Fig. 1. Schematic of the GSP single-focal metalens for linear-polarization conversion and focusing. (a) Artistic rendering of the working principle, which converts a normally incident x-polarized beam to a y-polarized focused beam in reflection. The right panel shows the meta-atom that consists of an Au nanoantenna on top of a SiO2 spacer and an Au substrate. (b) Simulated cross-polarized reflectivity Rcr, co-polarized reflectivity Rco, and orthogonal linear-polarization-conversion ratio PCR of the four elements at the design wavelength of λ=850  nm. (c) Simulated cross-polarized reflection phase φcr of the four elements at the design wavelength of λ=850  nm. The dimensions (l,w) of elements 1–4 are (228 nm, 90 nm), (155 nm, 360 nm), (90 nm, 228 nm), and (360 nm, 155 nm), respectively.

Fig. 2. Experimental demonstration of the GSP single-focal metalens for linear-polarization conversion and focusing. (a) Top-view SEM images of the fabricated sample with different magnifications. (b) Measured focal spot profiles in cross-polarization. (c) Measured intensity distributions in cross-polarization along the horizontal (x) and vertical (y) line cutting through the center of the focal spot in comparison with diffraction-limited focal spot profile. (d) Measured intensity profiles of the reflected beam in the x-z plane in cross-polarization. The x-polarized Gaussian beam is normally incident on the central part of the sample at the design wavelength of λ=850  nm.

Fig. 3. Measured PCR and absolute efficiencies at the corresponding focal plane as functions of wavelength. The PCR and absolute efficiency were integrated and averaged over two measured intensity profiles with the relative error being within 3%.

Fig. 4. Design, fabrication, and optical characterization of the dual-focal metalens for linear-polarization conversion, focusing, and beam splitting. (a) Schematic of the dual-focal metalens that converts a normally incident x-polarized beam into two y-polarized focused spots in reflection. (b) Top-view SEM image of the fabricated dual-focal sample. (c) Measured focal spot profiles in cross-polarization. (d) Measured intensity distributions in cross-polarization along the horizontal (x) line cutting through the centers of the focal spots. (e) Measured intensity profiles of the reflected beam in the x-z plane in cross-polarization. The x-polarized Gaussian beam is normally incident on the central part of the sample at the design wavelength of λ=850  nm.

Fig. 5. Generation of switchable power distributions with different distances between the incident beam and the dual-focal metalens. (a) Schematic diagram of the distance between the incident beam and the dual-focal metalens. (b) Measured power distributions with different distances between the incident beam and the dual-focal metalens.

Fig. 6. (a) Simulated cross-polarized reflectivity Rcr and orthogonal linear-polarization-conversion ratio PCR for elements 1 and 2. (c) Simulated cross-polarized reflection phase φcr for elements 1–4 that provide a linear phase variation spanning a 2π range.

Fig. 7. (a) Calculated phase profile and (b) designed geometry of the single-focal metalens with the diameter of D=50  μm and the focal length of f=60  μm at the wavelength of λ=850  nm.

Fig. 8. Additional SEM image of the fabricated single-focal metalens, which shows meta-atoms with different shapes more clearly.

Fig. 9. Schematic of the experimental setup for characterizing the metalens. HWP, half-wave plate; LP, linear polarizer; BS, beam splitter; Ob, objective; L, lens.

Fig. 10. Measured intensity profiles of the single-focal metalens at wavelengths of (a) 775 nm, (b) 800 nm, (c) 900 nm, and (d) 950 nm. All the intensity profiles are measured in cross-polarization, and the incident light is a normally incident x-polarized Gaussian beam.

Fig. 11. Optical characterization of the single-focal metalens at the wavelength of 850 nm when a y-polarized Gaussian beam is normally incident on the metalens. All the intensity profiles are measured in cross-polarization.

Fig. 12. (a) Calculated phase profile and (b) designed geometry of the dual-focal metalens with the diameter of D=50  μm, the focal length of f0=60  μm, and the separation of s=20  μm at the wavelength of λ=850  nm. The phase is calculated with φ(x,y)={2πλ[(x+s2)2+y2+f02−(s2)2+f02](x≪0)2πλ[(x−s2)2+y2+f02−(s2)2+f02](x>0).