Reverse Saturable Absorption Induced by Phonon-Assisted Anti-Stokes Processes.

In materials showing reverse saturable absorption (RSA), optical transmittance decreases at intense laser irradiation. One approach to application of these materials is to protect the sensors or human eyes from laser damage. To date, research has mainly concentrated on thin films and suspensions of graphite and its nanostructure (including nanotubes, graphene, and graphene oxides), which are mainly used as an optical limiter for nanosecond laser pulses. Moreover, thin individual pieces of semiconductor usually exhibit increased transmittance due to saturable absorption when the laser energy (E ) is higher than the band gap (E ). Here, it is shown that indirect gap semiconductor WSe exhibits high RSA on exposure to a femtosecond laser under E > E near band gap excitation, which is attributed to the longitudinal optical phonon-assisted anti-Stokes transition by the annihilation of phonons and the absorption of photons. An optical limiting threshold (≈21.6 mJ cm ) lower than those reported for other optical-limiting materials currently for femtosecond laser at 800 nm is observed.