Ultrafast asymmetric Rosen-Zener-like coherent phonon responses observed in silicon

TL;DR

This study reveals ultrafast asymmetric coherent phonon responses in silicon, driven by Rosen-Zener coupling between LO phonons and plasmons, with experimental and theoretical agreement on spectral asymmetry dynamics.

Contribution

The paper introduces a novel observation of ultrafast asymmetric phonon spectral profiles in silicon caused by Rosen-Zener coupling, supported by a combined experimental and theoretical analysis.

Findings

Spectral asymmetry occurs when LO phonons resonate with plasmons within 100 fs after laser excitation.

Asymmetry diminishes as carrier density deviates from resonance conditions.

Optical nutation influences coherent phonon signals, with comparisons to Fano resonance effects.

Abstract

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.