Ultrafast optical limiting based on non-degenerate two-photon absorption in Ga2O3 single crystal

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.