Metamaterials are artificial composites that acquire their electromagnetic properties from embedded subwavelength metallic structures. In theory, the effective electromagnetic properties of metamaterials at any frequency can be engineered to take on arbitrary values, including those not appearing in nature. As a result, this new class of materials can dramatically add a degree of freedom to the control of electromagnetic waves. The emergence of metamaterials fortunately coincides with the intense emerging interest in terahertz radiation (T-rays), for which efficient forms of electromagnetic manipulation are sought. Considering the scarcity of naturally existing materials that can control terahertz, metamaterials become ideal substitutes that promise advances in terahertz research. Ultimately, terahertz metamaterials will lead to scientific and technological advantages in a number of areas. This article covers the principles of metamaterials and reviews the latest trends in terahertz metamaterial research from the fabrication and characterization to the implementation.

Metamaterials in the Terahertz Regime

This review summarizes the recent trends and developments of infrared and Raman spectroscopy applied to the identification of explosives that have been published over the past decade, focusing on the different fields where explosives were studied: homeland and international security, forensics, environmental, characterization of explosives, trace detection and fluorescence-free Raman analysis of explosives.

Infrared and Raman spectroscopy techniques applied to identification of explosives

Microscopic split-ring-resonator (SRR) arrays are fabricated on 100 μm thick polyethylene naphthalate (PEN) films by femtosecond laser micro-lens array (MLA) lithography. The transmission properties of these metamaterials are characterized by THz Time Domain Spectroscopy (THz-TDS). Tunable resonance responses can be achieved by changing SRR structural design parameters. By stacking 2D PEN metamaterial films with different frequency responses together, a broadband THz filter with full width at half maximum (FWHM) of 0.38 THz is constructed. The bandwidth of the resonance response increases up to 4.2 times as compared to the bandwidths of single layer metamaterials. Numerical simulation reveals that SRR layers inside the multi-layer metamaterials are selectively excited towards specific frequencies within the broadband response. Meanwhile, more than one SRR layers respond to the chosen frequencies, resulting in the enhancement of the resonance properties. The multi-layer metamaterials provide a promising way to extend SRR based metamaterial operating region from narrowband to broadband with a tunable feature.

Broadband multi-layer terahertz metamaterials fabrication and characterization on flexible substrates

During the past decade electromagnetic metamaterials have realized many exotic phenomena that are difficult or impossible using naturally occurring materials. It is their resonantly enhanced interaction with electromagnetic waves that underpins their attractive qualities, which are increasingly important in the terahertz frequency range. Passive and active terahertz metamaterials and devices have enabled novel functionality and unprecedented terahertz device performance. These demonstrations prove their potential to address the so-called terahertz gap, a technology vacuum associated with the deficiency of natural materials with a desirable terahertz response.

Manipulation of terahertz radiation using metamaterials

We report true left-handed (LH) behavior in a composite metamaterial consisting of a periodically arranged split ring resonator (SRR) and wire structures. We demonstrate the magnetic resonance of the SRR structure by comparing the transmission spectra of SRRs with those of closed SRRs. We have confirmed experimentally that the effective plasma frequency of the LH material composed of SRRs and wires is lower than the plasma frequency of the wires. A well-defined LH transmission band with a peak value of 21.2 dB 20.3 dB cm was obtained. The experimental results agree extremely well with the theoretical calculations. © 2004 Optical Society of America OCIS codes: 120.7000, 260.2110.

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Experimental observation of true left-handed transmissionpeaks in metamaterials

Post-blast detection of traces of explosives by means of Fourier transform infrared spectroscopy

Fourier transform-infrared (FT-IR) spectroscopy has gained considerable attention among the forensic scientists because it shows high sensitivity and selectivity and offers near real time detection of analyzed samples. However, the amount of obtained information due to complexity of the measured spectra forces the use of additional data processing. Application of the multivariate statistical techniques for the analysis of the FT-IR data seems to be necessary in order to enable feature extraction, proper evaluation, and identification of obtained spectra. In this article, an attempt to develop a feasible procedure for characterization of spectroscopic signatures of the explosive materials in the remnants after explosion has been made. All spectra were derived after analysis of samples from debris after especially prepared and performed blasts with the use of three various highly explosive materials: C-4, 2,4,6-trinitrotoluene (TNT), and pentaerythritol tetranitrate (PETN). Two well-known multivariate stati...

Multivariate analysis techniques in the forensics investigation of the postblast residues by means of Fourier transform-infrared spectroscopy.

Presently existing THz electromagnetic metamaterials were mostly produced as planar single layer structures, either deposited on a substrate or embedded in a polymer matrix, and patterned by a primary pattern generator such as an electron beam writer or a laser writer. Some attempts to produce more voluminous structures were made using ultraviolet photolithography and a stacking process. We explore deep x-ray lithography and subsequent stacking of chips to fabricate, with good yield, substantial quantities of rod-split-ring structures that come closer to three dimensions than before. Samples were characterized layer-by-layer using Fourier transform interferometry in the far infrared.

Towards three-dimensional and multilayer rod-split-ring metamaterial structures by means of deep x-ray lithography

This paper addresses the 2D and 3D micro- and nanofabrication of ElectroMagnetic MetaMaterials (EM3) for the terahertz range. EM3 refers to artifbial composite materials which consist of a collection of repeated metal elements designed to have a strong response to applied electromagnetic felds, so that near resonance both the effective permittivity and magnetic permeability µ become simultaneously negative. This unusual situation leads to exotic consequences such as a negative index of refraction and an inverse Doppler and Cerenkov effect. EM3 fabricated so far have been mostly two-dimensional and in this respect are highly anisotropic. By anisotropic, it is inferred that the response of the system depends on the direction of illumination. The anisotropic nature of the metamaterials impedes eventual real-life applications of the negative media as it places constraints on the impinging electromagnetic waves. Ways of producing three-dimensional (3D) or more isotropic EM3 by means of tilted x-ray exposures will be introduced. Basic geometry tells us that if the structures are inclined at 30-45°, this would lead to an improvement of the coupling of the vector by 50-70%

https://iopscience.iop.org/article/10.1088/1742-6596/34/1/147/pdf

Fabrication of 2D and 3D Electromagnetic Metamaterials for the Terahertz Range.

Singapore Synchrotron Light Source (SSLS) is commissioning its new beamline for Infrared Spectro/Microscopy (ISMI). The infrared light is extracted from the edge region of dipole D1 of the compact superconducting electron storage ring Helios 2. The nominal source point is located at half the maximum field, i.e., at 2.25 T. The end station comprises both, a medium and a high resolution Fourier transform infrared spectrometer (FTIR), the former featuring an infrared microscope as well as a UHV chamber for catalysis experiments. Synchrotron Radiation Workshop (SRW) calculations and a preliminary experimental evaluation of ISMI show the capability of this beamline to deliver a bright flux of photons in the Far and Mid infrared spectral regions.

M. Bahou

Papers

Post-blast detection of traces of explosives by means of Fourier transform infrared spectroscopy

Multivariate analysis techniques in the forensics investigation of the postblast residues by means of Fourier transform-infrared spectroscopy.

Towards three-dimensional and multilayer rod-split-ring metamaterial structures by means of deep x-ray lithography

Fabrication of 2D and 3D Electromagnetic Metamaterials for the Terahertz Range.

Infrared Spectro/Microscopy at SSLS — Edge Effect Source in a Compact Superconducting Storage Ring