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”.