Recently, 2D materials have attracted considerable research attention in nanophotonic and integrated optoelectronic devices due to their intriguing thermal, mechanical, exciton and nonlinear optical response. Despite extensive efforts on various 2D materials (graphene, transition metal dichalcogenides, h-BN, etc.), however, a candidate material with large nonlinear optical (NLO) properties, ultrafast response speed, broadband response window and good thermal stability remains elusive. Black Phosphorus (BP) is a representative 2D semiconductors, which has huge potential for photodetectors, photo-catalysis, thin-film transistors and photodynamic therapy. In this study, we investigated the ultrafast NLO response of black phosphorus quantum dots (BP QDs)/water dispersions via Z-scan measurements with both femtosecond and picosecond laser pulses. We found a sign change of nonlinear refractive index n2 of BP QDs from femtosecond to picosecond timescale. The dynamic response mechanism of BP QDs/water dispersions was studied by using femtosecond transient absorption spectroscopy under 355 nm excitation. A broadband absorption signal was observed ~0.5 ps after pump excitation, and the decay lifetime of this signal was rather slow (several nanoseconds). Our results indicate that BP QDs is a promising NLO material for ultrafast all-optical signal processing applications.
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.
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