Fig. 1. (a) Schematic of the off-axis multi-wavelength dispersion controlling metalens. The metasurface is illuminated by three lights with different wavelengths and incident angles, and then focuses the reflected lights into the same point. (b) Schematic of a 3-chips DMD laser projection system with the dispersion-engineered metalens. The three monochromatic (R, G and B) images can be synthesized into a color image after reflected by the metasurface. BS: beam splitter.

Fig. 2. (a) Top-view of part of the designed metalens. (b) The element consists of a titanium dioxide (TiO₂) nanopillar (h₃=530 nm) on top of a silicon dioxide (SiO₂) layer (h₂=180 nm) above a silver substrate (h₁=200 nm). The period p=400 nm, the nanopillar has a square cross-section with width w. By adjusting the width, the reflection phase can be controlled. (c) Numerically calculated reflection phase shift as a function of w at three wavelengths of 473 nm, 532 nm, and 632 nm. As the nanopillar width w changes from 90 nm to 350 nm, the corresponding phase undergoes multiple 0-2π phase cycles. The refractive indexes of TiO₂ are 2.493, 2.43 and 2.373 for 473 nm, 532 nm, and 632 nm in the simulations, respectively.

Fig. 3. Comparison between the target phase profiles (a) and optimized results (b) at three wavelengths of 473 nm (left), 532 nm (middle), and 632 nm (right). The designed metasurface consists of 101×101 pixels. Abscissa and ordinate indicate the number of pixels.

Fig. 4. (a-c) Numerically calculated normalized intensity distributions in the yz-plane at the wavelengths of 473 nm, 532 nm, and 632 nm under TE polarized incidence. The corresponding incident angles are 0°, -17° and 17°, respectively. (d-f) Theoretical (dotted lines) and calculated (solid lines) normalized intensity curves of the three lights along the y-axis across the focal plane. (g-i) Normalized intensity distributions in the xy-plane at the focal plane for the wavelengths of 473 nm, 532 nm, and 632 nm.

Fig. 5. (a–c) Simulated monochromatic imaging by the metalens for three wavelengths of 473 nm, 532 nm, and 632 nm with incident angles of 0°, -17° and 17°, respectively. (d) Synthetic color image by the metalens.