Title: Irregular Oscillatory-Patterns in the Early-Time Region of Coherent Phonon Generation in Silicon,
Abstract: Coherent phonon (CP) generation in an undoped Si crystal is theoretically
investigated to shed light on unexplored quantum-mechanical effects in the
early-time region immediately after the irradiation of ultrashort laser pulse.
One examines time signals attributed to an induced charge density of an ionic
core, placing the focus on the effects of the Rabi frequency $\Omega_{0cv}$ on
the signals; this frequency corresponds to the peak electric-field of the
pulse. It is found that at specific $\Omega_{0cv}$'s where the energy of
plasmon caused by photoexcited carriers coincides with the longitudinal-optical
phonon energy, the energetically {\it resonant } interaction between these two
modes leads to striking anticrossings, revealing irregular oscillations with
anomalously enhanced amplitudes in the observed time signals. Also, the
oscillatory pattern is subject to the Rabi flopping of the excited carrier
density that is controlled by $\Omega_{0cv}$. These findings show that the
early-time region is enriched with quantum-mechanical effects inherent in the
CP generation, though experimental signals are more or less masked by the
so-called coherent artifact due to nonlinear optical effects.