Microwatts continuous-wave pumped second harmonic generation in few- and mono-layer GaSe

Low-power laser beams can realize efficient nonlinear optics in an ultrathin gallium selenide flake by coating it on a photonic crystal cavity. Second-harmonic generation (SHG) has been demonstrated in two-dimensional materials previously, but pulsed lasers with high peak powers were required. For practical applications, SHG realized using low-power, low-cost light sources is highly desirable. Xue-Tao Gan and co-workers from Northwestern Polytechnical University in Xi’an, China, have achieved this by coating a silicon photonic-crystal cavity with a 7.8-nanometer-thick layer of gallium selenide, corresponding to nine monolayers. The structure supported SHG when pumped with sub-milliwatt powers from a continuous-wave infrared laser. A similar experiment with a monolayer of gallium selenide also worked, but resulted in about 75 times weaker SHG. Further enhancement of SHG is expected by using a photonic-crystal cavity with a larger Q factor.