Tunable edge and depth sensing via phase-change nonlocal metasurfaces

TL;DR

This work tackles the challenge of combining depth-of-field extension and edge detection in a single optical element. It introduces a wavelength-tunable nonlocal Huygens’ metasurface that performs spin-multiplexed processing, with the converted LCP component delivering depth information via a PB-phase vortex PSF and the unconverted RCP component providing edge detection through nonlocal momentum-space filtering. The device achieves over 40% polarization-conversion efficiency through Q-BIC–MDR coupling and attains ~100 nm spectral tunability by modulating the phase-change material Sb2S3. Demonstrations include depth mapping in the 15–40 cm range and robust edge enhancement validated by resolution targets, highlighting potential for compact, real-time imaging in autonomous navigation and biomedical applications.

Abstract

Performing simultaneous depth-of-field (DoF) extension and edge enhancement within a single optical element remains a fundamental challenge in advanced imaging. Here, we propose a wavelength-tunable nonlocal Huygens' metasurface capable of simultaneously extracting depth and edge features of images in a single-shot exposure. Using the selective polarization response of the Huygens' metasurfaces, the circularly polarized converted component undergoes geometric phase modulation for wavefront shaping to extend the DoF, while the non-converted component acts as a spatial frequency filter to enhance edge contrast. The integration of a phase-change material, Sb$_{2}$S$_{3}$, enables continuous tuning of the resonance wavelength across a range of 100 nm by modulating its refractive index, granting the system excellent broadband spectral adaptability. This work offers a novel and compact solution for real-time depth sensing and feature extraction in applications such as autonomous navigation and biomedical imaging.

Figures (6)

• Figure 1: Schematic illustration of the wavelength-tunable nonlocal Huygens’ metasurface for spin-multiplexed edge-enhanced imaging and depth sensing. (a) Under right-circularly polarized (RCP) illumination, the converted left-circularly polarized (LCP) light, modulated by the metasurface's geometric phase profile, outputs the computationally extracted image depth information. The unconverted RCP component undergoes momentum-space filtering by the metasurface, generating an edge-enhanced image. The color bar on the left indicates the resonant wavelength shift associated with the refractive index change of Sb$_{2}$S$_{3}$. (b) Phase and amplitude profiles corresponding to RCP and LCP for spin-multiplexed imaging. (c) Metasurface unit cell comprising crescent-shaped Sb$_{2}$S$_{3}$ nanopillars deposited on a glass substrate in a hexagonal array. The rotation angle of the nanopillars imparts a geometric phase gradient.
• Figure 2: Optical responses of output converted LCP light. (a) Transmission spectra of the converted LCP component for Sb$_{2}$S$_{3}$ metasurfaces with refractive indices of 3.0 and 3.3. The red and blue shaded regions denote the bandwidths of the Q-BIC and MDR, respectively. (b) Electric field and current vector profiles within the unit cell at the resonance wavelengths for metasurfaces with the corresponding refractive indices. (c) Phase profiles of the metasurface for output RCP and LCP light. (d) Spectral variation of the refractive index (n, k) of Sb$_{2}$S$_{3}$.
• Figure 3: Optical responses of output unconverted RCP light. (a) Transmission spectra of the Sb$_{2}$S$_{3}$ metasurface with refractive indices varying from 3 to 3.3. (b) Transmittance variation with incident angle for Sb$_{2}$S$_{3}$ metasurfaces ($n = 3$ and $n = 3.3$) at their respective resonant wavelengths. (c) Two-dimensional transmission dispersion maps at the operating wavelengths for Sb$_{2}$S$_{3}$ metasurfaces with refractive indices of 3 and 3.3, respectively.
• Figure 4: Design of the metasurfaces for depth sensing. (a) Phase profile of multiplexed vortex phase profiles with multiple topological charges. (b) Mapping curve between the azimuthal angle of the twin images and the axial defocus displacement of the incident image. (c) Schematic illustration of the two focal spots’ variation at diffraction distances of 15 cm, 20 cm, 25 cm, 30 cm, 35 cm, and 40 cm.
• Figure 5: Performance of the metasurface for edge detection using a resolution target. (a) Schematic diagram of the experimental setup. Laser: light source; P$_1$ and P$_2$: polarizer and analyzer; QWP: quarter-wave plate; Target: resolution mask; CCD: imaging sensor. Incident light passes through the designed metasurface, and by rotating the analyzer, different optical functionalities are captured by the CCD. (b) Edge-enhanced images obtained from unconverted light after transmission through metasurfaces with Sb$_{2}$S$_{3}$ refractive indices of 3.3, 3.15, and 3, respectively. (c) Cross-sectional intensity profiles along the blue dashed lines in (b), showing edge contrast performance under three refractive indices.