Absorption Spectra and Electron-Vibration Coupling of Ti:Sapphire From First Principles

摘要

First-principles calculations are performed to study the absorption spectra and electron-vibration coupling of titanium-doped sapphire (Ti:Al_2O_3). Geometry optimization shows a local structure relaxation after the doping of Ti. Electronic band structure calculation shows that five additional dopant energy bands are observed around the band gap of Al_2O_3, and are attributed to the five localized d orbitals of the Ti dopant. The optical absorption spectra are then predicted by averaging the oscillator strength during a 4 ps first-principles molecular dynamics (MD) trajectory, and the spectra agree well with the experimental results. Electron-vibration coupling is further investigated by studying the response of the ground and excited states to the E_g vibrational mode, for which a configuration coordinate diagram is obtained. Stokes shift effect is observed, which confirms the red shift of emission spectra of Ti :sapphire. This work offers a quantitative understanding of the optical properties and crystal-field theory of Ti-doped sapphire. The first-principles calculation framework developed here can also be followed to predict the optical properties and study the electron-vibration coupling in other doped materials.