Two-dimensional layered materials (2DLMs), such as transition metal dichalcogenides and black phosphorus, have attracted intensive interests for numerous photonic and optoelectronic applications including lasers, photodetectors, modulators and plasmonics. In particular, 2DLMs have been demonstrated for various nonlinear optical applications, such as second, third and even high-order harmonic generation as well as four-wave mixing . Among various frequency conversion methods, difference frequency generation (DFG), employing the second-order nonlinear optical susceptibility, has a great potential for flexible and broadband tuneable wavelength generation using parametric interactions. Here, we demonstrate the DFG in few-layer Gallium Selenide (GaSe) and mono-layer molybdenum disulfide (MoS2) with the pump at 400 nm and the idler at 1020 nm. The generated signal in GaSe at a wavelength around 658 nm increases linearly with the power of the idler. Our results pave the way toward optical parametric generation in atom-thickness level for various applications (e.g., quantum photonics [8], THz generation [6]).
