We present the design for an absorbing metamaterial (MM) with near unity absorbance A(omega). Our structure consists of two MM resonators that couple separately to electric and magnetic fields so as to absorb all incident radiation within a single unit cell layer. We fabricate, characterize, and analyze a MM absorber with a slightly lower predicted A(omega) of 96%. Unlike conventional absorbers, our MM consists solely of metallic elements. The substrate can therefore be optimized for other parameters of interest. We experimentally demonstrate a peak A(omega) greater than 88% at 11.5 GHz.

Perfect metamaterial absorber.

Research into terahertz technology is now receiving increasing attention around the world, and devices exploiting this waveband are set to become increasingly important in a very diverse range of applications. Here, an overview of the status of the technology, its uses and its future prospects are presented.

Cutting-edge terahertz technology

Metamaterials are artificially fabricated materials that allow for the control of light and acoustic waves in a manner that is not possible in nature. This Review covers the recent developments in the study of so-called metasurfaces, which offer the possibility of controlling light with ultrathin, planar optical components. Conventional optical components such as lenses, waveplates and holograms rely on light propagation over distances much larger than the wavelength to shape wavefronts. In this way substantial changes of the amplitude, phase or polarization of light waves are gradually accumulated along the optical path. This Review focuses on recent developments on flat, ultrathin optical components dubbed 'metasurfaces' that produce abrupt changes over the scale of the free-space wavelength in the phase, amplitude and/or polarization of a light beam. Metasurfaces are generally created by assembling arrays of miniature, anisotropic light scatterers (that is, resonators such as optical antennas). The spacing between antennas and their dimensions are much smaller than the wavelength. As a result the metasurfaces, on account of Huygens principle, are able to mould optical wavefronts into arbitrary shapes with subwavelength resolution by introducing spatial variations in the optical response of the light scatterers. Such gradient metasurfaces go beyond the well-established technology of frequency selective surfaces made of periodic structures and are extending to new spectral regions the functionalities of conventional microwave and millimetre-wave transmit-arrays and reflect-arrays. Metasurfaces can also be created by using ultrathin films of materials with large optical losses. By using the controllable abrupt phase shifts associated with reflection or transmission of light waves at the interface between lossy materials, such metasurfaces operate like optically thin cavities that strongly modify the light spectrum. Technology opportunities in various spectral regions and their potential advantages in replacing existing optical components are discussed.

Flat Optics With Designer Metasurfaces

Graphene’s success has shown that it is possible to create stable, single and few-atom-thick layers of van der Waals materials, and also that these materials can exhibit fascinating and technologically useful properties. Here we review the state-of-the-art of 2D materials beyond graphene. Initially, we will outline the different chemical classes of 2D materials and discuss the various strategies to prepare single-layer, few-layer, and multilayer assembly materials in solution, on substrates, and on the wafer scale. Additionally, we present an experimental guide for identifying and characterizing single-layer-thick materials, as well as outlining emerging techniques that yield both local and global information. We describe the differences that occur in the electronic structure between the bulk and the single layer and discuss various methods of tuning their electronic properties by manipulating the surface. Finally, we highlight the properties and advantages of single-, few-, and many-layer 2D materials in...

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Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene

Plasmons describe collective oscillations of electrons. They have a fundamental role in the dynamic responses of electron systems and form the basis of research into optical metamaterials 1–3 . Plasmons of two-dimensional massless electrons, as present in graphene, show unusual behaviour 4–7 that enables new tunable plasmonic metamaterials 8–10 and, potentially, optoelectronic applications in the terahertz frequency range 8,9,11,12 .H ere we explore plasmon excitations in engineered graphene microribbon arrays. We demonstrate that graphene plasmon resonances can be tuned over a broad terahertz frequency range by changing micro-ribbon width and in situ electrostatic doping. The ribbon width and carrier doping dependences of graphene plasmon frequency demonstrate power-law behaviour characteristic of two-dimensional massless Dirac electrons 4–6 . The plasmon resonances have remarkably large oscillator strengths, resulting

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Graphene plasmonics for tunable terahertz metamaterials

High power THz pulses induce near transparency in superconductive YBCO thin films below the critical temperature. THz pump/THz probe measurements reveal a decay of the induced transparency on the time scale of a few picoseconds.

