Fig. 6. Angular nondispersive wavefront shaping using cascaded metasurface doublet.

(A) Schematic of angular nondispersive design using two layers of metasurfaces on both sides of a sapphire substrate. The phase gradient of the two-layer metasurfaces are $\frac{\mathit{d}{\mathrm{\phi }}^{\prime }}{\mathit{d}{\mathit{x}}^{\prime }}=\frac{2{\mathrm{\delta }}_{\mathrm{d}}^{\prime }}{\mathit{P}}$ on the top and $\frac{\mathit{d}\mathrm{\phi }}{\mathit{dx}}=\frac{2{\mathrm{\delta }}_{\mathrm{d}}}{\mathit{P}}$ on the bottom with ${\mathrm{\delta }}_{\mathrm{d}}=-{\mathrm{\delta }}_{\mathrm{d}}^{\prime }=30°$. A linearly broadband light is normally illuminated from the top metasurface and diffracted to the bottom metasurface. Two spots are generated from the top deflector as shown in the green optical path. (B) Transmitted angle with single-layer metasurface (red curve) and double-layer metasurfaces (blue curve). A nondispersive angle response is realized in unlimited bandwidth using double-layer metasurfaces. Measurement results of the holographic image of spade using (C) single-layer metasurface and (D) double-layer metasurfaces. The holograms are imaged at the entrance slit of a spectrometer for spectrum characterization. A slit with 20 μm in width is placed to select a line of the image light at specific position of P1, P2, … P5. The corresponding spectra for (E) dispersive case and (F) nondispersive case at position P1 to P5 show a red shift of the bandwidth for large angle in (E), while (F) spectra are essentially broadband.