Igor S. Nefedov

Optimization of radiative heat transfer in hyperbolic metamaterials for thermophotovoltaic applications

Using our recently developed method we analyze the radiative heat transfer in micron-thick multilayer stacks of metamaterials with hyperbolic dispersion. The metamaterials are especially designed for prospective thermophotovoltaic systems. We show that the huge transfer of near-infrared thermal radiation across micron layers of metamaterials is achievable and can be optimized. We suggest an approach to the optimal design of such metamaterials taking into account high temperatures of the emitting medium and the heating of the photovoltaic medium by the low-frequency part of the radiation spectrum. We show that both huge values and frequency selectivity are achievable for the radiative heat transfer in hyperbolic multilayer stacks.

Effects of Spatial Dispersion on Reflection From Mushroom-Type Artificial Impedance Surfaces

In a spatially dispersive medium, the electric dipole moment of an inclusion cannot be related to the macroscopic electric field through a local relation. Several recent works have emphasized the role of spatial dispersion in wire media, and demonstrated that arrays of parallel metallic wires may behave very differently from a uniaxial local material with negative permittivity. Here, we investigate the effect of spatial dispersion on reflection properties of the mushroom structure introduced by Sievenpiper, based on local and nonlocal homogenization methods. The objective of this paper is to clarify the role of spatial dispersion in the mushroom structure and demonstrate that, under some conditions, it is suppressed. The metamaterial substrate, or metasurface is modeled as a wire medium covered with an impedance surface. Surprisingly, it is found that, in such a configuration, the effects of spatial dispersion may be nearly suppressed when the slab is electrically thin, and that the wire medium can be modeled very accurately using a local model.

Perfect absorption in graphene multilayers

We demonstrate that 100% light absorption can be achieved in a graphene-based hyperbolic metamaterial, consisting of periodically arranged graphene layers which are tilted with respect to the interface. The geometrical parameters of the multilayered structure and the chemical potential of graphene are chosen in such a way that the in-plane relative effective permittivity is close to 1. Under this condition, the graphene multilayer exhibits asymmetry which appears as a very large difference between waves propagating upward and downward with respect to multilayer boundaries. One of them has a very high attenuation constant and neither of the waves undergo reflection at slab interfaces, resulting in total absorption even for an optically ultra-thin slab.

Giant radiation heat transfer through micron gaps

Near-field heat transfer between two closely spaced radiating media can exceed in orders radiation through the interface of a single black body. This effect is caused by exponentially decaying (evanescent) waves which form the photon tunnel between two transparent boundaries. However, in the mid-infrared range it holds when the gap between two media is as small as few tens of nanometers. We propose a new paradigm of the radiation heat transfer which makes possible the strong photon tunneling for micron thick gaps. For it the air gap between two media should be modified, so that evanescent waves are transformed inside it into propagating ones. This modification is achievable using a metamaterial so that the direct thermal conductance through the metamaterial is practically absent and the photovoltaic conversion of the transferred heat is not altered by the metamaterial.

Characterization of Surface-Wave and Leaky-Wave Propagation on Wire-Medium Slabs and Mushroom Structures Based on Local and Nonlocal Homogenization Models

In this paper, a nonlocal homogenization model is proposed for the analysis of the spectrum of natural modes on sub-wavelength mushroom-type high-impedance surfaces composed of a capacitive grid connected to a grounded wire-medium (WM) slab. Modal characteristics of mushroom structures are studied in conjunction with the surface-wave and leaky-wave propagation on WM slabs based on local and nonlocal homogenization models, showing the importance of spatial dispersion (SD) in WM. It is shown that mushroom structures support proper real (bound) forward and backward modes, whose dispersion determines the stopband properties of the mushroom structure, and proper (exponentially decaying from the surface) and improper (exponentially growing from the surface) complex leaky-wave modes related to the backward and forward radiation, respectively. Results obtained by different homogenization models are compared leading to important conclusions. Specifically, an interesting observation concerns the mushroom structures with short vias, wherein the SD of the WM slab is significantly reduced, and the results of local and nonlocal homogenization models are in excellent agreement.

Electromagnetic mode density for finite quasi-periodic structures

The transmission properties of quasi-periodic Cantor-like and Fibonacci photonic bandgap structures that have the same optical paths are studied and compared with one-dimensional, finite, N-period stacks. The electromagnetic mode densities and group velocities are also discussed. We show that the density of modes at the edge of the bandgap is greater for Cantor-like multilayers, so at the band edge a sharp decrease in the group velocity is found.

Total absorption in asymmetric hyperbolic media

Finite-thickness slabs of hyperbolic media with tilted optical axes exhibit asymmetry properties for waves propagating upward and downward with respect to slab interfaces. Under certain conditions, asymmetric hyperbolic media acquire extreme permittivity parameters and the difference between upward and downward propagating waves becomes very large. Furthermore, both waves can be perfectly matched with the free space; such a feature makes possible the development of optically ultra thin perfect absorbers. The proposed approach is unified and allows the use of different -negative materials. Of particular interest is the asymmetric hyperbolic medium, made of silicon nanowires, since it can be directly applicable to solar cell systems.

Ultrabroadband electromagnetically indefinite medium formed by aligned carbon nanotubes

Anisotropic materials with different signs of components of the permittivity tensor are called indefinite materials. The known realizations of indefinite media at ir and optical frequencies suffer from high absorption losses and narrow bandwidth. Here we show that periodic arrays of parallel carbon nanotubes (CNTs) can behave as low-loss indefinite media in the infrared range. We demonstrate that a finite-thickness slab of CNTs supports propagation of backward waves with small attenuation in an ultrabroad frequency band.

Infrared properties of randomly oriented silver nanowires

We experimentally investigated the infrared properties of a set of randomly oriented silver nanowires films deposited onto glass substrate. Infrared emission of the obtained films was characterized in the long infrared range, i.e., 8–12 μm, by observing their temperature evolution under heating regime with a focal plane array infrared camera as well as a thermocouple. The obtained experimental results showed that the infrared emission from a mesh composed of silver nanowires might be tailored by opportunely assessing preparation condition, such as the metal filling factor. From the theoretical point of view, the real and imaginary part of the electrical permittivity components were retrieved from the calculations of effective permittivities of in-plane randomly oriented metallic wires, thus giving the refractive index and extinction coefficients for the four different silver nanowires meshes. Due to the correspondence between emissivity and absorbance, the experimental results are interpreted with the rec...

Electromagnetic response and homogenization of grids of ferromagnetic microwires

This contribution presents an analytical formulation for the electromagnetic response of grids of ferromagnetic microwires, where the electromagnetic fields produced by the structure are found by means of the local field method. In addition, a circuit analogy is introduced for a better understanding of the grid response, where a single ferromagnetic microwire is modeled as an impedance-loaded wire, and the transmission-line approach is used for the whole grid. Moreover, the homogenization of the structure is considered to provide more physical insight into internal polarizations of the grid. Contrary to the previous experiments of left-handed transmission in grids of ferromagnetic microwires, it is found that such structures can be modeled as artificial dielectric slabs with a frequency dispersive permittivity.