Nonlocal, High-Q Metasurfaces for Precise Control of Light Waves in Energy-Momentum Space

Over the centuries, a wide range of optical components has been realized to control intrinsic properties of light waves, including frequency, linear momentum, and polarization. More recently, we see the emergence of flat optical elements termed metasurfaces that can reshape optical fields with the help of dense array of metallic and semiconductor nanostructures. Such structures support strong optical resonances and this has opened up a number of functionalities beyond the capability of their bulky counterparts. However, many fundamental challenges still remain. As one concrete example, it is a formidable task to manipulate optical wavefront at any desired wavelength and incident angle from overlapping beams. If solved, it will enable a new class of optical devices for emerging technologies such as augmented and virtual reality (AR/VR), light detection and ranging (LiDAR), optical computation and information processing, and ultrafast nonlinear optics. Conventional optics (e.g., a lens or a grating) and metasurfaces constructed from low quality factor (<i>Q</i>) plasmonic- and Mie-nanostructures cannot achieve such functions. They perform similar, highly-correlated optical behaviors across broad range of the wavelengths and incident angles.<br/>Precise spectral and angular control has been the domain of photonic crystals, guided-mode resonator (GMR), and bound states in the continuum devices that host nonlocal optical resonances. Their properties have enabled narrow-band optical filters, high-sensitivity sensors, low-threshold lasers, and elements that only shape beams at certain wavelengths and incident angles. However, their nonlocal optical responses at different wavelengths are thus far not fully independent and most of them operate in a static manner. As such, it is highly desirable to realize novel optical systems that can offer fully independent optical functions for different wavelength and incident angle and can facilitate such functions in a dynamic manner. Here, we demonstrate a new class of nonlocal, high-<i>Q</i> metasurfaces that enables 1) fully decoupled optical functions and 2) dynamic wavefront control only at a desired wavelength and incident angle. Our approach is based on a unique design of GMR structures that incorporates atomically-thin, patterned materials as the surface-relief gratings on the top of dielectric slabs. Atomically-thin gratings make the GMR structures exhibit distinct optical transfer functions only for a narrow range of wavelengths and angles and let these functions extremely sensitive to the optical properties of the grating materials.<br/>As the first example, we showcase a highly transparent, rainbow-free diffractive optical element by placing a 3-nm-thick Si diffraction grating on the top of a Si₃N₄ slab waveguide. Spectrally-selective optical absorption properties of the Si facilitate efficient (&gt;10%) resonant diffractions in a near-infrared wavelength (850 nm) as well as highly suppressed rainbow diffraction in the visible wavelengths which is one of the most severe issues in contemporary AR/VR technologies. We also demonstrate an eye tracking prototype that allows precise eye gaze tracking (1° accuracy) as well as significantly suppressed rainbow diffractions below 0.1%. Next, we demonstrate ultracompact, free-space optical modulators based on monolayer transition metal dichalcogenide (TMDC) as a switching medium. TMDC monolayers host strong exciton resonances that enable highly tunable dielectric constant by various external stimuli such as electric bias, local strain, and chemical/optical doping. We fabricate GMR structures with monolayer WS₂ gratings that accomplish high-contrast modulation (from 10% to 50%, and vice versa) of both transmissivity and reflectivity on resonance. The dispersion properties of the fabricated structure manifest the exciton features of the WS₂ monolayer and initial modulation experiments exhibit clear modulation of the optical signal.