Abstract-Extraordinary THz Transmission with a Small Beam Spot: The Leaky Wave Mechanism

Miguel Navarro-Cía, Víctor Pacheco-Peña, Sergei A. Kuznetsov, Miguel Beruete

http://onlinelibrary.wiley.com/doi/10.1002/adom.201701312/full

The discovery of extraordinary optical transmission (EOT) through patterned metallic foils in the late 1990s was decisive for the development of plasmonics and cleared the path to employ small apertures for a variety of interesting applications all along the electromagnetic spectrum. However, a typical drawback often found in practical EOT structures is the large size needed to obtain high transmittance peaks. Consequently, practical EOT arrays are usually illuminated using an expanded (mimicking a plane wave) beam. Here, it is shown with numerical and experimental results in the THz range that high transmittance peaks can be obtained even with a reduced illumination spot exciting a small number of holes, provided that the structure has a sufficient number of lateral holes out of the illumination spot. These results shed more light on the prominent role of leaky waves in the underlying physics of EOT and have a direct impact on potential applications.

Abstract-Experimental Realization of an Epsilon-Near-Zero Graded-Index Metalens at Terahertz Frequencies

Victor Pacheco-Peña, Nader Engheta, Sergei Kuznetsov, Alexandr Gentselev, and Miguel Beruete

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.8.034036

The terahertz band has been historically hindered by the lack of efficient generators and detectors, but a series of recent breakthroughs have helped to effectively close the “terahertz gap.” A rapid development of terahertz technology has been possible thanks to the translation of revolutionary concepts from other regions of the electromagnetic spectrum. Among them, metamaterials stand out for their unprecedented ability to control wave propagation and manipulate electromagnetic response of matter. They have become a workhorse in the development of terahertz devices such as lenses, polarizers, etc., with fascinating features. In particular, epsilon-near-zero (ENZ) metamaterials have attracted much attention in the past several years due to their unusual properties such as squeezing, tunneling, and supercoupling where a wave traveling inside an electrically small channel filled with an ENZ medium can be tunneled through it, reducing reflections and coupling most of its energy. Here, we design and experimentally demonstrate an ENZ graded-index (GRIN) metamaterial lens operating at terahertz with a power enhancement of 16.2 dB, using an array of narrow hollow rectangular waveguides working near their cutoff frequencies. This is a demonstration of an ENZ GRIN device at terahertz and can open the path towards other realizations of similar devices enabling full quasioptical processing of terahertz signals.

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Abstract-Wideband backscattering reduction at terahertz using compound reflection grating

Bakhtiyar Orazbayev, Pablo Rodríguez-Ulibarri, and Miguel Beruete

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-19-22905&origin=search

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.

© 2017 Optical Society of America

Research demonstrates various possibilities for controlling light in the terahertz frequency range

        

                Detail of a researcher working in the laboratory Mario Sorolla. Credit: Teralab at the UPNA
http://phys.org/news/2014-09-possibilities-terahertz-frequency-range.html

The Journal of Optics has devoted the front page of its special edition on Mid-infrared and THz Photonics to the work produced by the NUP/UPNA-Public University of Navarre researchers Víctor Pacheco-Peña, Víctor Torres, Miguel Beruete and Miguel Navarro-Cía, together with Nader Engheta (University of Pennsylvania). In their research they have proposed various devices capable of redirecting electromagnetic waves with efficiency levels close to 100%.

To explain what their work consists of they have put forward the following example: "If we shine a torch on a wall in which we have made a hole, experience tells us that the bigger the hole is, the greater the amount of light that will pass through to the other side. However, if we fill the hole with an ENZ metamaterial, something that appears to defy logic happens: the smaller the hole is, the greater the amount of light that passes through. This phenomenon has a tremendous practical implication because it opens up new ways of miniaturising numerous components and for light control."

Metamaterials are artificial materials with properties that go beyond those of natural means. To understand how they work, we can take a look at nature itself: while natural elements acquire their physical properties from the atoms that form them and the way in which they are ordered, metamaterials use natural means, like small metal fragments that fit together like parts of a Meccano model to artificially synthesise properties that are impossible to find otherwise. Initially put forward to control electromagnetic radiation, right now their use has become widespread and has extended to other areas like mechanical waves (sound, for example).

The piece of work referred to above proposes various compact devices comprising rectangular metal tubes with extremely narrow openings of dimensions designed in such a way that they are capable of redirecting the electromagnetic waves with levels of efficiency close to 100%. These gaps are capable of imitating an ENZ (Epsilon Near Zero, which means permittivity close to zero) metamaterial so that it is not necessary to "fill them" with anything in order to obtain amazing results.

Amazing properties

Among the electromagnetic metamaterials, the above-mentioned ENZ ones make it possible to achieve the super coupling of the light, the tunnel effect and the confining of energy in tiny spaces. "Going back to the first example," say the authors, "super coupling means that all the light will be transferred from one side of the wall to the other through any shape of hole we want to make; tunnel effect refers to light passing through a hole of any length, no matter how long we want to make it; and the confining of energy is due to the fact that the light is transferred even through very small holes, so the energy inside the hole is squeezed enormously."

This work has shown theoretically and by means of simulations how beam steerers and power splitters work for terahertz waves, and is of tremendous importance in view of their huge potential in sectors like security, biomedical engineering, pharmacy, space, etc. Right now, the authors of this piece of research are working to confirm the study through experimental means. In this respect, they stress that "this constitutes another milestone in an initiative of an international nature that has been going on for nearly four years."

 Explore further: Invisibility cloaks closer thanks to 'digital metamaterials'

More information: Pacheco-Peña V., Torres V., Beruete M., Navarro-Cía M., Nader Engheta. 2014. "Near-zero (ENZ) graded index quasi-optical devices: steering and splitting millimeter waves". Journal of Optics, 16: 094009. DOI: 10.1088/2040-8978/16/9/094009

Abstract-Terahertz epsilon-near-zero graded-index lens

Víctor Torres, Víctor Pacheco-Peña, Pablo Rodríguez-Ulibarri,Miguel Navarro-Cía, Miguel Beruete, Mario Sorolla, and Nader Engheta  »View Author Affiliations

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-21-7-9156

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 Fermat’s 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.

© 2013 OSA