Tensor-driven geometric phase in nonlinear AlGaAs metasurfaces

TL;DR

The work addresses nonlinear geometric phase control in metasurfaces by exploiting rotation-sensitive $\chi^{(2)}$ in AlGaAs. It combines a TAM-based phase analysis $m^{2\omega} = 2 m^{\omega} \pm 2 + \nu$ with tensor-rotation effects to obtain a composite SH phase $\varphi(\beta)$, enabling broadband, resonance-independent wavefront shaping. Experimentally, two metasurfaces demonstrate distinct nonlinear functionalities: nonlinear beam steering with SH deflection of ±12° and structured-light generation yielding a unit-charge vortex ($\ell=1$) at the SH, with SH efficiencies comparable to uniform designs. The results expand nonlinear metasurface design space, offering compact, high-contrast control of SH wavefronts for applications in communications, quantum optics, sensing, and optical manipulation, without reliance on linear resonances.

Abstract

Dielectric metasurfaces provide a unique platform for efficient harmonic generation and optical wavefront manipulation at the nanoscale. While several approaches are available for performing wavefront shaping, the one exploiting geometric phase streamlines significantly the design and fabrication process. It has been recently shown that, in III-V semiconductor alloys, the rotation of the crystal axes affects the phase and amplitude of second-harmonic generation (SHG) induced by circularly polarized light [1]. Based on this notion, we fabricated and characterized two aluminum gallium arsenide metasurfaces displaying the versatility of the geometric phase design approach through nonlinear beam steering and structured-light generation on the harmonic field.

Figures (5)

• Figure 1: (a) Schematic of the AlGaAs nanoantenna geometry, with $\beta$ the in-plane rotation angle. The circularly polarized pump and SH beams are shown in red and green, respectively. (b) Normalized SH intensity emitted in the vertical direction (i.e., orthogonal to the substrate plane) versus $\beta$. (c) Phase of the SH electric field emitted in the vertical direction versus $\beta$. (d,e) SH electric field amplitude in the plane bisecting the nanoantenna for $\beta=\qty{0}{\degree}$, (d), and $\beta=\qty{45}{\degree}$, (e).
• Figure 2: (a) Numerically calculated reflectance as a function of wavelength and meta-atom rotation angle $\beta$ for periodic array of resonators with period 900. (b) Electric field amplitude in the $xy$-plane bisecting the resonator. (c) normalized SH conversion efficiency and (d) phase as a function of the meta atom rotation angle $\beta$.
• Figure 3: (a) Experimental setup. (b) SEM image of the beam steering metasurface. Metasurface supercell highlighted with dashed rectangle. (c) Fourier space image of the metasurface SH signal. (d) Polarization analysis of the various diffraction orders.
• Figure 4: (a) SEM image of the structured light metasurface. (b) Near-field SH interference pattern. Inset shows a zoom-in of the fork-like pattern associated to a topological charge +1. (c) Measured and (d) simulated phase profile of the SH field emitted from the structured light metasurface. (e) Measured and (f) simulated far-field intensity map of the SH emitted by the metasurface.
• Figure 5: SH power as a function of pump power for different phase profiles.