  We report on an 800 nm femtosecond laser beam induced giant refractive index
modulation and enhancement of near-infrared transparency in topological
insulator material Bi2Te3 thin films. An ultrahigh refractive index of up to
5.9 was observed in the Bi2Te3 thin film in near-infrared frequency. The
refractive index dramatically decreases by a factor of ~ 3 by an exposure to
the 800 nm femtosecond laser beam. Simultaneously, the transmittance of the
Bi2Te3 thin films markedly increases to ~ 96% in the near-infrared frequency.
The Raman spectra provides strong evidences that the observed both refractive
index modulation and transparency enhancement result from laser beam induced
photooxidation effects in the Bi2Te3 thin films. The Bi2Te3 compound transfers
into Bi2O3 and TeO2 under the laser beam illumination. These experimental
results pave the way towards the design of various optical devices, such as
near-infrared flat lenses, waveguide and holograms, based on topological
insulator materials.
