In this paper, we report demonstration of sub-wavelength high-index contrast gratings which exhibit guided mode resonances for enhancing nonlinear optical effects from 2D materials transferred on top of the structures. Twodimensional hexagonal arrays of c-Si nanodisks on a silicon-on-insulator wafer have been designed to have normal incidence resonance in the 1550-1650 nm wavelength region. Numerical simulations were performed to show resonance variations with structural parameters and corresponding field enhancements outside the structure to aid nonlinear optical response from materials placed on top of the structures. The fabricated structures were characterized for linear reflection using an external cavity tunable laser as the incident light source at close to normal incidence and compared with simulated reflection. As a proof-of-concept, we transferred few-layer Gallium Selenide (GaSe) flake on to the grating using a dry transfer method to examine second harmonic generation response of GaSe in presence of the grating. Second harmonic generation measurements showed strong SHG signal from the GaSe on top of the grating structure, with enhancement of ~ 15x observed at 1645 nm close to fundamental resonance wavelength. No SHG emission was observed from the silicon nanodisks withput the GaSe overlayer. Spectral and power of the SHG were also characterized. This work shows that the potential of heterogeneous integration of high nonlinearity 2D materials on to silicon based resonant optical structures to realize high efficiency nonlinear metasurfaces.
We report spatially resolved measurement of third-harmonic generation (THG) emission from a Tin diselenide (SnSe2) multi-layer flake at a fundamental excitation wavelength of 1550 nm using a nonlinear optical microscopy system and study its thickness dependence. We also estimate the magnitude of the real part of the electronic nonlinearity susceptibility (χ(3) coefficient) by analyzing the thickness-dependence and found to be approximately 1.6×10-19 m2/V2, which is around 1500 times higher than that of the glass when measured with the same settings. We find excellent agreement between the measured THG thickness dependence and the analytical model considering absorption of harmonic emission in SnSe2 medium, phase mismatch and the multipath interference due to the underlying oxide/Si substrate. We also measure the second harmonic generation from same flake and find this to be maximum for thickness in the range of 10-12nm.