Two-photon saturable absorption properties and laser Q-switch application of carbon quantum dots

In this Letter, high-quality carbon quantum dots (C-QDs) with an average size of 15 nm are synthesized by using a solvothermal method. A C-QD saturable absorber mirror (SAM) was prepared, characterized, and employed as an ultrafast optical switch successfully in a 1.0 μm solid-state laser. The saturable absorption effect (at 1 μm) far away from the linear absorption band of the C-QDs could be attributed to two-photon saturable absorption, which has a native characteristic of wavelength selectivity. The modulation depth (ΔT) and saturable energy intensity (ϕ_s) of the C-QD-SA was measured to be about 4% and 15.34  W/mm², respectively. By using this SA, a Q-switched Nd:GdVO₄ laser at 1 μm were first realized with the shortest pulse width of 66.8 ns and a maximum repetition rate of 1.13 MHz, respectively. The results indicate that C-QDs may found to be a decent carbon SA material for the high-repetition-rate pulsed laser applications.