Optical nonlinear absorption characteristics of crystalline Ge2Sb2Te5 thin films.

In this work, the optical nonlinear absorption characteristics of the crystalline Ge2Sb2Te5 thin films were investigated by performing z-scan measurements with nanosecond laser pulse. The experimental results showed that the nonlinear saturable absorption coefficient was as large as -10-2 m/W for the excitation intensity lower than 0.17 GW/m2. The nonlinear saturable absorption changed to the nonlinear reverse-saturable absorption with a coefficient of about + 10-2 at the excitation intensity of above 0.17 GW/m2. To explore the internal mechanisms, the first-principle theory was employed to calculate the electronic structure, and a five-level structure suitable for explanation of nonlinear absorption reversal was suggested. The theoretical calculation and analysis indicated that for the excitation intensity smaller than 0.17 GW/m2, the contribution to the nonlinear saturable absorption is mainly from band-filling effect; for the excitation intensity larger than 0.17 GW/m2, the nonlinear reverse saturable absorption results from the thermal-induced nonlinearity, which is further confirmed by picosecond laser pulse z-scan measurement and the variable-temperature spectroscopy ellipsometric analysis. [ABSTRACT FROM AUTHOR]