Reconfigurable all-dielectric metalens with diffraction-limited performance

Abstract

Active metasurfaces, whose optical properties can be modulated post-fabrication, have emerged as an intensively explored field in recent years. The efforts to date, however, still face major performance limitations in tuning range, optical quality, and efficiency, especially for non-mechanical actuation mechanisms. In this paper, we introduce an active metasurface platform combining phase tuning in the full 2π range and diffraction-limited performance using an all-dielectric, low-loss architecture based on optical phase change materials (O-PCMs). We present a generic design principle enabling binary switching of metasurfaces between arbitrary phase profiles and propose a new figure-of-merit (FOM) tailored for reconfigurable meta-optics. We implement the approach to realize a high-performance varifocal metalens operating at 5.2 μm wavelength. The reconfigurable metalens features a record large switching contrast ratio of 29.5 dB. We further validate aberration-free and multi-depth imaging using the metalens, which represents a key experimental demonstration of a non-mechanical tunable metalens with diffraction-limited performance.

Fig. 1. Artistic rendering of a reconfigurable varifocal metalens.

Incident light is focused on the first focal plane (f₁ = 1.5 mm) when the meta-atoms are in the amorphous state and the second focal plane (f₂ = 2.0 mm) in the crystalline state.

Fig. 2. Phase maps of the metalens in amorphous and crystalline states.

a, f Ideal target phase profiles with continuous phase distribution; b, g four-level discretized phase profiles; and c, h final design taking into account phase responses of the meta-atoms. Difference between the ideal and final design phase maps at the d, i primary and e, j phantom focal planes. k 16 meta-atoms selected to construct the reconfigurable metalens. Colors correspond to the phase values shown in c, h.

Fig. 3. SEM scans of the metalens.

The images show the GSST meta-atoms with vertical sidewalls and excellent pattern fidelity.

Fig. 4. Optical characterization.

Focal spot profiles for the metalens in two states: a amorphous and b crystalline. Each plot contains the focal spot intensity distributions for the f₁ = 1.5 mm and f₂ = 2 mm focal planes. All the focal spots are diffraction-limited. The focal spots produced by ideal, aberration-free lenses of the same numerical aperture are marked with black dashed-curves. The insets show the 2-D images of the focal spots: f₁ = 1.5 mm and f₂ = 2 mm. Power contrast ratios are 10:1 and 90:1 for the a- and c-states, respectively.

Fig. 5. Imaging using the GSST varifocal metalens.

a Well-resolved lines of USAF-1951 resolution charts: the patterns have half periods close to the Rayleigh resolution limits of 7 µm and 9 µm in the a-state (f₁) and c-state (f₂), respectively. b Schematic of the setup for imaging multi-depth targets. Top-view photograph of the target consisting of two patterned samples overlapped at an angle of 45°. Camera images of the dual-depth target acquired by stationary metalens in a- and c-states.