Enhancement of mid-infrared saturable absorption of graphene by compositing with PbSe for ultra-short pulse lasers in the ∼ 2.8 to ∼ 4 μm band

The suitably large modulation depth of the saturable absorber (SA) is crucial to obtaining short pulse lasers via passive Q-switching or mode-locking techniques. Graphene could theoretically be used as a mid-infrared (mid-IR) SA due to its zero band-gap. However, the low modulation depth of graphene in the mid-IR region limits its applications. In this work, to improve the mid-IR modulation depth of graphene, a strategy of functionalizing graphene with narrow-band gap semiconductors was adopted. Graphene/PbSe (G/PbSe) composite was synthesized by a facile hydrothermal process and the reflective G/PbSe SA was prepared by spin-coating method. The saturation absorption properties were investigated by the open aperture Z-scan technique. The results indicated that the saturation absorption properties of G/PbSe SA were largely enhanced, and the modulation depth was increased from 3.02 % to 5.4 % at ∼ 2.8 μm and from 2.5 % to 6.4 % at ∼ 4 μm compared with pure graphene. Moreover, with the as-prepared reflective G/PbSe SA, passively Q-switched Er:YAP laser operation at ∼ 2.8 µm was realized. At the pump power of 6.78 W, the pulse train with pulse width of 326 ns and repetition rate of 107 kHz was obtained, and the average output power was 498 mW. This study presents a way for preparing graphene-based mid-IR SAs for short pulse mid-IR lasers.