[Paper Review] From Fano resonances to bound states in the continuum in dipole arrays at THz frequencies

Diego R. Abujetas, Niels van Hoof+3 more|Figshare|Jan 10, 2019

Plasmonic and Surface Plasmon Research40 references44 citations

TL;DR

The paper demonstrates experimentally and theoretically that 2D metasurfaces of asymmetric gold rod dimers exhibit narrow Fano resonances at THz frequencies, which progressively sharpen into bound states in the continuum (BICs) as the rods become identical, supported by a universal detuned-dipole model.

ABSTRACT

Fano resonances and bound states in the continuum (BICs) exhibit a rich phenomenology stemming from, respectively, their asymmetric line shapes and infinite quality factors. Here, we show experimentally and theoretically that rod dimer metasurfaces exhibit narrow (high-Q) Fano resonances at THz frequencies. These resonances evolve continuously into a BIC as the rods in each dimer become identical. We demonstrate analytically that this is a universal behavior occurring in arrays of dimers consisting of detuned resonant dipoles. Fano resonances arise as a result of the interference between broad and narrow lattice dipole resonances, with high-Q factors tending to infinity in the detuning parameter space as the narrow lattice resonance becomes a BIC for identical resonant dipoles. Similar configurations can be straightforwardly envisioned throughout the electromagnetic spectrum leading to ultrahigh-Q Fano resonances and BICs of interest in photonics applications such as sensing and lasing.

Motivation & Objective

• It investigate Fano resonances and bound states in the continuum (BICs) in THz rod-dimer metasurfaces.
• It demonstrate that narrowing resonances occur as the two rods in a unit cell approach identical dimensions.
• It develop a simple analytic model for arrays of detuned resonant dipoles to explain the observed phenomena.
• It establish the universality of the detuned-dipole mechanism across lattice parameters and dipole displacements.
• It discuss implications for sensing and lasing applications using ultrahigh-Q resonances and BICs.

Proposed method

• Fabrication of 2D square lattices of gold rod dimers on quartz with varied second rod length L2 while keeping L1 fixed.
• THz transmission measurements at normal incidence and comparison with numerical SCUFF simulations using planar perfectly conducting rods.
• Development of a coupled-dipole-dimer model with dipole polarizabilities along the y-axis and a lattice depolarization dyadic to describe the array response.
• Analytical derivation showing how detuning DeltaAlpha_y controls the widths of the two lattice modes and yields a zero-width BIC at zero detuning.
• Extraction of polarizabilities from simulated scattering cross sections to reproduce transmission spectra and phase behavior.
• Near-field and full-wave simulations to illustrate the BIC field confinement in the lattice.

Experimental results

Research questions

• RQ1 Can a detuned-dipole array support both a broad and a narrow lattice resonance whose interference yields Fano line shapes in THz transmission?
• RQ2 Does making the two dipoles identical (zero detuning) collapse the narrow resonance into a bound state in the continuum that is symmetry-protected and non-radiative?
• RQ3 Is the BIC condition robust to variations in lattice constants and intra-unit-cell dipole displacement, i.e., universal across geometries?

Key findings

• Experimentally observed strong, narrow Fano resonances in THz transmittance for L2 ≠ L1 that become vanishingly narrow as rods approach identical dimensions.
• Analytical coupled-dipole model shows Im[Lambda+], the broad/dark mode, tends to zero with vanishing detuning, indicating a BIC at zero detuning.
• Demonstration that the Fano resonance arises from interference between a broad bright mode and a narrow dark mode whose coupling is controlled by detuning DeltaAlpha_y.
• Universality claim: the BIC condition is robust against lattice parameter changes a, b and intra-unit-cell displacement dx, dy as long as higher multipoles are neglected.
• Near-field maps and phase analyses reveal field enhancements and π-phase differences between rods near resonance, consistent with dark lattice resonance behavior.