Large-amplitude chirped coherent phonons in tellurium mediated by ultrafast photoexcited carrier diffusion

TL;DR

This study investigates femtosecond coherent phonons in tellurium, revealing temperature-dependent frequency anomalies, large lattice displacements at high carrier densities, and the dominant role of carrier diffusion in reflectivity dynamics.

Contribution

It provides the first observation of anomalous temperature dependence of coherent phonons at high carrier densities and separates electron-phonon coupling effects from anharmonicity in tellurium.

Findings

Coherent phonons exhibit positive chirp increasing with carrier density.

Anomalous increase in phonon frequency with temperature at high excitation.

Carrier diffusion dominates electronic reflectivity response.

Abstract

We report femtosecond time-resolved reflectivity measurements of coherent phonons in tellurium performed over a wide range of temperatures (3K to 296K) and pump laser intensities. A totally symmetric A$_{1}$ coherent phonon at 3.6 THz responsible for the oscillations in the reflectivity data is observed to be strongly positively chirped (i.e, phonon time period decreases at longer pump-probe delay times) with increasing photoexcited carrier density, more so at lower temperatures. We show for the first time that the temperature dependence of the coherent phonon frequency is anomalous (i.e, increasing with increasing temperature) at high photoexcited carrier density due to electron-phonon interaction. At the highest photoexcited carrier density of $\sim$ 1.4 $\times$ 10$^{21}$cm$^{-3}$ and the sample temperature of 3K, the lattice displacement of the coherent phonon mode is estimated to be as high as $\sim$ 0.24 \AA. Numerical simulations based on coupled effects of optical absorption and carrier diffusion reveal that the diffusion of carriers dominates the non-oscillatory electronic part of the time-resolved reflectivity. Finally, using the pump-probe experiments at low carrier density of 6 $\times$ 10$^{18}$ cm$^{-3}$, we separate the phonon anharmonicity to obtain the electron-phonon coupling contribution to the phonon frequency and linewidth.