Fig. 1. (a)–(c) Schematic of (a) a transmissive achromatic metasurface that enables a light source with a continuously changing wavelength to have (b), (c) the same focal point. (d)–(f) Schematic of (d) a reflective achromatic metasurface that bestows a twice tangential momentum to (e), (f) the incoming light source with continuously changed wavelength. (g)–(i) Schematic of a broadband retroreflector comprised of a transmissive achromatic metasurface combined with a reflective achromatic metasurface. (j), (k) Spectral phase profiles (left panel) for the central meta-unit and spatial phase profiles (right panel) along the radial direction for a (j) transmissive/(k) reflective achromatic metalens within an arbitrary wavelength range of $\lambda \in [{\lambda }_{\min },{\lambda }_{\max }]$.

Fig. 2. (a), (b) Schematics of (a) the transmissive meta-units comprised of silicon square pillars and square holes, and (b) the reflective meta-units comprised of silicon square pillars and square holes on a gold film. (c) Theoretical transmission phase profiles for the transmissive metalens at r₀$=6.3\mathrm{\mu }\mathrm{m}$ (red dashed line), $4.5\mathrm{\mu }\mathrm{m}$ (azure dashed line), and 0 (black dashed line), associated with the simulated transmission efficiencies (red, azure, and black dotted lines) and phase profiles (red, azure, and black solid lines) of three meta-units with $W_p^L=0.18\mathrm{\mu }\mathrm{m}$, $W_h^L=0.34\mathrm{\mu }\mathrm{m}$, and $W_h^L=0.10\mathrm{\mu }\mathrm{m}$, respectively. (d) Theoretical reflection phase profiles for the reflective metalens at R₀$=5.4\mathrm{\mu }\mathrm{m}$ (red dashed line), $3.6\mathrm{\mu }\mathrm{m}$ (azure dashed line), and 0 (black dashed line), associated with the simulated reflection efficiencies (red, azure, and black dotted lines) and phase profiles (red, azure, and black solid lines) of three meta-units with $W_p^R=0.20\mathrm{\mu }\mathrm{m}$, $W_h^R=0.32\mathrm{\mu }\mathrm{m}$, and $W_h^R=0.18\mathrm{\mu }\mathrm{m}$, respectively. The lattice constants along the x and y directions are P$=0.45\mathrm{\mu }\mathrm{m}$, and the thickness of silicon is H₁$=0.6\mathrm{\mu }\mathrm{m}$. The silicon layer is covered by SU-8 polymer, with the thickness and refractive index being H₂$=2\mathrm{\mu }\mathrm{m}$ and 1.555, respectively, and the refractive indices of other materials are extracted from Ref. [34].

Fig. 3. (a) Distributions of $|$E${|}^2$ in the x–z plane for the transmissive metalens of 13.95 µm in diameter under the illumination of x-polarized light waves with 0° and 16° at 1.35, 1.55, 1.75, and 1.95 µm. (b) Foci offsets and (c) focal lengths versus incidence angle for four wavelengths (1.35, 1.55, 1.75, and 1.95 µm). (d) Distributions of $|$E${|}^2$ in the x–z plane for the reflective metalens of 12.15 µm in diameter under the normal illumination of x-polarized light waves at 1.35, 1.55, 1.75, and 1.95 µm. (e) Focal length of the reflective metalens versus light wavelength.

Fig. 4. (a) Distributions of the real part of E_ref,x for the broadband metasurface retroreflector under the illumination of x-polarized light waves for four wavelengths and three incidence angles (5°, 10°, and 15°). (b) In the upper semicircles, the normalized $|A_x(\phi ,k_y=0,\theta ,\lambda ){|}^2$ (solid lines, with respect to its maximum value) versus spatial angle $\phi$ and $\min [|r_{\mathrm{ret}}(\theta ,\lambda ){|}^2,|r_{\mathrm{nor}}(\theta ,\lambda ){|}^2]/\max [|r_{\mathrm{ret}}(\theta ,\lambda ){|}^2,|r_{\mathrm{nor}}(\theta ,\lambda ){|}^2]$ (dotted arcs) under the x-polarized incidence for different wavelengths and incidence angles: 5° (red), 10° (orange), and 15° (green). In the lower semicircles, the normalized $|\mathrm{\Delta }{A}_x(\phi ,k_y=0,\theta ,\lambda ){|}^2$ (dashed lines, with respect to its maximum value) versus spatial angle $\phi$ for different wavelengths and incidence angles: 5° (red), 10° (orange), and 15° (green).

Fig. 5. (a) Reflection angles and (b) the difference between the reflection angles and the incidence angles versus incidence angle and wavelength under x- and y-polarized incidence. (c) Real and imaginary parts of the retroreflection coefficients r_ret$(\theta ,\lambda )$ versus incidence angles under x- and y-polarized incidence. (d) Values of $|$r_ret$(\theta ,\lambda ){|}^2$ versus incidence angle and wavelength under x- and y-polarized incidence.