Polarization independent enhancement of zeroth order diffracted second harmonic from multilayer gallium selenide on a silicon resonant metasurface

Abstract

We demonstrate polarization-independent resonant-enhancement of second harmonic generation (SHG) from multilayer Gallium Selenide (GaSe) on a silicon-based resonant metasurface. Two-dimensional hexagonal photonic lattice with circularly symmetric silicon meta-atoms are designed to achieve resonant field enhancement at the fundamental wavelength independent of the incident polarization direction. Such structures are however found to exhibit strong resonant field depolarization effects at the fundamental excitation fields resulting in modified nonlinear polarization components when compared to the native GaSe layer. Furthermore, the sub-wavelength metasurface designed to have resonances at the fundamental wavelengths act as a higher order diffraction grating at the second harmonic wavelength. Nonlinear wave propagation simulations show that the higher order diffracted SHG exhibit strong polarization dependent enhancement with characteristics very different from the native GaSe layer. In this context, polarization independent enhancement of the second harmonic signal is achieved only for the zeroth order diffracted component. Experimental study of second harmonic generation from the GaSe layer integrated with the silicon metasurface shows maximum nonlinear signal enhancement on-resonance with polarization dependence identical to the native GaSe layer by selectively detecting the zeroth-order diffracted component. This work shows that it is not sufficient to use symmetric meta-atoms in such 2D material integrated resonant metasurfaces for achieving polarization independent nonlinear optical enhancement. Depolarization of the resonant fields and higher-order diffraction at the nonlinear signal wavelength need to be considered as well.

Giant Enhancement of Continuous Wave Second Harmonic Generation from Few-Layer GaSe Coupled to High-Q Quasi Bound States in the Continuum

“…On the contrary, the y-polarized SHG intensity follows a 4-fold rotational symmetry when pumping off the metasurface (Figure 4b), resulting from the crystalline symmetry of GaSe. 50 It should be noted that we deliberately aligned the polarization of the quasi-BIC mode to the armchair crystallographic axis of the GaSe flake in order to obtain the maximal SHG signal along the y-axis.…”

“…The y -polarized SHG intensity of the GaSe flake on the metasurface is dominated by the polarization of the quasi-BIC, which comes to the maximum under a pump laser with y -polarized as well. On the contrary, the y -polarized SHG intensity follows a 4-fold rotational symmetry when pumping off the metasurface (Figure b), resulting from the crystalline symmetry of GaSe . It should be noted that we deliberately aligned the polarization of the quasi-BIC mode to the arm-chair crystallographic axis of the GaSe flake in order to obtain the maximal SHG signal along the y -axis.…”

Nonlinear Dielectric Nanoantennas and Metasurfaces: Frequency Conversion and Wavefront Control

“…Unpatterned mid-index hybrid organic–inorganic 2D perovskite crystalline films of sub-100 nm thickness have demonstrated resonant visible TH emission at their excitonic wavelengths with conversion efficiencies close to 10 –4 at <10 GW/cm 2 . Integrating monolayer or few-layers semiconductors with dielectric metasurfaces may also become a favorable route to endow additional nonlinear functionalities to the metasurface. − For example, few-layers GaSe coupled to THG-emitting Fano-resonant or quasi-BIC Si metasurfaces have demonstrated enhancement of SHG and SFG of several orders of magnitude at CW excitation compared to the bare 2D material. Note that given the centrosymmetric structure of Si, its second-order emission is negligible compared to that of GaSe, which possesses a noncentrosymmetric ε-type stacking of the constituting layers.…”

Estimation of the nonlinear-optical coefficient of GaSe:S crystals according to electro-optical measurements

“…[10][11][12] GaSe has recently been taken into consideration as a material for the optical range used in telecommunications, specifically the 1.5 -1.6 microns spectral region. [13][14][15][16][17] However, there are limitations to the use of GaSe related to its features. Firstly, due to the high refraction, the internal angle of phase matching exceeds a critical value, for example, while generating harmonics of radiation in the telecommunication range in samples with a cut orthogonal to the crystal's z axis.…”

“…Matching optical elements 18,19 or the use of nanoclops of material in the area of increased concentration of the optical field can act as a solution: the core of the fiber 14,16 or the surface of the metamaterial. 15 A different approach that enhances the crystal's optical and mechanical qualities involves doping it with isovalent elements [20][21][22] particularly S. [23][24][25] This in turn makes it possible to prepare the necessary optical cuts of the crystal by fixing the crystal sample in an epoxy compound with subsequent processing. 26 The direct interaction of near-IR wavelengths with sub-THz frequencies in a nonlinear medium is one of the approaches discussed in the scientific community, which presumably promises a breakthrough in telecommunications technologies of the future generation.…”