Engineering GaS crystal anisotropy via ultrafast laser excitation

Abstract

Gallium sulphide (GaS) is an emerging monochalcogenide material that has recently attracted interest in optical technologies due to its tunable bandgap in the near-UV region. In this work, we employ in situ, in-operando X-ray diffraction to investigate local atomic modifications in GaS induced by 400-nm femtosecond laser pulses. We identify the energy threshold at which irreversible structural changes occur and observe a laser-induced elongation of the unit cell along the c-axis. This elongation is expected to enhance the anisotropy of the material's physical properties. Ab initio calculations further reveal that the experimentally observed ≈ 10% elongation along the c-axis leads to a transition from a direct to an indirect bandgap, accompanied by a bandgap increase of approximately 0.45 eV. Additional ab-initio optical simulations show that this structural transformation results in a nearly constant in-plane refractive index contrast of Dn ≈ 0.1 across a wide spectral range, from the visible to the near-infrared, with negligible optical losses, which could be of interest for reconfigurable photonics applications.