed in a GaAs crystal in the text.
