Wavelength-selective metamaterial absorber and emitter

Electromagnetic absorbers and emitters have been attracting interest in lots of fields, which are significantly revitalized because of the novel properties brought by the development of the metamaterials, the artificially designed materials. Metamaterials broadens the approaches to design the electromagnetic absorbers and emitters, making it possible to obtain the perfect absorption or emission at the wavelengths covering a wide range. Metamaterial absorbers and emitters are promising for various applications, including solar thermal-photovoltaics and thermal-photovoltaics for energy harvesting, chemical and biomedical sensors, nanoscale imaging and color printing. This work focuses on three aspects (materials, structures and design methods) to improve the experiment realizations of visible and infrared absorbers and emitters. Firstly, this work investigates simple structures based on aluminum and tungsten materials for the metamaterial absorber and emitter, which results in the realization of the all-metal visible color printing with square resonators and wavelength selective mid-infrared absorber (emitter) with cross resonators, respectively. Secondly, we explore the thermal emission properties of the quasi-periodic metal-dielectric multilayer metamaterials, which show the ability of engineering emissivity by different lattice structures. Finally, this work demonstrates the use of micro-genetic algorithm to realize efficient design and optimization for broadband metasurface absorbers, as well as wavelength-selective metasurfaces with giant circular dichroism. This work is believed to facilitate the development and application of metamaterial absorbers and emitters --Abstract, page iv

Large-area, wide-angle, spectrally selective plasmonic absorber

A simple metamaterial-based wide-angle plasmonic absorber is introduced, fabricated, and experimentally characterized using angle-resolved infrared spectroscopy. The metamaterials are prepared by nano-imprint lithography, an attractive low-cost technology for making large-area samples. The matching of the metamaterial's impedance to that of vacuum is responsible for the observed spectrally selective "perfect" absorption of infrared light. The impedance is theoretically calculated in the single-resonance approximation, and the responsible resonance is identified as a short-range surface plasmon. The spectral position of the absorption peak (which is as high as 95%) is experimentally shown to be controlled by the metamaterial's dimensions. The persistence of "perfect" absorption with variable metamaterial parameters is theoretically explained. The wide-angle nature of the absorber can be utilized for sub-diffraction-scale infrared pixels exhibiting spectrally selective absorption/emissivity.Comment: 7 pages, 6 figures, submitted to Phys. Rev.

Design for optical metamaterial design for optical metamaterial absorber

“Optical metamaterlal (MM) absorbers in the visible or near-infrared range have been widely investigated in these years since they are crucial in many promising applications, such as solar energy harvesting systems, thermo-photovoltaic energy conversion devices, thermal imaging and emissivity control. This dissertation aims to design and investigate various optical metamaterial absorbers based on different mechanisms and theories, such as cavity resonance, impedance match, equivalent circuit model and waveguide stop light mode. First, via utilizing the cavity resonance, a tunable narrowband MM absorber/emitter for thermophotovoltaic (TPV) is designed and analyzed based on gold nanowire cavities to improve the overall efficiency of TPV systems. Second, a broadband absorber made of ultrathin silica-chromium-silica film working in visible and near-infrared (NIR) range is proposed and demonstrated using impedance transformation method. To further broaden the absorption range and enhance the absorption performance, another broadband absorber covering the visible and near-infrared (NIR) is proposed firstly utilizing the combination of the multilayer impedance match in the short wavelength range and the double resonances in the long wavelength range. Finally, an ultra-broadband multilayer waveguide absorber is designed and studied via the stop light trapped at different waveguide width. The stop light mode is analyzed based on the waveguide mode theory considering the guided forward mode and backward mode at the same waveguide width position”--Abstract, page iv

