Scanning nonlinear absorption in lithium niobate over the time regime of small polaron formation

Abstract

Nonlinear absorption is studied in presence of small polaron formation in lithium niobate using the z-scan technique and ultrashort laser pulses with pulse durations of 70 - 1,000 fs. A model for the analysis of the transmission loss as a function of pulse duration is introduced that considers (i) the individual contributions of two-photon and small polaron absorption, (ii) the small polaron formation time and (iii) an offset time between the optical excitation of free carriers by two-photon absorption and the appearance of small polarons. It is shown that the model allows for the analysis of the experimentally determined z-scan data with high precision over the entire range of pulse durations using a two-photon absorption coefficient of beta = (5.6 /- 0.8) mm/GW. A significant contribution by small polaron absorption to the nonlinear absorption is uncovered for pulse durations exceeding the characteristic small polaron formation time of approximate to 100fs. It can be concluded that the small polaron formation time is as short as (70 - 110) fs and the appearance of small polaron formation is delayed with respect to two-photon absorption by an offset of about 80 fs. (C)2015 Optical Society of America