Free-standing THz electromagnetic metamaterials

Broadband negative-refractive index terahertz metamaterial with optically tunable equivalent-energy level

An optically tunable terahertz negative-refractive index metamaterial (NIM) is proposed. The NIMs are composed of two aluminum rings and two photosensitive ring-shaped silicon apertures coaxially coated on the both sides of Teflon substrate. The NIMS are also designed to realize wide incident angle, polarization insensitivity, and tunability. Similar to the real atom, the unit cell of NIMs is equivalent to the Teflon nucleus surrounded by top and bottom resonator electrons, which indicates that the equivalent-energy level of NIMs can be dynamically controlled by the resonator electrons, once the scale of substrate nucleus is fixed. Using the LC-circuit model, the dynamic control of the equivalent-energy level of NIMs is studied in detail. Simulation results indicate that the transmission of NIMs is tuned from lowpass to highpass when the conductivity of silicon is increased, and the corresponding phase at lower frequency can be continually tuned. Correspondingly, the negative refractive index of NIMs represents dynamically tunable property, and the tunable negative refraction is simulated by classical wedge prism model. Besides, the phase flow indicates that the direction of phase velocity of NIMs is negative for the single-negative index.

Negative refractive index metamaterial with high transmission, low reflection, and low loss in the terahertz waveband

The refractive index is a basic parameter of materials which it is essential to know for the manipulation of electromagnetic waves. However, there are no naturally occurring materials with negative refractive indices, and high-performance materials with negative refractive indices and low losses are demanded in the terahertz waveband. In this paper, measurements by terahertz time-domain spectroscopy (THz-TDS) demonstrate a metamaterial with a negative refractive index n of -4.2 + j0.17, high transmitted power of 81.5%, low reflected power of 4.3%, and a high figure of merit (FOM = |Re(n)/Im(n)|) of 24.2 at 0.42 THz. The terahertz metamaterial with these unprecedented properties can provide various attractive terahertz applications such as superlenses with resolutions beyond the diffraction limit in terahertz continuous wave imaging.

Terahertz metamaterials and systems based on rolled-up 3D elements: designs, technological approaches, and properties

Electromagnetic metamaterials opened the way to extraordinary manipulation of radiation. Terahertz (THz) and optical metamaterials are usually fabricated by traditional planar-patterning approaches, while the majority of practical applications require metamaterials with 3D resonators. Making arrays of precise 3D micro- and nanoresonators is still a challenging problem. Here we present a versatile set of approaches to fabrication of metamaterials with 3D resonators rolled-up from strained films, demonstrate novel THz metamaterials/systems, and show giant polarization rotation by several chiral metamaterials/systems. The polarization spectra of chiral metamaterials on semiconductor substrates exhibit ultrasharp quasiperiodic peaks. Application of 3D printing allowed assembling more complex systems, including the bianisotropic system with optimal microhelices, which showed an extreme polarization azimuth rotation of 85° with drop by 150° at a frequency shift of 0.4%. We refer the quasiperiodic peaks in the polarization spectra of metamaterial systems to the interplay of different resonances, including peculiar chiral waveguide resonance. Formed metamaterials cannot be made by any other presently available technology. All steps of presented fabrication approaches are parallel, IC-compatible and allow mass fabrication with scaling of rolled-up resonators up to visible frequencies. We anticipate that the rolled-up meta-atoms will be ideal building blocks for future generations of commercial metamaterials, devices and systems on their basis.

Magnetic Fano resonances by design in symmetry broken THz meta-foils

Magnetic Fano resonances in there-dimensional symmetry broken meta-foils at THz frequencies are theoretically and experimentally studied. Sharp Fano resonances occur due to the interference between different resonances and can be designed by choosing geometric parameters of the meta-foil. At the Fano resonances, the meta-foil supports antisymmetric modes, whereas, at the main resonance, only a symmetric mode exists. The meta-foil is left-handed at the Fano resonances and shows sharp peaks of the real part of the refractive index in transmission with small effective losses opening a way to very sensitive high-speed sensing of dielectric changes in the surrounding media and of mechanical configuration.

Compensating substrate-induced bianisotropy in optical metamaterials using ultrathin superstrate coatings

In this work, we propose an efficient approach to compensate for the commonly observed substrate-induced bianisotropy that occurs in on-wafer optical metamaterials at normal incidence. First, the consequence of placing a finite thickness substrate underneath a metamaterial is analyzed, indicating that the induced bianisotropy is a near-field effect. The properties of metamaterials sandwiched between an infinitely thick substrate and a finite-thickness superstrate with different permittivity and thickness values are then investigated. It is demonstrated from full-wave simulations that by adding an ultrathin superstrate with a judicious choice of its thickness and permittivity value, the substrate-induced bianisotropy of the system can be suppressed and even eliminated. In addition to the extracted nonlocal effective medium parameters, the induced electric and magnetic dipole moments calculated from the volumetric microscopic fields are also presented, validating that the magnetoelectric coupling compensation is a real physical phenomenon. This study will benefit future optical metamaterial design and implementation strategies as well as the corresponding fabrication and characterization methodologies.

All-metal self-supported THz metamaterial--the meta-foil

Modern metamaterials face functional constraints as they are commonly embedded in or deposited on dielectric materials. We provide a new solution by microfabricating a completely free-standing all-metal self-supported metamaterial. Using upright S-string architecture with the distinctive feature of metallic transverse interconnects, we form a locally stiff, globally flexible space-grid. Infrared Fourier transform interferometry reveals the typical double-peak structure of a magnetically excited left-handed and an electrically excited right-handed pass-band that is maintained under strong bending and heating, and is sensitive to dielectrics. Exploiting UV/X-ray lithography and ultimately plastic moulding, meta-foils can be mass manufactured cost-effectively to serve as optical elements.

Magnetically tunable left handed metamaterials by liquid crystal orientation

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 degrees. 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 - 12 GHz frequency band.

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

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.