硅超构表面上强烈增强的三次谐波

We report the enhanced third-harmonic generation (THG) from a silicon metasurface consisting of an array of L-shaped nanoresonators. The L-shaped nanoresonator is designed as a small cuboid with a notch cut from one corner. And 16×15 L-shaped nanoresonators are arranged into an array with a square lattice. In order to fabricate the structure, a 600-nm-thick silicon layer is first deposited on a 500-μm-thick sapphire substrate, which is then patterned into the metasurface structure by using electron beam lithography and inductively coupled plasma dry etching process. To evaluate the linear optical property of the fabricated metasurface, a continuous-wave narrow band laser with a tunable wavelength range of 1530-1560 nm is employed to measure the transmission and reflection spectrum. The measurement results show a Fano resonance at a wavelength of 1548 nm when the incident laser is linearly polarized along the long arm of the L-shaped nanoresonator. Pumping at the resonant wavelength, the metasurface shows significant polarization sensitivity for the transmitted light and the reflected light. To excite the THG signal from the metasurface, a femtosecond pulsed laser with a tunable wavelength range of 1540-1560 nm is then employed as the pump. Strong THG signal is observed when the laser wavelength is tuned on the resonant wavelength (1548 nm), indicating a conversion efficiency of ~3×10^-7. By comparing the THG signals triggered on- and off-resonance, an enhancement factor of 220 is extracted, which is attributed to the field-enhancement of the Fano resonance. The resonance enhanced THG signal also has polarization-dependence with an extinction ratio of 15 dB. These experimental results are verified well by numerical simulations based on a finite-element technique, including the Fano resonance and the enhanced THG process. By combining the numerically calculated electrical field of the resonant mode and the calculation of nonlinear polarizations, the resonance enhanced THG as well as its polarization-dependence are confirmed numerically. The realized strongly enhanced THG from the silicon metasurface promises to extend their linear optical functionalities into nonlinear regime.