a, Schematic illustration of spin-multiplexing nonlocal Huygens’ meta-lens. Upon illumination with RCP light, the excited nonlocal resonance generates wavelength-selective properties and incident-angle dependence. By geometrically rotating each unit, the output LCP light exhibits a focusing phase profile for bright-field imaging, while the output RCP light without phase modulation is employed for edge detection. Insets are schematic diagrams of spectral responses output LCP and RCP lights required by spin-multiplexing imaging. b, Functional relationship between transmission efficiencies TRL and TRR in an ultrathin metasurface.
a,b, Experimental and simulated transmission TRL (a) and TRR (b) spectra. Red and blue areas indicate the bandwidth of q-BIC and MDR, respectively. Blue dashed lines denote wavelengths of two dips. c,d, Simulated transmission coefficients as a function of the normalized in-plane wavevector for Dips 1 (c) and 2 (d).
a-c, Experimental xz-plane (a) and xy-plane (b) intensity distributions and bright-field imaging (c) at the nonresonant wavelength of 1500 nm and the resonant wavelength of 1560 nm. d, Experimental setups for edge-enhanced imaging. e, Experimental output images of reference, Dip 1, and Dip 2. The reference is the original object imaging without metasurface. Dips 1 and 2 are the scenarios with metasurface filtering. Scale bars are 20 μm. f, Intensity distributions along the white dashed lines.
Newswise — Bright-field and edge-enhanced imaging can provide morphological information of amplitude and phase objects. Combining these two imaging techniques facilitates the detailed visualization of intricate structures, such as biological tissues and cells. Multiplexing meta-lenses present promising candidates for achieving this functionality. However, the existing solutions are based on broadband local responses, which lack effective modulation of narrowband spectral responses. This limitation can degrade imaging quality due to crosstalk between different wavelengths under broad-spectrum illumination, especially in biomedical samples requiring a specific excitation wavelength.
In a new published in , a team of scientists, led by Professor Din Ping Tsai from Department of Electrical Engineering, City University of Hong Kong, Kowloon, Hong Kong SAR, China, and co-workers have experimentally demonstrated a spin-multiplexing high-quality-factor (high-Q-factor) meta-lens for simultaneous bright-field imaging and edge detection in the near-infrared region. The proposed nonlocal Huygens’ meta-lens consists of silicon crescent-shaped integrated-resonant units (IRUs) on a silica substrate. By introducing asymmetry within the in-plane parametric space, the symmetry-protected quasi-bound state in the continuum (q-BIC) is excited, achieving a high Q factor of 90 and a notable incident-angle dependence. The Fano-like interaction between q-BICs and in-plane Mie-type magnetic dipole resonance then results in the generalized Kerker condition, achieving a transmission polarization conversion TRL peak with efficiency up to 65% accompanied by a geometric phase robust to the rotation angle of IRUs. The unconverted output polarization TRR shows a transmission dip possessing a low value without the geometric phase, surpassing the theoretical limit of traditional nonlocal metasurfaces. These two output spin states are suitably utilized for bright-field imaging based on focusing phase control and edge detection through spatial frequency filtering, respectively, with wavelength-selective properties. This approach ensures minimal interference from other wavelengths, thereby enhancing the accuracy and reliability of the imaging and sensing processes.
These scientists summarize the operational principle of their meta-lens:
“We simultaneously target a high Q factor and dual functionalities: (1) bright-field imaging, requiring a transmission polarization conversion TRL peak with a geometric phase that remains robust to IRU rotation; and (2) edge-enhanced imaging, necessitating an unconverted output polarization TRR with a low-value transmission dip, no geometric phase, and strong incident-angle dependence driven by nonlocal effects. To achieve this, symmetry-protected q-BICs are excited, and their Fano-like interaction with in-plane Mie-type magnetic dipole resonances establishes the generalized Kerker condition, surpassing the theoretical limitations of traditional nonlocal metasurfaces.”
“Benefiting from these effects, the wavelength-selective focusing and bright-field imaging are demonstrated to have an efficiency enhanced by at least ten folds at resonant wavelength compared to the nonresonant one. The imaging quality is also improved by manipulating the nonlocal effect. The other output spin state is utilized for edge-enhanced imaging, capable of resolving micrometer-scale objects,” they added.”
“The proposed nonlocal Huygens’ meta-lens paves the way for performant high-Q-factor wavefront shaping and image processing. Spin-multiplexing imaging with wavelength-selective properties holds promise for practical applications in complex biomedical imaging, sensing, and microscopy,” the scientists forecast.
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This work is supported by the University Grants Committee / Research Grants Council of the Hong Kong Special Administrative Region, China [Project No. AoE/P-502/20, CRF Project: C1015-21E; C5031-22G, GRF Project: CityU15303521; CityU11305223; CityU11300224, and Germany/Hong Kong Joint Research Scheme: G-CityU 101/22], City University of Hong Kong [Project No. 9380131 and 7005867], and National Natural Science Foundation of China [Grant No. 62375232]. S.X. acknowledges financial support from National Key R&D Program of China (Grant Nos. 2021YFA1400802), the National Natural Science Foundation of China (Grant Nos. 62125501, and 6233000076), Fundamental Research Funds for the Central Universities (Grant No. 2022FRRK030004), and Shenzhen Fundamental Research Projects (Grant Nos. JCYJ20220818102218040).
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Credit: Light: Science & Applications
Caption: a, Schematic illustration of spin-multiplexing nonlocal Huygens’ meta-lens. Upon illumination with RCP light, the excited nonlocal resonance generates wavelength-selective properties and incident-angle dependence. By geometrically rotating each unit, the output LCP light exhibits a focusing phase profile for bright-field imaging, while the output RCP light without phase modulation is employed for edge detection. Insets are schematic diagrams of spectral responses output LCP and RCP lights required by spin-multiplexing imaging. b, Functional relationship between transmission efficiencies TRL and TRR in an ultrathin metasurface.
Credit: Light: Science & Applications
Caption: a,b, Experimental and simulated transmission TRL (a) and TRR (b) spectra. Red and blue areas indicate the bandwidth of q-BIC and MDR, respectively. Blue dashed lines denote wavelengths of two dips. c,d, Simulated transmission coefficients as a function of the normalized in-plane wavevector for Dips 1 (c) and 2 (d).
Credit: Light: Science & Applications
Caption: a-c, Experimental xz-plane (a) and xy-plane (b) intensity distributions and bright-field imaging (c) at the nonresonant wavelength of 1500 nm and the resonant wavelength of 1560 nm. d, Experimental setups for edge-enhanced imaging. e, Experimental output images of reference, Dip 1, and Dip 2. The reference is the original object imaging without metasurface. Dips 1 and 2 are the scenarios with metasurface filtering. Scale bars are 20 μm. f, Intensity distributions along the white dashed lines.
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