Single Nanoflake Hexagonal Boron Nitride Harmonic Generation with Ultralow Pump Power

The strong nonlinear optical response of two-dimensional materials has applications in bioimaging and integrated optical information processing; however, past experiments were on diffraction limited samples or required intense pulsed lasers, which is a detriment to potential applications due to cost, power and complexity. Here we show that second harmonic generation can be achieved from single subwavelength two-dimensional material nanoflakes smaller than the diffraction limit using a plasmonic optical tweezer with a low-power (down to 3 mW) laser diode operating in continuous-wave mode. A double nanohole plasmonic tweezer enhances the local field and the local density of optical states to allow for trapping and significant nonlinear generation at the nanoscale. Also, the fact that it is a two-dimensional material means that it can be positioned closer to the highest field regions, realizing 2 orders of magnitude higher power second harmonic generation than other nonlinear materials like lithium niobate. The ability to have simple high efficiency nonlinear generation at the nanoscale will benefit future nonlinear optics applications of these emerging materials.