The physics and applications of a broad class of artificial electromagnetic materials composed of lattices of aligned metal rods embedded in a dielectric matrix are reviewed. Such structures are here termed wire metamaterials. They appear in various settings and can operate from microwaves to THz and optical frequencies. An important group of these metamaterials is a wire medium possessing extreme optical anisotropy. The study of wire metamaterials has a long history, however, most of their important and useful properties have been revealed and understood only recently, especially in the THz and optical frequency ranges where the wire media correspond to the lattices of microwires and nanowires, respectively. Another group of wire metamaterials are arrays and lattices of nanorods of noble metals whose unusual properties are driven by plasmonic resonances.
One of the basic functionalities of photonic devices is the ability to manipulate the polarization state of light. Polarization components are usually implemented using the retardation effect in natural birefringent crystals and, thus, have a bulky design. Here, we have demonstrated the polarization manipulation of light by employing a thin subwavelength slab of metamaterial with an extremely anisotropic effective permittivity tensor. Polarization properties of light incident on the metamaterial in the regime of hyperbolic, epsilon-near-zero, and conventional elliptic dispersions were compared. We have shown that both reflection from and transmission through λ/20 thick slab of the metamaterial may provide nearly complete linear-to-circular polarization conversion or 90° linear polarization rotation, not achievable with natural materials. Using ellipsometric measurements, we experimentally studied the polarization conversion properties of the metamaterial slab made of the plasmonic nanorod arrays in different dispersion regimes. We have also suggested all-optical ultrafast control of reflected or transmitted light polarization by employing metal nonlinearities.
1 Introduction Metal-dielectric nanocomposites (wire metamaterial) based on porous alumina matrices fabricated by electrochemical eatching attract increasing attention for many reasons, ranging from fundamental research problems (study of nonlinear properties [1]) to applications in many different fields (biosensors [2], optical metamaterials [3], nanophotonics [4], etc.). Other materials which have similar structural parameters (e.g. aspect ratio) and optical properties are metallic nanotubes based on dielectric [5] and semiconductors matrices [6,7]. However, as is shown in the theoretical [8] and experimental [9] works, the medium with nanotubes is not a suitable material to realise the properties of wire metamaterials because the latter require the thickness of the tubes to be greater than the skin depth of a metal for proper operation (at least 15-20 nm for operation in the visible domain). The investigation and application of the unique properties of these materials face significant challenges, such as small values of the aspect ratio (less than 100) and low values of electric permittivity, ε, in dielectric matrices. Another material in which porous matrices can be created by an anodic electrochemical etching [10,11] is an A III B V semiconductor. In this case the nanoporous matrices give a unique set of parameters: the aspect ratio greater than 10000, the thickness of porous matrices up to 150 μm and more [12]. Using different orientations of the original semiconductor crystal, etching regimes and types of electrolyte, one can control the param-
In this paper, we have studied the emission of terahertz radiation from nanoporous semiconductor matrices of GaP excited by the femtosecond laser pulses. We observe 3–4 orders of magnitude increase of terahertz radiation emission from the nanoporous matrix compared to bulk material. The effect is mainly related to drastic increase of the sample surface and pinning of conducting electrons to surface states. This result opens up a promising way to create powerful sources of terahertz radiation using nanoporous semiconductors.
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