Ultrafast all-optical second harmonic wavefront shaping

Abstract

Optical communication can be revolutionized by encoding data into the orbital angular momentum of light beams. However, state-of-the-art approaches for dynamic control of complex optical wavefronts are mainly based on liquid crystal spatial light modulators or miniaturized mirrors, which suffer from intrinsically slow (µs-ms) response times. Here, we experimentally realize a hybrid meta-optical system that enables complex control of the wavefront of light with pulse-duration limited dynamics. Specifically, by combining ultrafast polarization switching in a WSe₂ monolayer with a dielectric metasurface, we demonstrate second harmonic beam deflection and structuring of orbital angular momentum on the femtosecond timescale. Our results pave the way to robust encoding of information for free space optical links, while reaching response times compatible with real-world telecom applications.

Fig. 1. Schematic representation of the operating principle of the cascaded TMD-metasurface structure for ultrafast wavefront shaping.

In the first step, the polarization axes of two orthogonally linear polarized pulse replicas with the same pulse duration τ are aligned along the main crystal axes (armchair (AC) and zig-zag (ZZ) directions, respectively) of the WSe₂ monolayer. As a result of the D_3h symmetry of the crystal lattice and the associated contributions of the nonlinear susceptibility, depending on the time offset (Δt) of the two pulse replicas, the signal of the generated second harmonic (SH) in the WSe₂ monolayer is emitted either along the AC (Δt > τ) or the ZZ direction (Δt = 0). In the second step of the cascaded structure, a quarter-wave plate (QWP), whose fast axis is oriented at a 45° angle with respect to both, AC and ZZ direction, leads to a left (right) handed circular polarization of the SH for an emission along AC (ZZ), depending on the temporal delay in the aforementioned SHG process. Finally, the designed silicon metasurface manipulates the SH wavefront depending on the helicity of the incident radiation, which, in the displayed example, leads either to a Gaussian or a vortex beam shape. E and k denote the electric field and the wave vector, respectively. “pol” is the abbreviation of “polarization”.

Fig. 2. Components of the fabricated hybrid meta-optical system.

a Optical microscope image of the employed WSe₂ monolayer. The white dotted line highlights the boundaries. b, c SH spectra of the WSe₂ monolayer for ∆t > τ (b) and ∆t ≈ 0 (c). d–f The top row shows the phase distribution for different circular polarizations of the incident field. The bottom row shows optical images of the metasurfaces (left) and SEM images of the central areas of the structures (right) for d beam deflection, e Gaussian-to-vortex beam switching, and f topological charge switching, respectively.

Fig. 3. Theoretical (left) and experimental (right) far-field distributions.

a–c, g–i LCP and d–f, j–l RCP light transmitted through the polarization-dependent metasurfaces in Fourier space. The first row shows results for beam deflection, the middle row for Gaussian-to-vortex beam switching, and the bottom row for topological charge switching. For (c), (f), (i), (l) propagation of the vortex beam through a cylindrical lens was additionally considered.

Fig. 4. Ultrafast wavefront shaping dynamics.

a Cross section of the far-field intensity as a function of the delay time ∆t for the case of Gaussian-to-vortex beam switching (full interferometric trace). The inset shows a fragment of the interferometric fringes obtained from the region marked by the white dashed lines. b Reduced interferometric trace for frames corresponding to a linear output polarization only. The inset shows the intensity in the center of the map (dashed white line) versus time. The red curve represents a guide to the eye obtained by smoothing the experimental data using a Savitzky–Golay filter.