# Energy dependent amplitude of Brillouin oscillations in GaP

## TL;DR

This study uses ultrafast pump-probe spectroscopy to analyze how the amplitude of Brillouin oscillations in GaP varies with probe energy, revealing insights into its electronic structure and optical transitions.

## Contribution

The paper develops a theoretical model that explains the energy-dependent amplitude of coherent phonons in GaP, linking phonon spectroscopy to electronic and dielectric properties.

## Key findings

- Amplitude of coherent phonons varies non-monotonically near the direct gap.
- Model accurately explains experimental amplitude dependence.
- Spectroscopy provides detailed electronic structure information.

## Abstract

Gallium phosphide is an important indirect band gap material with variety of applications in optics ranging from LEDs to applications in GaP/Si based solar cells. We investigated GaP using ultrafast, pump-probe coherent acoustic phonon spectroscopy (time-domain Brillouin scattering). We measured the dependence of the amplitude of the differential reflectivity as modulated by coherent acoustic phonons (CAPs) as a function of laser probe energy and found that the amplitude of the coherent phonon oscillations varies non-monotonically near the direct gap transition at the $\Gamma$ point. A theoretical model is developed which quantitatively explains the experimental data and shows that one can use coherent phonon spectroscopy to provide detailed information about the electronic structure, the dielectric function and optical transitions in indirect band gap materials. Our calculations show that the modelling of experimental results is extremely sensitive to the wavelength dependent dielectric function and its derivatives.