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 demonstrate active tuning of coupled inductive-capacitive resonance in a multi-layer metamaterial. Our experiment reveals that one resonance mode of a coupled pair can be selectively switched off by driving the metamaterial with infrared light.

Active tuning of coupled resonance modes in terahertz metamaterials

Bilayer split ring resonators as a function of separation (∼λ /500) and orientation are measured. Terahertz measurements match simulations showing frequency shifting of the resonances with implications for electrically small antenna design.

Passive resonance tuning through closely spaced coupled split ring resonators

The interrelation between transmission amplitude and phase spectra of anisotropic metamaterials enables us to develop realistic designs for a terahertz quarter-wave plate and a polarizing terahertz beam splitter based on an anisotropic metamaterial structure.

THz polarimetric components based on metamaterials

This work demonstrates a non-contact diffuse reflectance approach with a working distance of ~1.1 meters for the potential of glucose sensing. Non-contact diffuse reflectance over 1.1-1.3 micrometers was developed according to a center-illumination-area-detection (CIAD) geometry. The modeled response of diffuse reflectance in the CIAD geometry was examined with phantoms by altering independently the size of the collection geometry and the reduced scattering and absorption properties of the medium. When applied to aqueous turbid medium containing glucose control solutions with the cumulative volume varying over three orders of magnitude, a linear relationship expected for the diffuse reflectance as a function of the medium absorption/reduced-scattering property was observed for four conditions of the glucose-medium composition that differed either in the effective glucose concentration or the host medium scattering property. The cumulation of glucose up to 17.8mg/dL and 8.9mg/dL in the host medium having the same optical properties resulted in linear regression slopes of 0.0032 and 0.0030, respectively. The cumulation of the glucose up to 17.8mg/dL in an aqueous host medium that differed two folds in the reduced scattering property caused the linear regression slope to differ between 0.0032 and 0.0019. The R^2 values of all cases were all greater than 0.987. A provisional patent (US#63/053,004) has been obtained for this work.

Towards Non-Contact Glucose Sensing in Aqueous Turbid Medium at ~1.1 Meters Distance

Summary from only given. Improvements in resolution are beneficial to any imaging system. Since they are usually incoherent, optical imaging systems infrequently utilize synthetic aperture and phased-array techniques to improve resolution. Imaging with coherent, broadband THz pulses, however, can benefit from these well-known techniques. Using these techniques, we demonstrate resolution enhancement in an established THz imaging system.

John F. O'Hara

Papers

Active tuning of coupled resonance modes in terahertz metamaterials

Passive resonance tuning through closely spaced coupled split ring resonators

THz polarimetric components based on metamaterials

Towards Non-Contact Glucose Sensing in Aqueous Turbid Medium at ~1.1 Meters Distance

Synthetic phased-array THz imaging