Fig. 1. Schematic function tunability and designs of the metalens. (a) The broadband achromatic focusing state without bias. (b) The dispersive focusing state with a saturated bias. The superstrate depicts the dielectric metasurface while the blue ellipsoids between the superstrate and substrate denote LCs. (c) Dependencies of φR (red curve) and PCR of a silicon pillar metaunit on f. Inset shows the dimensions of the unit. Dependency of φR on l and w at (d) 0.9 and (e) 1.4 THz, respectively. (f) Dispersion ratio of the silicon pillar as a function of l and w under LCP incidence. Dependency of PCR on l and w at (g) 0.9 and (h) 1.4 THz. The region circled by the black dashed line indicates a PCR over 12% from 0.9 to 1.4 THz. (i) Designed l and w of the silicon pillars along r from the center to the edge. (j) φA and φR at 0.9 (blue solid and dashed curves) and 1.4 THz (red solid and dashed curves) and residual φG. (k) Target φG and corresponding LC director distributions are labeled by the orange lines.

Fig. 2. Characterization of the metalens. (a) SEM image of the metasurface (top view). Scale bar: 300μm. Zoomed-in images of the silicon pillars labeled by the (b) orange and (c) blue squares in (a), respectively. (d) Cross-section image of the metasurface. Scale bars in (b)–(d): 100μm. (e) Optical image of the LC layer under crossed polarizers. (f) A zoomed-in image labeled by the orange square in (e). Scale bars in (e) and (f) are 1 mm and 500μm, respectively.

Fig. 3. Focusing performance of the metalens. (a) Simulated THz fields in the xz plane and (b) measured THz fields in the xz and xy plane (z=12.0mm) at 0.9, 1.1, and 1.4 THz, respectively, when no bias is applied. (c) Simulated and (d) measured THz fields in the xz and xy plane (z=13.0mm) of the same sample with a saturated bias. Dashed lines in (a)–(d) label the corresponding focal planes. White curves in (b) and (d) depict the transverse intensity distributions at z=12.0 and 13.0 mm, respectively. (e) Simulated and measured F, (f) FWHM at z=12.0 and 13.0 mm in (b) and (d), separately, and (g) measured device efficiency, as a function of f. The black dashed line in (f) depicts the diffraction-limited FWHM.

Fig. 4. Broadband THz imaging. (a) Schematic of the SNTM setup. The inset exhibits the micrograph of the object “smiling face,” where the scale bar indicates 1 mm. Measured intensity and phase distributions at (b) 0.9 and (c) 1.4 THz without bias. The white and orange curves depict the intensity profiles along the white and orange dashed lines, respectively. The outline of the object “smiling face” is labeled by black dashed lines in the phase diagrams. Measured intensity and phase distributions at (d) 0.9 and (e) 1.4 THz with a saturated bias.