# Irregular Oscillatory-Patterns in the Early-Time Region of Coherent Phonon Generation in Silicon

## TL;DR

This paper investigates quantum-mechanical effects in the early-time coherent phonon generation in silicon, revealing irregular oscillations caused by resonant interactions and Rabi flopping influenced by the laser pulse's electric field.

## Contribution

It uncovers the role of resonant plasmon-phonon interactions and Rabi flopping in shaping early-time coherent phonon signals in silicon, highlighting quantum effects in ultrafast laser excitation.

## Key findings

- Resonant interaction causes anticrossings and irregular oscillations.
- Oscillations are influenced by Rabi flopping of excited carriers.
- Quantum effects are prominent in early-time coherent phonon signals.

## 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.