[Paper Review] Nonlocal metasurfaces for spectrally decoupled wavefront manipulation and eye tracking

Jung‐Hwan Song, Jorik van de Groep+2 more|arXiv (Cornell University)|Feb 11, 2021

Advanced Optical Imaging Technologies79 references127 citations

TL;DR

This paper introduces high-Q, nonlocal metasurfaces based on guided-mode resonators (GMRs) with spectrally decoupled optical functions, enabling unperturbed visible light transmission and selective redirection of near-infrared (NIR) light for eye tracking. By engineering spectrally dependent absorption in atomically thin Si grating layers, the metasurface achieves >90% visible transparency, <0.1% rainbow suppression, and 10%+ diffraction efficiency at 870 nm, enabling compact, front-view eye tracking in smart glasses.

ABSTRACT

Metasurface-based optical elements typically manipulate light waves by imparting space-variant changes in the amplitude and phase with a dense array of scattering nanostructures. The highly-localized and low optical-quality-factor (Q) modes of nanostructures are beneficial for wavefront-shaping as they afford quasi-local control over the electromagnetic fields. However, many emerging imaging, sensing, communication, display, and non-linear optics applications instead require flat, high-Q optical elements that provide notable energy storage and a much higher degree of spectral control over the wavefront. Here, we demonstrate high-Q, nonlocal metasurfaces with atomically-thin metasurface elements that offer notably enhanced light-matter interaction and fully-decoupled optical functions at different wavelengths. We illustrate a possible use of such a flat optic in eye tracking for eye-wear. Here, a metasurface patterned on a regular pair of eye-glasses provides an unperturbed view of the world across the visible spectrum and redirects near-infrared light to a camera to allow imaging of the eye.

Motivation & Objective

• Develop a flat, high-Q optical element that spectrally decouples visible and near-infrared (NIR) wavefront manipulation for wearable AR/VR devices.
• Overcome the spectral coupling limitation in conventional metasurfaces, where optical functions at different wavelengths are intrinsically linked by band structure.
• Enable high-transparency, rainbow-free visible imaging while efficiently redirecting NIR light to a camera for eye tracking in a single-layer, compact optic.
• Design a metasurface that functions as a transparent window for visible light and a high-efficiency, narrowband beam deflector for NIR, suitable for integration into eyeglass frames.

Proposed method

• Employ guided-mode resonator (GMR) structures with atomically thin (3–7 nm) polycrystalline silicon (pSi) grating elements on a Si3N4 waveguide to achieve high optical quality factor (Q) and nonlocal response.
• Engineer spectrally dependent absorption using Lorentzian oscillators in the dielectric grating to decouple optical functions at different wavelengths, enabling independent control of visible and NIR responses.
• Use full-field rigorous coupled-wave analysis (RCWA) simulations to model and optimize diffraction efficiency, field enhancement, and spectral response across multiple GMR layers.
• Design asymmetric GMR structures to guide +1st order and reflect -1st order, enabling directional control of NIR light while preserving broadband visible transmission.
• Stack multiple high-Q GMR metasurfaces with independently tuned grating periods and absorption profiles to achieve wavelength-division multiplexing of optical functions.
• Implement a multi-layer GMR stack where each layer selectively interacts with a distinct wavelength (700 nm, 705 nm, 710 nm), with absorption toggling the function of each layer independently.

Experimental results

Research questions

• RQ1 Can high-Q, nonlocal metasurfaces with spectrally decoupled functions be realized in a single, compact, and transparent optical element?
• RQ2 Can spectrally dependent absorption in atomically thin dielectric grating layers break the spectral coupling inherent in conventional GMR and metasurface designs?
• RQ3 Can such a metasurface simultaneously provide >80% visible transparency, <0.1% rainbow suppression, and >10% diffraction efficiency for NIR light in a single-layer format?
• RQ4 Can the metasurface enable front-view eye tracking in smart glasses without obstructing the user’s view or introducing chromatic artifacts?
• RQ5 Can multiple optical functions (e.g., beam steering at different wavelengths) be independently controlled in a stacked, multi-layer GMR metasurface architecture?

Key findings

• The metasurface achieves >90% transmission across the visible spectrum (400–700 nm) with minimal spectral distortion, enabling an unperturbed view of the world.
• Rainbow artifacts are suppressed to less than 0.1% intensity relative to the main peak, eliminating chromatic dispersion that plagues conventional diffractive optics.
• At 870 nm, the metasurface achieves a diffraction efficiency of over 10% into the +1st order, enabling efficient redirection of NIR light to a camera for eye tracking.
• The use of spectrally tuned absorption in the dielectric grating enables full decoupling of optical functions: the top layer can be selectively activated to reflect 710 nm light while transmitting 705 nm and 700 nm.
• In the stacked GMR design, each layer selectively interacts with its target wavelength (710 nm, 705 nm, 700 nm), with independent control over reflection and transmission spectra via localized absorption.
• Simulations confirm strong local electric field enhancement (>10×) at the GMR resonance frequency (870 nm), indicating high light-matter interaction and efficient energy storage in the high-Q mode.