Strongly resonant metasurfaces supported by reflective substrates for highly efficient second- and high-harmonic generations with ultralow pump intensity.

Metasurfaces composed of plasmonic nanoantennas generate a highly enhanced local field when supported by reflective substrates, enabling efficient harmonic generation with a pump of ultralow peak intensity. Second-harmonic generation efficiency from the metasurfaces of bowtie silver nanoantennas embedded with lithium niobate reaches up to about 0.4% for pumping with peak intensity of 100 MW cm at 1050 nm. The above conversion efficiency is orders-of-magnitude higher compared with the preceding results and the required pump intensity is an order-of-magnitude lower than the previous results, originating from the strong intensity enhancements both at fundamental and second-harmonic frequencies. Numerical results also show that the silver nanoantenna array surrounded by a noble gas and supported by a reflective substrate can generate high-order harmonics with dramatically low pump threshold reaching down to a few GW cm. For a peak intensity 10 GW cm of the pump, the broadband spectrum with harmonic orders up to about 90 can be generated from the metasurface. The nanostructures that we proposed can find broad applications for biosensing and nonlinear surface spectroscopy based on second/third-harmonic generation. They also enable us to use compact femtosecond pump sources for obtaining coherent extreme ultraviolet and soft-X rays by high-harmonic generation.