Artificially engineered metamaterials are now demonstrating unprecedented electromagnetic properties that cannot be obtained with naturally occurring materials. In particular, they provide a route to creating materials that possess a negative refractive index and offer exciting new prospects for manipulating light. This review describes the recent progress made in creating nanostructured metamaterials with a negative index at optical wavelengths, and discusses some of the devices that could result from these new materials.

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Optical negative-index metamaterials

The diffraction limit of light, which is causd by the loss of evanescent waves in the far field that carry high spatial frequency information, limits the resolution of optical lenses to the order of the wavelength of light. We report experimental demonstration of the optical hyperlens for sub-diffraction-limited imaging in the far field. The device magnifies subwavelength objects by transforming the scattered evanescent waves into propagating waves in an anisotropic medium and projects the high-resolution image at far field. The optical hyperlens opens up possibilities in applications such as real-time biomolecular imaging and nanolithography.

https://science.sciencemag.org/content/sci/315/5819/1686.full.pdf

Far-field optical hyperlens magnifying sub-diffraction-limited objects.

Plasmonics is a research area merging the fields of optics and nanoelectronics by confining light with relatively large free-space wavelength to the nanometer scale - thereby enabling a family of novel devices. Current plasmonic devices at telecommunication and optical frequencies face significant challenges due to losses encountered in the constituent plasmonic materials. These large losses seriously limit the practicality of these metals for many novel applications. This paper provides an overview of alternative plasmonic materials along with motivation for each material choice and important aspects of fabrication. A comparative study of various materials including metals, metal alloys and heavily doped semiconductors is presented. The performance of each material is evaluated based on quality factors defined for each class of plasmonic devices. Most importantly, this paper outlines an approach for realizing optimal plasmonic material properties for specific frequencies and applications, thereby providing a reference for those searching for better plasmonic materials.

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Searching for better plasmonic materials

Searching for Better Plasmonic Materials

Metamaterials (MMs) are artificial, engineered materials with rationally designed compositions and arrangements of nanostructured building blocks. These materials can be tailored for almost any application because of their extraordinary response to electromagnetic, acoustic, and thermal waves that transcends the properties of natural materials (1–3). The astonishing MM-based designs and demonstrations range from a negative index of refraction, focusing and imaging with sub-wavelength resolution, invisibility cloaks, and optical black holes to nanoscale optics, data processing, and quantum information applications (3). Metals have traditionally been the material of choice for the building blocks, but they suffer from high resistive losses—even metals with the highest conductivities, silver and gold, exhibit excessive losses at optical frequencies that restrict the development of devices in this frequency range. The development of new materials for low-loss MM components and telecommunication devices is therefore required.

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Low-Loss Plasmonic Metamaterials

The wave nature of light limits the spatial resolution in classical microscopy to about half of the illumination wavelength. Recently, a new approach capable of achieving subwavelength spatial resolution, called superlensing, was invented, challenging the already established method of scanning near-field optical microscopy (SNOM). We combine the advantages of both techniques and demonstrate a novel imaging system where the objects no longer need to be in close proxim-ity to a near-field probe, allowing for optical near-field microscopy of subsurface objects at sub-wavelength-scale lateral resolution.

http://science.sciencemag.org/content/sci/313/5793/1595.full.pdf

Near-Field Microscopy Through a SiC Superlens

Soft x-ray lasing with a gain length GL{approx_equal}5.5 was demonstrated in hydrogenlike LiIII at 13.5 nm (2-1 transition) in a 5 mm long LiF microcapillary using a 50 mJ, 250 fs UV laser beam at a 2 Hz repetition rate. The initial plasma was created in the 0.3 mm diameter microcapillary by a low power Nd/YAG laser. The strongly nonlinear increase of the 13.5 nm line intensity with increasing microcapillary length was compared with a linear increase of the LiIII 11.4 nm (3-1 transition) line intensity. {copyright} {ital 1996 The American Physical Society.}

Demonstration of Soft X-Ray Lasing to Ground State in Li III.

We demonstrate that a negative-permittivity material (silicon carbide) sandwiched between two layers of positive-permittivity material (silicon oxide) can be used for enhancement of the resolution of near-field imaging via the superlensing effect. The resulting three-layer metamaterial is also shown to exhibit an enhanced transmission when its effective dielectric permittivity matches that of the vacuum. Experimental far-field diagnostics of the superlensing based on measuring transmission coefficients through the metal-coated superlens is implemented using Fourier-transformed infrared microscopy. Superlensing is shown to be a highly resonant phenomenon manifested in a narrow frequency range.

Enhanced near-field resolution in midinfrared using metamaterials

We observe critical coupling to surface phonon-polaritons in silicon carbide by attenuated total reflection of mid-IR radiation. Reflectance measurements demonstrate critical coupling by a double scan of wavelength and incidence angle. Critical coupling occurs when prism coupling loss is equal to losses in silicon carbide and the substrate, resulting in maximal electric field enhancement.

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Critically coupled surface phonon-polariton excitation in silicon carbide.

We present the first demonstration of pL-scale analyte index sensing based on surface phonon polaritons in the midinfrared, which are excited at the silicon carbide/analyte interface in the Otto configuration. Attenuated total reflectance measurements reveal analyte index specificity through a double-scan of wavelength and incidence angle for analyte volumes as small as 100 pL. Midinfrared sensing tuned to surface phonon polariton resonance paves the way for index sensing of analytes beyond current volume−resolution limits.

Dmitriy Korobkin

Papers

Near-Field Microscopy Through a SiC Superlens

Demonstration of Soft X-Ray Lasing to Ground State in Li III.

Enhanced near-field resolution in midinfrared using metamaterials

Critically coupled surface phonon-polariton excitation in silicon carbide.

Midinfrared Index Sensing of pL-Scale Analytes Based on Surface Phonon Polaritons in Silicon Carbide†