Modulating the nonlinear absorption response of SnO x thin films via phase engineering.

Phase (composition) is known to play a key role in determining the electronic and optical properties of amorphous oxide semiconductors. In this work, modulating the ultrafast nonlinear optical (NLO) response of SnO and SnO thin films by tuning oxygen partial pressure during film sputtering is explored. Femtosecond Z-scan results demonstrate that intermediate phases have no profound impact on the two-photon absorption (TPA) response of SnO and SnO films. Interestingly, the magnitude of the effective nonlinear absorption coefficient (β) of both intermediate SnO and SnO are enhanced after the change of Sn/Sn composition ratio, as measured by picosecond Z-scan technique. Femtosecond degenerate pump-probe measurements show that intermediate phases accelerate the carrier trapping and improve the defect-related carrier absorption in SnO (SnO-rich) film, while intermediate phase suppress the TPA response of SnO (SnO-rich) films, therefore carrier-induced absorption dominates the NLO behavior of SnO film on picosecond regime. Our results indicate a simple and effective way to modulate the NLO response of transparent conductive oxide SnO and SnO.