We experimentally investigate coupling of the photoluminescence (PL) from monolayers of MoS2 to Mie-resonant metasurfaces consisting of silicon nanocylinders. By a systematic variation of the nanocylinder diameter, we sweep the metasurface resonances over the excitonic emission band of monolayer MoS2. We observe strong enhancement, as well as spectral and directional reshaping of the emission. By a comprehensive optical characterization, we unveil the different physical factors, including electronic, photonic, and mechanical influences, responsible for the observed PL changes. Importantly, we show that by geometrical tuning of the nanocylinder resonances, the emission can be tailored from occurring under very large angles to being directed out of the substrate plane. Our results highlight the need and potential of controlling not only the photonic, but also electronic and mechanical environmental factors for tailoring PL from TMD monolayers by integrating them in nanophotonic architectures.

Lithium niobate is an excellent and widely used material for nonlinear frequency conversion due to its strong optical nonlinearity and broad transparency region. Here, we report the fabrication and experimental investigation of resonant nonlinear metasurfaces for second-harmonic generation based on thin-film lithium niobate. In the fabricated metasurfaces, we observe pronounced Mie-type resonances leading to enhanced second-harmonic generation in the direction normal to the metasurface. We find the largest second-harmonic generation efficiency for the resonance dominated by the electric contributions because its specific field distribution enables the most efficient usage of the largest element of the lithium niobate nonlinear susceptibility tensor. This is confirmed by polarization-resolved second-harmonic measurements, where we study contributions from different elements of the nonlinear susceptibility tensor to the total second-harmonic signal. Our work facilitates establishing lithium niobate as a material for resonant nanophotonics.

Resonant semiconductor metasurfaces are an emerging versatile platform for nonlinear photonics. In this work, we investigate second-harmonic generation from metasurfaces consisting of two-dimensional square arrays of gallium arsenide nanocylinders as a function of the polarization of the fundamental wave. To this end, we perform nonlinear second harmonic microscopy, where the pump wavelength is tuned to the resonances of the metasurfaces. Furthermore, imaging the generated nonlinear signal in Fourier space allows us to analyze the spatial properties of the generated second harmonic. Our experiments reveal that the second harmonic is predominantly emitted into the first diffraction orders of the periodic arrangements, and that its intensity varies with the polarization angle of the fundamental wave. While this can be expected from the structure of the GaAs nonlinear tensor, the characteristics of this variation itself are found to depend on the pump wavelength. Interestingly, we show that the metasurface can reverse the polarization dependence of the second harmonic with respect to an unstructured GaAs wafer. These general observations are confirmed by numerical simulations using a simplified model for the metasurface. Our results provide valuable input for the development of metasurface-based classical and quantum light sources based on parametric processes.

The coupling of two-dimensional materials with optical metasurfaces is a promising avenue to enhance the advantageous properties of both platforms. Here we integrate an ultrathin monolayer of the transition metal dichalcogenide (TMD) MoS2, grown by chemical-vapor deposition, with a silicon metasurface, to obtain a hybrid system with enhanced nonlinear response. To this end, we utilize a metasurface exhibiting resonances with high quality factors, which provides increased optical fields. Using the nonlinearity of the TMD monolayer, these resonantly enhanced fields enable more efficient nonlinear frequency conversion. In particular, we experimentally observe an enhanced efficiency of second-harmonic generation in our hybrid structure. By comparing second-harmonic generation using different photonic resonances, we furthermore identify optimized conditions for the spatial distribution of the local optical fields to maximize the nonlinear response. Our results enable the precise design of hybrid structures consisting from TMDs and metasurfaces for future applications.

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A one‐dimensional periodic rectangular potential, also known as the Kronig‐Penney (KP) potential, transforms the parabolic dispersion of a free particle into a set of bands separated by bandgaps. However, if the potential wells are deep enough, the lowest bands converge into a set of single discrete states, numbered from j = 1 to j = jmax, which can be even or odd, describing the number of extrema. Here, discrete and continuous KP states are experimentally observed within a periodically modulated metal–organic microcavity. Depending on the width of the photonic wires, the thickness of the cavity, and the added metal grating, the parity of the highest localized state jmax can be either even or odd, leading to a complementary parity of the first continuous mode. The apex of this Bloch‐like state in turn either starts at k = 0, or a π‐state at the edges of the Brillouin zone, formed by the periodic metallic wires. An easy analytical explanation and numerical confirmation of zero‐ or π‐phase locking for laser modes in spatially modulated microcavities are provided.

We experimentally analyze enhancement of second-harmonic generation from CVD-grown MoS2 monolayers coupled to a Si metasurface exhibiting different resonances and reveal how different resonant fields influence the nonlinear conversion efficiency.

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