Generation of small bound polarons in lithium niobate crystals on the subpicosecond time scale.

Light pulses of 240 fs duration and 388-nm wavelength generate in lithium niobate crystals transient absorption. The absorption changes are observed with delayed ultra-short pulses (wavelength 776 nm) and with continuous-wave light (wavelength 785 nm) over 10 decades from 400 fs up to 10 s after the pump pulse. The decay is well described by one stretched-exponential function over the entire temporal range. We attribute the absorption changes to small bound polarons because of the temporal, spectral, and polarization fingerprints. The polarons are formed in less than 400 fs after the pump illumination, ruling out that phonons with wavenumbers below 100 cm^-1 are the main cause for polaron generation and indicating that the breathing mode of the oxygen octahedra is of relevance. An upper limit of the generated polaron concentration is estimated as ≈4.4×10¹⁸ cm^-3 with an intensity of ≈220 GW/cm². [ABSTRACT FROM AUTHOR]