Abstract
Abstract The nonlinear processes of frequency conversion such as second harmonic generation (SHG) usually obey certain selection rules, resulting from the preservation of different kinds of physical quantities, e.g. the angular momentum. For the SHG created by a monolayer of transition-metal dichalcogenides (TMDCs) such as WS2, the valley-exciton locked selection rule predicts an SHG signal in the cross-polarization state. By combining plasmonic nanostructures with a monolayer of TMDC, a hybrid metasurface is realized, which affects this nonlinear process because of an additional polarization conversion process. Here, we observe that the plasmonic metasurface modifies the light-matter interaction with the TMDC, resulting in an SHG signal that is co-polarized with respect to the incident field, which is usually forbidden for the monolayers of TMDC. We fabricate such hybrid metasurfaces by placing plasmonic nanorods on top of a monolayer WS2 and study the valley-exciton locked SHG emission from such system for different parameters, such as wavelength and polarization. Furthermore, we show the potential of the hybrid metasurface for tailoring nonlinear processes by adding additional phase information to the SHG signal using the Pancharatnam-Berry phase effect. This allows direct tailoring of the SHG emission to the far-field.
Highlights
In the past years, two-dimensional layered materials have been studied more intensely since the discovery of the properties of graphene [1]
We observe that the plasmonic metasurface modifies the light-matter interaction with the transition-metal dichalcogenides (TMDCs), resulting in an second harmonic generation (SHG) signal that is co-polarized with respect to the incident field, which is usually forbidden for the monol ayers of TMDC
We present experimental results of the nonlinear coupling in a hybrid metasurface consisting of plasmonic metasurfaces on top of a monolayer WS2
Summary
Two-dimensional layered materials have been studied more intensely since the discovery of the properties of graphene [1]. The nanorods are designed to be resonant at a fundamental wavelength of 1240 nm, which is half of the bandgap energy of the 1L-WS2 corresponding to a wavelength of 620 nm (Figure 1B) In such way, the localized surface plasmon polariton resonances of the Au nanorods can facilitate the coupling by a two-photon process to the WS2 to lead to enhanced SHG. To approach the best overlap of the nanorods’ resonance wavelengths with half of the bandgap energy of the 1L-WS2, we fabricate three metasurfaces with different nanorod lengths [23] In this way, the slightly modified coupling between the nanorods and the 1L-WS2 is obtained, and fabrication tolerances can be compensated. The inset shows a scanning electron microscopy (SEM) image of the Au-metasurface with antenna lengths of 220 nm (scale bar equals 1 μm)
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