Chiral Quasi-Bound States in the Continuum

Adam Overvig, Nanfang Yu, and Andrea Alù

Phys. Rev. Lett. 126, 073001 – Published 17 February 2021

Abstract

Quasi-bound states in the continuum (QBICs) are Fano resonant states with long optical lifetimes controlled by symmetry-breaking perturbations. While conventional Fano responses are limited to linear polarizations and do not support tailored phase control, here we introduce QBICs born of chiral perturbations that encode arbitrary elliptical polarization states and enable geometric phase engineering. We thereby design metasurfaces with ultrasharp spectral features that shape the impinging wave front with near-unity efficiency. Our findings extend Fano resonances beyond their conventional limits, opening opportunities for nanophotonics, classical and quantum optics, and acoustics.

Received 8 June 2020
Accepted 13 January 2021

DOI:https://doi.org/10.1103/PhysRevLett.126.073001

Physics Subject Headings (PhySH)

Metamaterials Photonic crystals

Atomic, Molecular & Optical

Authors & Affiliations

Adam Overvig ^1,2, Nanfang Yu^2,†, and Andrea Alù ^1,3,*

¹Photonics Initiative, Advanced Science Research Center at the Graduate Center of the City University of New York, New York, New York 10031, USA
²Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, USA
³Physics Program, Graduate Center, City University of New York, New York, New York 10016, USA

^*Corresponding author. aalu@gc.cuny.edu
^†Corresponding author. ny2214@columbia.edu

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Issue

Vol. 126, Iss. 7 — 19 February 2021

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Images

Figure 1
Chiral QBICs. (a) Homogenized model of a metasurface supporting a chiral QBIC: Two interfaces scatter optical energy to linearly polarized free-space light with polarization angles $ϕ_{1}$ and $ϕ_{2}$ , respectively, resulting in chiral states leaked upward and downward, $| e_{u} ⟩$ and $| e_{d} ⟩$ . (b) Perturbation scheme involving two perturbations, $V_{1}$ and $V_{2}$ , for control over $ϕ_{1}$ and $ϕ_{2}$ , respectively. (c) Example spectra of a device based on the scheme in (b) (with $α = 70 °$ and $Δ α = 50 °$ ) showing full circular dichroism only at resonance. (d),(e) Unperturbed and perturbed structures, respectively. (f),(g) Unperturbed and perturbed magnetic field components, showing that the bound state (principally characterized by $H_{z}$ ) leaks to RCP light when perturbed.
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Figure 2
Fano resonances with arbitrary elliptical eigenpolarizations. (a) Map of reflectance for a device with $α = 47 °$ and $Δ α = 31 °$ as a function of the input polarization state (characterized by the latitude $2 χ$ and longitude $2 ψ$ on the Poincaré sphere). (b) Reflectance spectra for the extremal cases in (a) (marked by red and blue dots), with the insets showing the eigenpolarization states $e$ that are maximally and minimally resonant and their positions on the Poincaré sphere. (c),(d) Map of the coverage of the Poincaré sphere for the maximally resonant elliptical eigenpolarization as a function of $α$ and $Δ α$ .
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Figure 3
Full control over the amplitude and phase of resonantly reflected light. (a) Spectral reflectance near the resonance for $α = 19 °$ and sampled values of $Δ α$ . (b) Spectral phase near the resonance for $Δ α \approx 50 °$ and sampled values of $α$ . The orange curve highlights the conventional phase function expected for a Fano resonance. (c),(d) Amplitude $A$ and phase $Φ$ at the resonant wavelength for reflected RCP light as a function of $α$ and $Δ α$ , with the contours sampled in (a),(b) highlighted.
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Figure 4
Chiral phase-gradient resonant metasurface with near-unity diffraction efficiency. (a) Sublibrary of Figs. 3 and 3 with near-unity amplitude and phase varying over $2 π$ . (b) Device with a spatially tailored leaky mode encoding a coupling phase gradient of $k_{g} = π / 8 a$ . (c) Mode profile on resonance when excited at normal incidence. The ellipses denote the geometry of the structure. (d) Far field for RCP incidence at each wavelength, demonstrating near-unity diffraction efficiency (96.1%) to the anomalously reflected angle (20.17°). The spin is preserved in transmission and reflection. (e) Far field for LCP incidence at each wavelength, showing no resonance.
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