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

We demonstrate the first achievement of continuous-wave (CW) pumped second harmonic generation (SHG) in few- and mono-layer gallium selenide (GaSe) flakes, which are coated on silicon photonic crystal (PC) cavities. Because of ultrahigh second order nonlinearity of the two-dimensional (2D) GaSe and localized resonant mode in the PC cavity, SHG’s pump power is greatly reduced to microwatts. In a nine-layer GaSe coated PC cavity, while the optical power inside the GaSe flake is only 1.5% of that in the silicon PC slab, the SHG in GaSe is more than 650 times stronger than the third harmonic generation in silicon slab, indicating 2D GaSe’s great potentials to strengthen nonlinear processes in silicon photonics. Our study opens up a new view to expand 2D materials’ optoelectronic applications in nonlinear regime and chip-integrated active devices. 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.