• Advanced Photonics
  • Vol. 3, Issue 2, 026002 (2021)
Adam Overvig1 and Andrea Alù1、2、*
Author Affiliations
  • 1City University of New York, Advanced Science Research Center, Photonics Initiative, New York, United States
  • 2City University of New York, Graduate Center, Physics Program, New York, United States
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    Fano resonances are conventionally understood as sharp spectral features with selectivity in the momentum-frequency domain, implying that they can be excited only by plane waves with specific frequencies and incident angles. We demonstrate that Fano resonances can be made generally selective in the space-frequency domain. They can be tailored to resonate only when excited by a frequency, polarization, and wavefront of choice. This generalization reveals that Fano systems are characterized by eigenwaves that scatter to their time-reversed image upon reflection. Although in conventional Fano systems this trivially occurs for normally incident plane waves, we show that, in general, the selected wavefront is locally retroreflected everywhere across the device. These results show that conventional Fano resonances are a subset of a broader dichroic phenomenon with spin, spatial, and spectral selectivity. We demonstrate these concepts with nonlocal metasurfaces whose governing principles are deeply rooted in the symmetry features of quasi-bound states in the continuum. Enhanced light–matter interactions and symmetry-protection make these phenomena uniquely suited for enriching applications in quantum optics, non-linear optics, augmented reality, and secure optical communications, laying the groundwork for a range of novel compact optical sources and devices.

    1 Introduction

    Optical Fano resonance is a phenomenon born of interference between a discrete (dark) state and a broad (bright) continuum of states. Observed in many photonic systems,1 it is characterized by an asymmetric lineshape originally described by Fano in 1961.2 Wood’s anomaly is a prominent early example of such a phenomenon, wherein sharp resonant features were observed in the spectra of super-wavelength metallic gratings at discrete angles and frequencies at which diffractive orders emerge.3,4 Modern micro- and nano-technology has enabled subwavelength patterning of surfaces that exhibit more complex Fano resonant responses. For instance, in weakly corrugated dielectric slabs, weak coupling of light to a discrete waveguide mode can interfere with strong coupling to a broadband thin-film resonance, enabling a guided mode resonance.5 Such devices behave as notch filters, selective to a specific combination of angles and frequencies that track the dispersion of the underlying waveguide mode.6,7 Similar responses are observed in photonic crystal slabs and strongly corrugated gratings,8 well-modeled by temporal coupled mode theory that can accurately predict the Fano lineshape.9