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 .
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 (red dashed line),
(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
,
, and
, respectively. (d) Theoretical reflection phase profiles for the reflective metalens at
R0 (red dashed line),
(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
,
, and
, respectively. The lattice constants along the
x and
y directions are
P, and the thickness of silicon is
H1. The silicon layer is covered by SU-8 polymer, with the thickness and refractive index being
H2 and 1.555, respectively, and the refractive indices of other materials are extracted from Ref. [
34].
Fig. 3. (a) Distributions of E 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 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 Eref,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 (solid lines, with respect to its maximum value) versus spatial angle and (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 (dashed lines, with respect to its maximum value) versus spatial angle 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 rret versus incidence angles under x- and y-polarized incidence. (d) Values of rret versus incidence angle and wavelength under x- and y-polarized incidence.