Interface second harmonic generation enhancement in hetero-bilayer van der Waals nanoantennas

TL;DR

The study tackles how to induce and amplify second-harmonic generation at interfaces of van der Waals TMDC heterostructures. It fabricates WS$_2$/MoS$_2$ dual-layer nanoantennas that support anapole states to confine energy and couple to excitonic resonances, enabling interfacial SHG with strong enhancement. The authors observe up to $10^2$× SHG enhancement when $2\omega$ overlaps excitonic resonances and the anapole condition aligns with the fundamental wavelength, demonstrating a synergistic effect between material resonances and photonic confinement. This vdW-based nanoantenna approach opens a versatile path toward engineered multilayer metamaterials and nonlinear nanophotonics with tunable stacking and twist-angle degrees of freedom.

Abstract

Layered van der Waals (vdW) materials have emerged as a promising platform for nanophotonics due to large refractive indexes and giant optical anisotropy. Unlike conventional dielectrics and semiconductors, the absence of covalent bonds between layers allows for novel degrees of freedom in designing optically resonant nanophotonic structures down to the atomic scale, from the precise stacking of vertical heterostructures to controlling the twist angle between crystallographic axes. Specifically, while transition metal dichalcogenides monolayers exhibit giant second order nonlinear responses, their bulk counterparts with 2H stacking have zero second order response. In this work, we show second harmonic generation (SHG) arising from the interface of WS$_2$/MoS$_2$ hetero-bilayer thin films with an additional SHG enhancement in nanostructured optical antennas mediated by both the excitonic resonances and the anapole condition. When both conditions are met, we observe up to $10^2$ SHG signal enhancement. Our results highlights vdW materials as a platform for designing unique multilayer optical nanostructures and metamaterial, paving the way for advanced applications in nanophotonics and nonlinear optics.

Figures (11)

• Figure 1: Hetero-bilayer WS$_2$/MoS$_2$ van der Waals hexagonal nanoantennas sustaining anapole states. (a) Schematic of the dual-layer van der Waals (vdW) nanoantenna positioned on a SiO$_2$ substrate, composed of a bottom MoS$_2$ layer (blue) and a top WS$_2$ layer (red), each approximately 100 nm thick. Inset shows the second harmonic generation process where the absorption of two photons at the fundamental frequency $\omega$ leads to the generation of a frequency-doubled photon at $2\omega$. (b) Illustration of the in-plane TMDC crystalline honeycomb symmetry. Metal atoms are depicted in blue and chalcogenide ones in yellow. (c) Illustration of the interface between the two TMDC layers aligned at zero degrees, showing broken inversion symmetry region resulting in the second order non-linear signal. (d) Real and imaginary parts of the in-plane dielectric function for WS$_2$ (red) and MoS$_2$ (blue). Adapted from Ref.Munkhbat2022. (e) Far field illustration of an electric dipole (top) and a toroidal dipole (bottom) whose interference generates the anapole state. (f) Numerical FDTD simulations of the electric field intensity, $(|E|/|E_0|)^{2}$, at the anapole wavelength for a WS$_2$/MoS$_2$ disk with layer thicknesses of 92nm and 115nm, respectively, and radius of 280nm. The data is displayed along the z-plane of the TMDCs interface. Scale bar: 200 nm. (g) Numerical FDTD simulations of the normalized scattering cross section (in red) exhibiting a minima, and the normalized internal electromagnetic energy (in blue) exhibiting a maximum at the anapole wavelength. (h) Normalized scattering cross section for a WS$_2$/MoS$_2$ disk on a glass substrate, with radial size from 260nm to 360nm, and height of 92nm for the MoS$_2$ layer and 115nm for the WS$_2$ one. The sample is illuminated with normal incidence light from the air side. The dashed white line indicates the dip in far-field scattering attributed to the anapole state.
• Figure 1: (a) Bright field microscopy image of the fabricated stack before nanofabrication. (b) Image of the final sample after nanofabrication.
• Figure 2: Fabrication and linear optical characterization of WS$_2$/MoS$_2$ anapole nanoantennas (a) Fabrication steps of the dual TMDC layer nanoantennas. From left to right: exfoliation and transfer of the thin TMDC films, deposition of the electron beam resist, electron beam lithography writing, gold etching mask deposition, resist lift-off and final dry etching step and mask removal (not shown). (b) Height profile of the exfoliated and stacked TMDC layers, before nanofabrication, revealing a total height of 207nm. (c) Large area atomic force microscopy (AFM) scan of the final sample after nanofabrication. (d-e) Electron microscope images of the fabricated sample, for different tilting angles of 50 (d) and 60 (e) degrees. Scale bars: 200nm. (f) Linear reflectance spectra of the exfoliated and unpatterned reference of MoS$_2$ and WS$_2$, with the relative A ($X^{A}$) and B ($X^{B}$) exciton energies, and that of the reference double layer unpatterned stack. (g) Reflectance of a set of WS$_2$/MoS$_2$ antennas with radius ranging from 260nm to 360nm. The black dashed lines are a Gaussian fit to the relative anapole spectral dip. (h) Spectral position of the anapole condition, extracted from the fit in panel (g), as a function of the radius. The grey line correspond to the linear dependence predicted from numerical simulations.
• Figure 2: Atomic force microscopy profiles of three fabricated nanoantennas. The thickness of the final nanostructures is consistent with the pre-fabrication observations, confirming that the overetching has not affected the height of the TMDC hetero-bilayer.
• Figure 3: Exciton and anapole driven enhancement of WS$_2$/MoS$_2$ interface second harmonic generation (a) Raw counts of the SHG intensity as a function of the fundamental wavelength, for nanoantennas with radii of 290nm, 300nm and 310nm (dot markers), and the reference sample (diamond markers). (b) Comparison between the SHG ratio for a nanoantenna with radius of 280nm and the reflectance spectrum of the reference hetero-bilayer, where the SHG enhancement is resonant with the exciton position of the TMDC layers. (c) Three-dimensional plot of the SHG ratio, as a function of the nanoantenna radius and fundamental wavelength. (d) Normalized SHG signal for the same set of nanoantennas in panel 3c, revealing a linear dependence of the maximum SHG emission and the anapole wavelength (dashed white line).