Nonlinear ultrafast dynamics of high temperature YBa2Cu3O7–δ superconductors probed with THz pump / THz probe spectroscopy

A subwavelength metal-dielectric structure for anti-reflection in the mid-wave infrared regime has been designed and experimentally verified. Applied to high index germanium substrates, less than 5% power reflection is achieved over a wide angular range.

Metal-dielectric Structure Anti-Reflective Coating in the Mid-Wave Infrared

Terahertz radiation—electromagnetic waves in the 0.1–10THz frequency range—occupies a middle ground between microwaves and infrared light. The unique properties of terahertz radiation make it very attractive for numerous applications in molecular spectroscopy, biomedical imaging, short-range ultrahigh-bandwidth wireless communications, non-destructive inspection, security screening, and even quantum computing in silicon devices. For example, in a similar way to how infrared radiation can probe the vibrations corresponding to the motion of atoms bound together in an individual molecule, terahertz spectroscopy can probe the much weaker forces governing the interaction between molecules. Its use could revolutionize our understanding of how complex biomolecules interact. Terahertz waves are also well suited to non-destructive testing and security screening applications because, like microwaves and millimeter waves, they are non-ionizing, making them much safer than x-rays, and they can travel through many materials including plastics and cloth. However, the shorter wavelength of terahertz waves compared to microwaves and millimeter waves enables significantly higher image resolution. Historically, the terahertz range has been largely unexplored due to the difficulty of generating, detecting, and manipulating terahertz radiation. Electronic devices that have driven the widespread use of microwaves are generally limited to much lower frequencies. In addition, a paucity of suitable materials means that photonic technologies that have been wildly successful in the infrared, visible, and ultraviolet regimes run into severe limitations in the terahertz range. One powerful way to circumvent the limitations of existing materials is to create artificial ‘metamaterials.’ These consist of arrays of conducting elements that are small enough to appear as an effectively continuous material to the electromagnetic waves (much as we can often ignore that natural materials are really composed of tiny atoms instead of being continuous). Metamaterials can also be engineered to exhibit exotic electromagnetic phenomena not observed in natural materials, including negative refraction and anomalous Figure 1. Microscope image of our superconducting metamaterial. YBCO: Yttrium barium copper oxide.

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Active terahertz metamaterials

We present a perfect terahertz absorber that operates over numerous incidence angles. The two-fold symmetry of rectangular fishnet structures allows either complete absorption or mirror-like reflection depending on the orientation of the electric field.

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Perfect terahertz absorber using fishnet based metafilm

Chiral metamaterials have shown extraordinary capability to change the polarization state of light It has been proposed chiral media with large chirality can be used to make negative index materials Recently, we demonstrate experimentally and numerically that metamaterials based on bi-layer rosette structure, bi-layer cross-wires and non-planar chiral SRRs, give giant optical activity, circular dichroism, and negative refractive index The presented chiral designs offer very simple geometry and an efficient way to realize negative refractive index in a large spectra range from microwave frequency up to optical regime We also develop a retrieval procedure for chiral materials which works successfully for circularly polarized waves By adding active materials in chiral metamaterials design, we propose an active chiral metamaterial with tunable optical activity

Hou-Tong Chen

Papers

Nonlinear ultrafast dynamics of high temperature YBa2Cu3O7–δ superconductors probed with THz pump / THz probe spectroscopy

Metal-dielectric Structure Anti-Reflective Coating in the Mid-Wave Infrared

Active terahertz metamaterials

Perfect terahertz absorber using fishnet based metafilm

Chiral metamaterials with negative refractive index