Browsing by Subject "Broadband absorption"
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Item Open Access Optically transparent metamaterial RF absorbers(Bilkent University, 2023-05) Şahin, FurkanRecent advances in metamaterials have allowed to impart unique properties to flat RF absorbers including broadband absorption, low thickness (in terms of the longest operating wavelength) and polarization insensitiveness, all essential to high-performance absorbers. For these RF absorbers, introducing additional properties of high optical transparency (in the visible range) and mechanical ro-bustness opens up also stealth window applications. However, achieving all of these critical characteristics in a single design is a challenging task. In this thesis, to address this challenge, we propose and demonstrate an optically transpar-ent, broadband, and polarization-insensitive RF absorbing metamaterial that is extremely thin (thickness = 0.079λL; λL: longest operating wavelength). Our design consists of a single dielectric layer of polymethyl methacrylate (PMMA) sandwiched between the top and bottom indium tin oxide (ITO) films, altogether providing high optical transmission. The bottom ITO film acts as a ground plane, which reduces the RF transmission significantly. On the other hand, the top ITO film adorns a unique pattern that minimizes the RF reflection across a particular frequency range. Here we obtained these customized ITO patterns using a novel design methodology. We developed the fabrication process specific to the pro-posed RF structure and fabricated their prototypes. To validate numerical simu-lation results, we measured experimentally the RF absorption of these fabricated prototypes. The experimental results show that the proof-of-concept absorbers achieve over 90% absorption between 4.4-11.2 GHz and over 95% absorption between 4.8-10.6 GHz. Furthermore, we found the fabricated absorbers to be in-sensitive to polarization angles and preserve 90% absorption for oblique incidence angles of 60° for TM and 40° for TE polarizations in agreement with the numer-ical predictions. Also, besides RF characterizations, we optically recorded the transmittance in the visible range to be 65% on average for the tested absorbers. These findings indicate that the proposed single-dielectric-layered architecture of optically transparent, broadband, polarization-insensitive RF absorbers, featur-ing a record relative thickness of 0.079λL, holds great promise for use in stealth window applications.Item Open Access Plasmonic absorbers for multispectral and broadband absorption(SPIE, 2012) Ayaş, Sencer; Güner, Hasan; Türker, Burak; Ekiz, Öner; Dana, AykutluWe present polarization dependent multispectral and broadband plasmonic absorbers in the visible spectrum. The spectral characteristics of these structures are tunable over a broad spectrum. Experimental results are verified with the FDTD and RCWA analysis methods. These structures are used as surface enhanced raman spectroscopy(SERS) substrates. Designed structures have resonances that span the Raman Stokes and excitation wavelength. Such structures can be used for energy, LED and other spectroscopy applications. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).Item Open Access Strong light-matter interaction in lithography-free planar metamaterial perfect absorbers(American Chemical Society, 2018) Ghobadi, Amir; Hajian, Hodjat; Bütün, Bayram; Özbay, EkmelThe efficient harvesting of electromagnetic (EM) waves by subwavelength nanostructures can result in perfect light absorption in the narrow or broad frequency range. These metamaterial-based perfect light absorbers are of particular interest in many applications, including thermal photovoltaics, photovoltaics, sensing, filtering, and photodetection applications. Although advances in nanofabrication have provided the opportunity to observe strong light-matter interaction in various optical nanostructures, the repeatability and upscaling of these nano units have remained a challenge for their use in large scale applications. Thus, in recent years, the concept of lithography-free planar light perfect absorbers has attracted much attention in different parts of the EM spectrum, owing to their ease of fabrication and high functionality. This Perspective explores the material and architecture requirements for the realization of light perfect absorption using these multilayer metamaterial designs from ultraviolet (UV) to far-infrared (FIR) wavelength regimes. We provide a general theoretical formulation to find the ideal condition for achieving near unity light absorption. Later, these theoretical estimations are coupled with findings of recent studies on perfect light absorbers to explore the physical phenomena and the limits of different materials and design architectures. These studies are categorized in three main class of materials; metals, semiconductors, and other types of materials. We show that, by the use of proper material and design configuration, it is possible to realize these lithography-free light perfect absorbers in every portion of the EM spectrum. This, in turn, opens up the opportunity of the practical application of these perfect absorbers in large scale dimensions. In the last section, we discuss the progress, challenges, and outlook of this field to outline its future direction.