Fig. 1. (a) Illustration of SHG and THG from the GaSe flake integrated on a moiré superlattice nanocavity constructed in a silicon photonic crystal slab. Inset: band diagram of the moiré superlattice and lattice of the ε-GaSe flake. (b) Calculated group velocity and group refractive index of the flat-band mode in the moiré superlattice nanocavity with a twisted angle of 2.646°. (c) In-plane (upper) and cross-section (bottom) distributions of electric field enhancement |Eloc|/|Ein| of the localized mode induced by mode locking in momentum space. (d) Calculated reflection spectrum of the moiré superlattice nanocavity integrated with the GaSe flake. Inset: Fourier spectrum of the flat-band mode obtained from the calculated real space electric field distribution of quasi-plane beam incidence excitations. (e) Calculated enhancement factors of SHG in the moiré superlattice nanocavity with excitation at different incident angles.

Fig. 2. (a) SEM image of the fabricated silicon moiré superlattice nanocavity with a twist angle of 2.6459°. Bottom: enlarged image of a unit cell of the moiré superlattice and central region of the nanocavity. (b) Optical microscope image of the device integrated with a few-layer GaSe crystal. (c) AFM image of the boundary of the GaSe flake [red frame in (b)]. (d) Reflection spectra for the resonance mode of the moiré superlattice nanocavities with (blue) and without (red) GaSe flakes.

Fig. 3. (a) Top: THG signal from the bare silicon moiré superlattice nanocavity. Middle: SHG and THG signals from the GaSe-integrated silicon moiré superlattice nanocavity. Bottom: SHG from the GaSe located on the bare silicon slab. Note: 0.5 s and 0.1 s refer to the exposure times used by the high-speed spectrometer in the testing system. (b) Dependence of SHG and THG intensities on the pump wavelength. (c) SHG intensities as a function of pump power. (d) THG intensities as a function of pump power.

Fig. 4. (a) Alignment of the moiré superlattice nanocavity with respect to the HWP. φ denotes the angle between the fast axis of the HWP and the x axis of the moiré superlattice nanocavity. (b) Polarization dependence of the reflection spectra of the pump laser. (c) On- and (d) off-resonance polarization dependence of the SHG from the GaSe flake.

Fig. 5. Spatial distributions of the resonance mode, SHG, and THG. (a)–(c) Experimentally measured spatial mappings of (a) the resonant mode and the corresponding (b) SHG and (c) THG from the fabricated GaSe-integrated moiré superlattice nanocavity. (d)–(f) Simulated near-field distributions of (d) |E|, (e) |P(2)|, and (f) |P(3)| for the resonance mode, SHG, and THG of the GaSe-integrated moiré superlattice nanocavity.