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 SnO2 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 SnO2 and SnO films. Interestingly, the magnitude of the effective nonlinear absorption coefficient (βeff) of both intermediate SnO2-x and SnOx are enhanced after the change of Sn2+/Sn4+ 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 SnOx (SnO-rich) film, while intermediate phase suppress the TPA response of SnO2-x (SnO2-rich) films, therefore carrier-induced absorption dominates the NLO behavior of SnO2-x film on picosecond regime. Our results indicate a simple and effective way to modulate the NLO response of transparent conductive oxide SnO2 and SnO.

Optical limiting (OL) materials are of great importance for protecting optoelectronic sensors and human eyes from high power laser illumination. Despite extensive efforts on various OL materials (organic molecules, metal clusters, low dimensional semiconductors, etc.), however, a candidate OL material with ultrafast response speed, broad laser protection window and large transmission modulation remains elusive. Gallium oxide (β-Ga2O3) is one of the third generation wide-band gap semiconductors. Due to its large band gap (~4.8 eV), high breakdown field strength, low preparation cost and compatibility with other III-N semiconductors, Ga2O3 has huge potential for high-power and lowloss photonic devices. In this study, we investigated the ultrafast optical limiting response in β-Ga2O3 (100) single crystal by using the femtosecond pump-probe technique. Using a 355 nm femtosecond laser pulse as the modulation source, ultrafast broadband transmission modulation was achieved across the white-light continuum probe spectrum (450-750 nm) via the non-degenerate two-photon absorption in Ga2O3. Furthermore, a slow Drude free-carrier plasma effect was observed <5 ps after pump excitation, which is related to the non-degenerate two-photon absorption induced carrier absorption. Our results indicate that Ga2O3 is a promising material platform for broadband optical limiting and highspeed all-optical signal processing.