Spectral and temporal evidence of robust photonic bound states in the continuum on terahertz metasurfaces

Diego R Abujetas,Jaime Gómez Rivas,Niels Van Hoof,Stan Ter Huurne,José A Sánchez-Gil

doi:10.1364/optica.6.000996

Diego R Abujetas, Jaime Gómez Rivas + Show 3 more

Open Access

https://doi.org/10.1364/optica.6.000996

Copy DOI

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.

Highlights

Bound states in the continuum (BICs) have attracted much interest lately in physics for their infinite Q factor
We have shown experimentally through the THz transmission spectra and transients that metasurfaces consisting of sub-wavelength gold-rod dimers support BICs when the rods are identical and that such BICs emerge from strong Fano resonances, which become narrower as the dimensions of the rods approach each other
Such experimental results have been fully explained in the theoretical context of detuned-dipole arrays as a universal condition for the emergence of symmetry-protected BICs and vanishing dipole detuning

Summary

INTRODUCTION

Fano resonances [20,21,22,23,24,25,26], and electromagnetically induced transparency at optical and THz frequencies [27,28,29,30,31,32,33,34,35,36], have been reported in metasurfaces; robust symmetry-protected BICs remain unexplored. We will unequivocally demonstrate experimentally that such BICs emerge at terahertz (THz) frequencies in gold-rod dimer metasurfaces formed by two rods per unit cell by observing the vanishing linewidth in the THz transmission spectra accompanied by the pronounced increase of the resonance lifetime, when rods are made identical. We show analytically a very general condition for the emergence of symmetry-protected BICs in the parameter space of dipole detuning, from a dark Fano resonance (hybridized dipole lattice mode) that becomes an infiniteQ-factor BIC for zero detuning. These calculations fully explain the experimental results in this wider theoretical context of detuned-dipole arrays

THZ TIME-DOMAIN SPECTROSCOPY

THEORETICAL MODEL

CONCLUSIONS