Near-Field Imaging and Spectroscopy of Optical Resonances in a Semiconductor Metasurface

Metasurfaces have emerged as a versatile platform to control light-matter interactions in sub-wavelength volumes, leading to “planar” optics with various potentially groundbreaking applications in bio-sensing, hyperspectral imaging, lasing, frequency mixing, and quantum information processing. Historically, metasurfaces tailored electromagnetic fields via localized surface plasmon resonances confined in metallic nanostructures. However, dielectric metasurfaces made of III-V semiconductors have recently garnered much interest owing to their low losses at optical wavelengths and the ability to support electromagnetic Mie-like resonances within individual resonator units. Recent efforts have further introduced symmetry breaking elements to individual resonator units lending access to high-Q symmetry-protected quasi bound states in the continuum (BICs) optical modes, that offer a promising avenue for enhanced light matter interactions. A technique capable of directly imaging the electromagnetic fields at nano-scale in these platforms is sought after to inform us on the origin and the spatial extent of these photonics processes and expand pathways to tailor nanostructures for chosen functionalities.<br/><br/>Imaging nanoscale optical phenomenon is non-trivial since light-matter interactions take place at length scales smaller than the wavelength of light, far below the resolution of traditional optical probes. Nonetheless, several techniques such as scanning probe-based near-field optical microscopy (SNOM), photo-induced force microscopy (PiFM), cathodoluminescence (CL), and electron energy loss spectroscopy (EELS) have begun to probe this space. An imaging technique that, despite broad applications in plasmonics research, has hitherto remained unexplored in the field of nanophotonics is photoemission electron microscopy (PEEM), a technique that capitalizes on a true optical excitation in a “photon in, electron out” approach to probe the near-field enhancement of the electric fields through photoelectron intensity. Here we employ PEEM to image broken-symmetry III-V semiconductor metasurfaces with high–quality factor, quasi-bound state in the continuum resonances. We find that through far-field optical excitation at normal incidence we can elicit highly selective mode excitation which enables high-resolution imaging of field profiles as a function of wavelength and polarization. These profiles are further used to corroborate finite difference time-domain (FDTD) simulations and develop a more complete understanding of the optical response of the metasurface. Additionally, the “electron out” portion of PEEM provides full-field spectroscopic imaging allowing us to collect near-field spectral responses at high spatial resolution. Through spectroscopic analysis of the field profiles, we reveal significant shifts in the excited resonances as a function of position within an individual resonator. Finally, we exhibit the ability of PEEM to image long range interactions among resonators as an entire array of structures can be excited and imaged simultaneously. This work highlights the powerful capabilities of PEEM for the investigation of semiconductor metasurfaces and sheds light on nature of the photonic processes occurring in this class of materials.<br/><br/>Supported by the Laboratory Directed Research and Development program at Sandia National Laboratories, a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525.