Pablo Rodríguez-Ulibarri

Terahertz epsilon-near-zero graded-index lens

An epsilon-near-zero graded-index converging lens with planar faces is proposed and analyzed. Each perfectly-electric conducting (PEC) waveguide comprising the lens operates slightly above its cut-off frequency and has the same length but different cross-sectional dimensions. This allows controlling individually the propagation constant and the normalized characteristic impedance of each waveguide for the desired phase front at the lens output while Fresnel reflection losses are minimized. A complete theoretical analysis based on the waveguide theory and Fermats principle is provided. This is complemented with numerical simulation results of two-dimensional and three-dimensional lenses, made of PEC and aluminum, respectively, and working in the terahertz regime, which show good agreement with the analytical work.

Fishnet metamaterial from an equivalent circuit perspective

An equivalent circuit for the double-fishnet metamaterial is proposed. All lumped elements are inferred from the interpretation of the electric and magnetic fields as well as the surface currents generated when the structure is excited with a normally incident plane wave. The approach provides a physical meaningful circuit representation of the electromagnetic response of the double-fishnet metamaterial and underlines the tight link between the fishnet metamaterial and the extraordinary transmission phenomenon.

Mid-infrared plasmonic inductors: Enhancing inductance with meandering lines

We present a mid-infrared inductor that when applied to an extraordinary transmission hole array produces a strong redshift of the resonant peak accompanied by an unprecedented enlargement of the operation bandwidth. The importance of the result is twofold: from a fundamental viewpoint, the direct applicability of equivalent circuit concepts borrowed from microwaves is demonstrated, in frequencies as high as 17 THz upholding unification of plasmonics and microwave concepts and allowing for a simplification of structure design and analysis; in practical terms, a broadband funnelling of infrared radiation with fractional bandwidth and efficiency as high as 97% and 48%, respectively, is achieved through an area less than one hundredth the squared wavelength, which leads to an impressive accessible strong field localization that may be of great interest in sensing applications.

Quarter-Wave Plate Based on Dielectric-Enabled Extraordinary Resonant Transmission

A compact quarter-wave plate (QWP) based on a doubly periodic subwavelength hole array lying over a dielectric slab is synthesized analytically under perfect electric conductor assumption and its validity is demonstrated numerically for real metals (aluminum and silver) at terahertz (THz) and near infrared. The form birefringence is achieved via the excitation of transverse-magnetic and transverse-electric extraordinary transmission resonances for the perpendicular and parallel polarization to the short in-plane period, respectively. To prove the generality of the approach, a prototype working at the near-infrared is designed and numerically optimized. By imposing a maximum phase deviation of ±10° from the ideal QWP response and an axial ratio smaller than 3 dB, the bandwidth of the designs are 0.6% and 0.4% for the 0.23-λ-thick THz and 0.33- λ-thick near-infrared prototype, respectively. Given the simplicity of the design, it holds promise for compact narrowband microwaves-to-visible polarizing devices.

Experimental demonstration of deflection angle tuning in unidirectional fishnet metamaterials at millimeter-waves

Asymmetric transmission enables high forward-to-backward transmission contrast in the Lorentz reciprocal framework, so that it is envisioned to be a backbone of future practical devices with strong directional selectivity. In this letter, we experimentally demonstrate efficient tuning and sign-switching capabilities of the deflection angle by varying the incidence angle and/or frequency in the unidirectional regime of a compact and simple structure comprising a stack of a fishnet metamaterial and a dielectric grating. The entire device operates at frequencies ranging from 45 to 75 GHz and has a total thickness of 0.77λ0 at 60 GHz.

Annular Apertures in Metallic Screens as Extraordinary Transmission and Frequency Selective Surface Structures

A 2-D periodic array of annular apertures (or ring slots) is studied using an accurate circuit model. The model accounts for distributed and dynamic effects associated with the excitation of high-order modes operating above or below cutoff but not far from their cutoff frequencies. This paper allows to ascertain the substantial differences of the underlying physics when this structure operates as a classical frequency selective surface or in the extraordinary-transmission (ET) regime. A discussion of two different designs working at each regime is provided by means of the equivalent circuit approach (ECA), full wave simulation results, and experimental characterization. The agreement between the equivalent circuit calculation applied here and the simulation and experimental results is very good in all the considered cases. This validates the ECA as an efficient minimal-order model and a low computational-cost design tool for frequency selective surfaces and ET-based devices. Additional scenarios such as oblique incidence and parametric studies of the structural geometry are also considered.

Hedgehog subwavelength hole arrays: control over the THz enhanced transmission*

By backing or sandwiching a holey metal layer with or between isotropic dielectric slabs, additional peaks of transmission within the long- wavelength regime arise as a result of the induced transverse magnetic (TM) or transverse electric (TE) grounded dielectric modes. A similar control of the complex surface wave modes, and thus of the extraordinary transmission (ET) peaks, is demonstrated here via anisotropic slabs in the form of a fakirs bed of nails. However, it is shown that those ET peaks formed from TE modes are suppressed because of the inherent dispersion characteristics of the free-standing grounded pins. This allows the red-shifting of the ET for the polarization parallel to the larger in-plane period of the hole array, but unlike the dielectric isotropic slab configuration, the orthogonal polarization remains inhibited.

Wide angle terahertz sensing with a cross-dipole frequency selective surface

In this work, a terahertz sensor based on a cross dipole frequency selective surface is analyzed and experimentally tested. The sensing structure is optimized for operation at the fundamental band-stop resonance near 0.7 THz and characterized under normal and oblique incidence. The sensing performance as a function of the incidence angle and the wave polarization is evaluated with good agreement between simulations and measurements. It is shown that a figure of merit for the proposed sensor can be enhanced from 0.2 up to 0.6 due to switching from normal to oblique excitation, which yields the maximum performance for TM polarization at the incidence angle of 70°. The presented results demonstrate a wide angle operation regime in THz sensing that opens up an alternative approach in improving capabilities of sensing devices.

Blind spot mitigation in phased array antenna using metamaterials

In this work, a metaradome based on a fakirs bed of nails is designed and tested in order to suppress the blind directions of a 100-element antenna array. The antenna is a microstrip array designed to operate in X-band. The fakirs bed metamaterial-like was first approximated using analytical formulas before a full-wave numerical optimization. Experimental results are exposed and confronted to numerical results. They show a significant reduction of the blind spot subsequent to the metaradome addition.

Wideband backscattering reduction at terahertz using compound reflection grating

Backscattering reduction is usually achieved by using either absorbers or diffractions gratings at the expense of a narrow bandwidth. In this paper, we propose a different strategy based on a metallic compound reflection grating (CRG). We demonstrate that this structure allows a strong and broadband (fractional bandwidth, FBW ≈57%) backscattering reduction in the terahertz (THz) range by efficiently transferring the incident energy to the diffracted modes. The design is analyzed in terms of equivalent circuit and numerical simulations and the results are corroborated by a manufactured prototype operating at 0.35 THz.