There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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.

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

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

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Principles Of Optics Electromagnetic Theory Of Propagation Interference And Diffraction Of Light

We study experimentally and numerically the electro-optic (EO) effect in a single metamaterial cell incorporating a nonlinear crystal. A metal split-ring fabricated on a ZnTe substrate resonating at 3 GHz was excited with a tunable microwave source. A linearly polarized laser beam probed the EO effect. Enhanced EO modulation was observed in the split-gap at the resonant frequency proportional to Q-factor. The electric field surrounding the metamaterial element in the ZnTe was mapped by the EO effect and agrees well with simulation. We show that high-Q metamaterials incorporating an EO crystal in the split-gap could lead to improved EO devices.

Direct observation of electro-optic modulation in a single split-ring resonator

We report the results of a study to model the behavior of nonlinear metamaterials in the microwave frequency range
composed of arrays of split-ring resonators combined with nonlinear circuit elements. The overall model consists of an
array of coupled damped oscillators whose inter-element coupling is a function of signal amplitude, similar to that which
exists in the Fermi-Pasta-Ulam system. [8] We note the potential occurrence of classical nonlinear effects including
parametric coupling, FPU recurrence and chaos. These effects lead to nonlinear waves on the array which are a type of
soliton particular to the form of nonlinearity that has been incorporated. We have studied, in particular, the nonlinear
effects that arise from tunnel diodes embedded in the resonant circuits. We carry out simulations of the resulting circuit
frequency response.

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Modeling of Nonlinear Metamaterials

Planar electric metamaterials are studied with terahertz time-domain spectroscopy in transmission and reflection Energy absorption of 5-20% due to Ohmic losses within the metal patterning is observed at resonant frequencies Finite-element simulations verify experimental results

Properties of Novel Terahertz Electric Metamaterials

We present experimental results of metamaterials operating at terahertz and mm-wave frequencies. Metamaterials consist of a single layer of 200 nm thick gold on a doped or undoped semiconducting substrate. By optical and electronic doping of supporting semiconducting substrates we show external control of planar arrays of metamaterials, characterized with terahertz time domain spectroscopy. Both methods yield meta-material / semiconductor devices which can be utilized as switches or modulators, enabling modulation of THz transmission by 50 percent. Experiments are supported by simulations and results agree well. Because of the universality of metamaterial response over many decades of frequency, these results have implications for other regions of the electromagnetic spectrum and will undoubtedly play a key role in future demonstrations of novel high-performance devices.

Metamaterials for Novel Terahertz and Millimeter Wave Devices

We use active metamaterials as external modulators for a 2.4 THz quantum cascade laser. We present initial optical modulation results with the goal of designing an electrically-driven modulator.

John F. O'Hara

Papers

Direct observation of electro-optic modulation in a single split-ring resonator

Modeling of Nonlinear Metamaterials

Properties of Novel Terahertz Electric Metamaterials

Metamaterials for Novel Terahertz and Millimeter Wave Devices

External modulation of Terahertz quantum cascade lasers using metamaterials.