Backaction in metasurface etalons

We consider the response of etalons created by a combination of a conventional mirror and a metasurface, composed of a periodic lattice of metal scatterers with a resonant response. This geometry has been used previously for perfect absorption, in so-called Salisbury screens, and for hybridization of localized plasmons with Fabry-Perot resonances. The particular aspect we address is if one can assume an environment-independent reflectivity for the metasurface when calculating the reflectivity of the composite system, as in a standard Fabry-Perot analysis, or whether the fact that the metasurface interacts with its own mirror image renormalizes its response. Using lattice sum theory, we take into account all possible retarded dipole-dipole interactions of scatterers in the metasurface amongst each other, and through the mirror. We show that while a layer-by-layer Fabry-Perot formalism captures the main qualitative features of metasurface etalons, in fact the mirror modifies both the polarizability and reflectivity of the metasurface in a fashion that is akin to Drexhage's modification of the radiative properties of a single dipole.Comment: 10 pages, 5 figure

Anomalies in Light Scattering

Scattering of electromagnetic waves lies at the heart of most experimental techniques over nearly the entire electromagnetic spectrum, ranging from radio waves to optics and X-rays. Hence, deep insight into the basics of scattering theory and understanding the peculiar features of electromagnetic scattering is necessary for the correct interpretation of experimental data and an understanding of the underlying physics. Recently, a broad spectrum of exceptional scattering phenomena attainable in suitably engineered structures has been predicted and demonstrated. Examples include bound states in the continuum, exceptional points in PT-symmetrical non-Hermitian systems, coherent perfect absorption, virtual perfect absorption, nontrivial lasing, non-radiating sources, and others. In this paper, we establish a unified description of such exotic scattering phenomena and show that the origin of all these effects can be traced back to the properties of poles and zeros of the underlying scattering matrix. We provide insights on how managing these special points in the complex frequency plane provides a powerful approach to tailor unusual scattering regimes

Angular behavior of the absorption limit in thin film silicon solar cells

We investigate the angular behavior of the upper bound of absorption provided by the guided modes in thin film solar cells. We show that the 4n^2 limit can be potentially exceeded in a wide angular and wavelength range using two-dimensional periodic thin film structures. Two models are used to estimate the absorption enhancement; in the first one, we apply the periodicity condition along the thickness of the thin film structure but in the second one, we consider imperfect confinement of the wave to the device. To extract the guided modes, we use an automatized procedure which is established in this work. Through examples, we show that from the optical point of view, thin film structures have a high potential to be improved by changing their shape. Also, we discuss the nature of different optical resonances which can be potentially used to enhance light trapping in the solar cell. We investigate the two different polarization directions for one-dimensional gratings and we show that the transverse magnetic polarization can provide higher values of absorption enhancement. We also propose a way to reduce the angular dependence of the solar cell efficiency by the appropriate choice of periodic pattern. Finally, to get more practical values for the absorption enhancement, we consider the effect of parasitic loss which can significantly reduce the enhancement factor

Mid-infrared chiral metsurface coupling with molecular vibration

“The mid-infrared design of chiral metamaterial shows a tremendous absorption capability of infrared rays, and it has the significant application of circular dichroism based device development. Plasmon-phonon coupling is one of the application of metamaterial that provides a new path for tailoring the surface in the nanoscale, which is also applicable in molecules detection. It is possible to change the Plasmon-Phonon coupling strength not only through the chemical change of molecules but also by changing the metamaterial light-matter interaction property. So far, linearly polarized light shows the strong coupling between metamaterial and molecules. However, it is possible to observe strong coupling in circularly polarized light by fabricating the chiral metasurface. In this research, we introduce two types of new chiral metasurface that has strong interaction with the molecules in different circular polarization of light, which exhibits over 58% and 65% circular dichroism (CD) in the mid-infrared region (5- 6 μm). By adjusting the geometric parameters of the new chiral structure of single-sized unit cells, it is possible to shift the absorption peak in the various midinfrared range. Besides that, we design the broadband resonator by combining the multiple chiral structures. For the molecule’s detection, It also shows a higher splitting gap in the right circularly polarized light in compare to Left circular polarized light. Our numerical and experimental result of the C=0 bands signals which emits from polymethyl methacrylate (PMMA) film using chiral metasurface unveils the effective way for tuning the coupling strength of molecules in circular polarization of light”--Abstract, page iii