Steering Second-Harmonic Beams in Nanophotonic Waveguides by Gratings

Efficient manipulation of the wavefront at the nanoscale is important for optical beaming, signal encoding, nonlinear phase matching, etc. Diffraction gratings are widely adapted for the control of the wavefront of light waves, but predominantly in linear optics. Here, we demonstrated the flexible steering of the second-harmonic (SH) beams in nanophotonic waveguides using gratings. The emission angle of the SH signals can be shifted by an amount of 60° from the waveguide axis into the collection cone, allowing the detection of the otherwise undetectable SH from co-propagating waveguide modes in the high-loss waveguides. The SH conversion efficiency (3 × 10–5 W–1) from co-propagating modes contributes up to 26.7% of the total SH signal. As the pump wavelength varies from 770 to 840 nm, the SH beam is swept by up to 22°. This design functions as a nonlinear grating coupler for nanophotonic systems and could be used for efficient nonlinear coherent light generation and integrated nonlinear nanophotonics.