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

We have fabricated a quarter-wave plate from a single layer of birefringent electric split-ring resonators (ELC). For comparison, an appropriately scaled double layer meanderline structure was fabricated. At the design frequency of 639 GHz, the ELC structure achieves 99.9% circular polarization while the meanderline achieves 99.6%. The me-anderline displays a larger bandwidth of operation, attaining over 99% circular polarization from 615 - 743 GHz, while the ELC achieves 99% from 626 - 660 GHz. However, both are broad enough for use with CW sources making ELCs a more attractive choice due to the ease of fabrication. Both samples are free standing with a total thickness of 70 microm for the meanderline structure and a mere 20 microm for the ELC highlighting the large degree of birefringence exhibited with metamaterial structures.

Comparison of birefringent electric split-ring resonator and meanderline structures as quarter-wave plates at terahertz frequencies.

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

The tunability of an omega–type left handed metamaterial was demonstrated at microwave frequencies via the magnetic control of liquid crystal (LC) orientation. From the experimental and simulation results, it is shown that the left handed pass-band can be tuned by 220 MHz by changing the orientation of LC molecules by 90°. A maximum index variation of 0.25 was obtained in the negative index regime with a measured LC birefringence of 0.05 in the 10 - 12GHz frequency band.

Magnetically tunable left handed metamaterials by liquid crystal orientation.

Metamaterials are engineered composite materials offering unprecedented control of wave propagation. Despite their complexity, effective properties can frequently be extracted by conceptualizing them as homogeneous and isotropic media with dispersive electric permittivity and magnetic permeability. For an ideal isotropic medium, strong dispersion in these properties causes wave and field vectors to form a left-handed (E,H,k)-frame involving backward waves, and offering control of quantities like the refractive index which may become negative. Experimental evidence exists from microwaves to the visible. Applications include sub-wavelength-resolution imaging, invisibility cloaking, plasmonics-based lasers, metananocircuits, and omnidirectional absorbers. As the engineered sub-structures must be smaller than their design wavelength, micro/nanomanufacturing is exploited from primary pattern generation over lithography to templating and molecular beam epitaxy. 3D metamaterials have been made by stacking of layers, multilayer structuring, and 3D primary pattern generation. Theory shows that full properties may build up over one or a very few layers.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/lpor.201000019

3D THz metamaterials from micro/nanomanufacturing

Using micromanufactured S-shaped gold strings suspended in free space by means of window-frames, we experimentally demonstrate an electromagnetic meta-material (EM3) in which the metallic structures are no longer embedded in matrices or deposited on substrates such that the response is solely determined by the geometrical parameters and the properties of the metal. Two carefully aligned and assembled window-frames form a bi-layer chip that exhibits 2D left-handed pass-bands corresponding to two different magnetic resonant loops in the range of 1.4 to 2.2 THz as characterized by Fourier transform interferometry and numerical simulation. Chips have a comparably large useful area of 56 mm2. Our results are a step towards providing EM3 that fulfill the common notions of a material.

Free-standing THz electromagnetic metamaterials.

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.

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

For successful operation of superconducting cavities in light sources, high reliability and minimal beam losses are essential. Diamond started operation with users in January 2007 and since then, the Diamond storage ring superconducting cavities have been the largest single contributor to unplanned beam trips. We have dedicated extensive effort to improve our data acquisition, numerical modelling and fault analysis to improve our understanding of the main causes of the trips and how to prevent trips or reduce their frequency. In the past seven months, the performance of the cavities has improved significantly. We present here our analysis of some of the trips and their underlying causes and discuss improvements carried out.

Reliability improvements of the diamond superconducting cavities

The phase and amplitude detector (PAD) is one of the key units of the phasing system for BEPCII linac. A new PAD based on I/Q demodulator has been constructed to measure the phase and amplitude of each klystron. To compensate the mismatches of the I/Q demodulator, a calibration algorithm using the adaptive LMS method has been developed to calibrate the I/Q demodulator, and most of the mismatches have been compensated after calibration. Some EMC methods have been adopted to minimize the influence of the noises in the klystron gallery, and the PAD has been running stably and successfully.

Pengda Gu

Papers

Free-standing THz electromagnetic metamaterials.

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

Reliability improvements of the diamond superconducting cavities

Design and Calibration of a Phase and Amplitude Detector