Dynamics of laser-induced defects by multiple femtosecond pulses in potassium dihydrogen phosphate crystals

Abstract

The dynamics of the formation and annihilation of point defects in potassium dihydrogen phosphate crystals induced by multiple femtosecond laser pulses at 800 nm and 266 nm wavelengths is addressed. It is investigated through time-resolved photo-luminescence (PL) experiments in the [4.3 eV; 6.3 eV] spectral region for sample temperatures ranging from 20 K to 300 K. For both laser excitation wavelengths, a 5.1 eV energy PL band is found, for which amplitude depends on the temperature, and exhibiting a particular behavior around the Curie temperature (123 K). The low-temperature PL band characteristics (energy, width, kinetics of PL decay) suggest that it is due to the radiative annihilation of self-trapped excitons. Above the Curie temperature, the PL band is centered around the same previous energy but exhibits a different width, suggesting radiative annihilation of self-trapped holes next to the [HPO4]− group. During the laser irradiation at 1 kHz repetition rate of a low-temperature sample, the PL signal at 5.1 eV decreases as a function of the irradiation time on a timescale going from a few seconds to a few minutes, depending on both the laser wavelength and the intensity. This behavior is attributed to the formation and annihilation of mobile hydrogen defects. A model based on rate equations provides the temporal evolution of the defects’ density, allowing us to reproduce and interpret the observed evolution of the PL signal. This model also provides timescales for the defects’ density evolution with respect to the laser wavelength and intensity.