THz Transition from Fano Resonances to Bound States in the Continuum in 3D Printed Photonic Structures

Bound states in the continuum (BIC) are discrete solutions of the single-particle Schrodinger equation embedded in the continuum of positive-energy states [1]. For electromagnetic waves, several physical conditions leading to optical BICs have been identified. Symmetry protected BIC (SP-BIC) are particularly important because are stable against small perturbations that preserve the symmetry. They possess a wave vector close to the resonant modes with high-quality factors (high-Q) that can be exploited for different applications [2, 3]. For these reasons BIC are particularly attractive to the THz community because they may be used to make THz filters, sensors and nonlinear sources. SP-BIC can be observed at in-plane momentum k? = 0 in the modes dispersion of a photonic crystal slab (PhCS) with a C2 rotational symmetry namely a 180o rotation around its normal axis. For non-normal incidence conditions, the symmetry properties are broken and the modes start to be resonantly coupled with freely propagating waves giving Fano resonances in the scattering field [4]. Our study deals with the design and realization of THz one-dimensional photonic crystal slab (PhCS) made of rectangular dielectric rods showing SP-BIC. The structures are realized by low-cost, rapid, and versatile fused deposition modelling 3D-printing [5]. An optimization design approach for smart engineering of such resonances is proposed by analyzing the link between BIC and Fano resonances by simulations performed by an analytical exact solution of the Maxwell equations [6]. In accordance with the simulations, THz time-domain spectroscopy measurements of the realized structures as a function of the incidence angle show the evolution of a Fano resonance from the weak to the strongly coupling regime that allows for strong confinement of light and high-Q resonances.