Design and analysis of metamaterial based perfect absorbers
Soydan, Mahmut Can
Ertürk, Vakur Behçet
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Subwavelength light absorbers have an enormous potential on applications such as photodetection, optoelectronics, solar cells and sensing. Scaling down the device dimensions provides artificial and advanced properties. That's why achieving higher performance devices with smaller sizes is the main trend in semiconductor technology. Design of an electromagnetic wave absorber has two dominant factors on the performance and spectral operation region: material selection and design configuration. Perfect light absorbers require an absorbing layer, such as a metal, semiconductor or any type of absorbing material, to achieve light confinement. While conventional metals have been mostly the primary choice in designs, there are various material types other than them which can have advantageous thermal properties in fabrication, integration or tunability besides having lossy nature. Although conventional metals are great absorbing materials due to lossy natures, they are not durable against erosion and oxidation. In the first work, we scrutinize unprecedented potential of transition metal carbides (TMCs) and nitrides (TMNs) as optional materials to conventional metals, for realization of light perfect absorption in an ultra-broad frequency range encompassing all of the visible (Vis) and near infrared (NIR) regions. To gain insight on the condition for light perfect absorption, a systematic modeling approach based on transfer matrix method (TMM) is firstly utilized. Our modeling findings prove that the permittivity data of these TMCs and TMNs are closely matched with the ideal data. Thus, they can have stronger and broader absorption behavior compared to metals. Besides, these ceramic materials are preferred to metals due to the fact that they have better thermal properties and higher durability against erosion and oxidation than metals. This could provide the opportunity for design of highly e cient light harvesting systems with long-term stability. Two different configurations which are planar and trapezoidal arrays are employed. Numerical simulations are conducted to optimize the device optical performance for each of the proposed carbides and nitrides. Our findings reveal that these ceramic coatings have the broadest absorption response compared to all lossy and plasmonic metals. In planar configuration, titanium carbide (TiC) has the largest absorption bandwidth (BW) where an absorption above 0.9 is retained over a broad wavelength range of 405 nm-1495 nm. In trapezoid architecture, vanadium nitride (VN) shows the widest BW covering a range from 300 nm to 2500 nm. The results of this study can serve as a beacon for the design of future high performance energy conversion devices including solar vapor generation and thermal photovoltaics where both optical and thermal requirements can be satisfied. Majority of existing designs necessitate a lithography-step during the fabrication, which hinders the repeatability, upscaling and large-scale compatibility of these designs. In the second work, we designed, fabricated and characterized a lithography free, double functional single Bismuth (Bi) metal nanostructure for ultra-broadband absorption in the visible and near-infrared, and narrowband response with ultra-high refractive-index sensitivity in mid-infrared (MIR) range. The superior permittivity data of Bi over conventional metals is comprehensively analyzed and explained using systematic modeling approaches based on TMM and Bruggeman's effective medium theory (EMT). To achieve a large scale fabrication of the design in a lithography-free route, oblique-angle deposition approach is used to obtain densely packed and randomly spaced/oriented Bi nanostructures. It has been shown that this fabrication technique can provide a bottom-up approach to control the length and spacing of the design. Our characterization findings reveal a broadband absorption above 0.8 in Vis and NIR, and a narrowband absorption centered around 6.54 m. Due to densely packed architecture of the Bi nanostructures and its extraordinary permittivity response, they can provide strong field confinement in their ultra-small gaps and this could be utilized for sensing application. An ultrahigh sensitivity of 2.151 m/refractive-index-unit (RIU) is acquired for this Bi nanostructured absorber, which is, to the best of our knowledge, the experimentally attained highest sensitivity so far. The simple and large scale compatible fabrication route of the design together with extraordinary optical response of Bi coating, makes this design promising for many optoelectronic and sensing applications.