Sub-bandgap laser-induced fluorescence in the CaF2 via a linear process

Calcium fluoride (CaF2) is an excellent material for deep-ultraviolet optics. However, exposure to intense UV laser light can induce new defects in the material, a phenomenon known as laser-induced fluorescence (LIF). In our study, a femtosecond laser with a wavelength of 257 nm (4.82 eV, i.e., below CaF2 bandgap) was used to induce the LIF. We thoroughly studied the phenomenon behind the photoluminescence (PL) and its variation. Based on its linear excitation intensity dependence, we assigned the PL below 3.4 eV to sub-bandgap defect emission. The rate of LIF in this spectral region also scaled linearly with the irradiation pulse intensity. At the same time, the LIF rate was strongly dependent on the repetition rate. Our results suggest that laser-induced heating is responsible for the observed PL changes. We rationalized our observations based on the formation of F- and M-centres in CaF2. •A new mechanism of laser-induced fluorescence (LIF) in calcium fluoride (CaF2) was observed. UV sub-bandgap excitation at 257 nm leads to prominent changes in defect-induced photoluminescence.•The LIF is irreversible and its rate scales linearly with the irradiation pulse intensity, indicating that the LIF originates from a linear sub-bandgap excitation.•Multiple Gaussian bands were employed to identify the various defects, including colour centres and extrinsic defects.•A strong dependence of LIF on the laser repetition rate was observed, suggesting that the photoluminescence changes result from heat generated by the laser absorption.