Electromagnetic properties of a dielectric grating. I. Propagating, evanescent, and guided waves

Observation of terahertz transition from Fano resonances to bound states in the continuum

Bound states in the continuum (BIC) in metamaterials have recently attracted attention for their promising applications in photonics. Here, we investigate the transition from Fano resonances to BIC, at terahertz (THz) frequencies, of a one-dimensional photonic crystal slab made of rectangular dielectric rods. Simulations performed by an analytical exact solution of the Maxwell equations showed that symmetry-protected, high-quality factor (Q), BIC emerge at normal incidence. For non-normal incidence, BIC couple with the freely propagating waves and appear in the scattering field as a Fano resonance. Simulations were verified by realizing the photonic crystal slab by 3D-printing technique. THz time-domain spectroscopy measurements as a function of the incidence angle matched the simulation to good accuracy and confirmed the evolution of Fano resonances to high-Q resonances typical of BIC. These results point out the design of highly sensitive and low-cost THz devices for sensing for a wide range of applications.

Numerical demonstration of low-reflective wire grid polarizers with a patterned Fe2O3 absorptive layer

Absorptive polarizers are pivotal components for realizing a low ambient reflection in liquid crystal displays (LCDs) and organic light-emitting diodes (OLEDs). Different types of absorptive polarizers have been proposed. Nevertheless, the realization of compact and efficient absorptive polarizers remains challenging. Wire grid polarizers (WGPs) are a promising solution because of their high durability and relatively thin thickness. In this paper, two structures of absorptive-WGPs have been proposed and optimized at the target wavelength of 532 nm: one is based on a patterned F e ₂ O ₃/A l bi-layer on top of a S i O ₂ substrate, and the second one builds on the first one by depositing a S i O ₂ layer in the gaps of Al. The optimal solutions exhibit a reflectance less than 5%, a transmittance over 45%, and an extinction ratio over 40 dB. To evaluate the manufacturing feasibility, their sensitivity to the wire's dimensional parameters is investigated. Their great spectral performance and large acceptance angles demonstrate that such polarizers have the potential to significantly promote the development of current display technologies.

Terahertz waves dynamic diffusion in 3D printed structures

Applications of metamaterials in the realization of efficient devices in the terahertz band have recently been considered to achieve wave deflection, focusing, amplitude manipulation and dynamical modulation. Terahertz metamaterials offer practical advantages since their structures have typical sizes of hundreds microns and are within the reach of current three-dimensional (3D) printing technologies. Here, we propose terahertz photonic structures composed of dielectric rods layers made of acrylonitrile styrene acrylate realized by low-cost, rapid, and versatile fused deposition modeling 3D-printing. Terahertz time-domain spectroscopy is employed for the experimental study of their spectral and dynamic response. Measured spectra are interpreted by using simulations performed by an analytical exact solution of the Maxwell equations for a general incidence geometry, by a field expansion as a sum over reciprocal lattice vectors. Results show that the structures possess specific spectral forbidden bands of the incident THz radiation depending on their optical and geometrical parameters. We also find evidence of disorder in the 3D printed structure resulting in the closure of the forbidden bands at frequencies above 0.3 THz. The size disorder of the structures is quantified by studying the dynamics diffusion of THz pulses as a function of the numbers of layers of dielectric rods. Comparison with simulations of light diffusion in photonic crystals with increasing disorder allows estimating the size distributions of elements. By using a Mean Squared Displacement model, from the broadening of the pulses’ widths it is also possible to estimate the diffusion coefficient of the terahertz radiation in the photonic structures.

Mechanism for designing metallic metamaterials with a high index of refraction

We introduce a mechanism for creating artificial high refractive index metamaterials by exploiting the existence of subwavelength propagating modes in metallic systems. As an example, we investigate analytically and numerically metal films with a periodic arrangement of cut-through slits. Because of the presence of TEM modes in the slits, for TM polarization such a system can be rigorously mapped into a high refractive index dielectric slab when the features are smaller than the wavelength of light. The effective refractive index is entirely controlled by the geometry of the metal films, is positive, frequency independent, and can be made arbitrarily large.