Browsing by Subject "Thermal emission"

Now showing 1 - 4 of 4

Open Access
Exceptional adaptable MWIR thermal emission for ordinary objects covered with thin VO2 film
(Elsevier Ltd, 2021-01-25) Durna, Yılmaz; Kocer, Hasan; Aydın, Koray; Cakir, Mehmet Cihan; Soydan, Mahmut Can; Odabasi, Oguz; Işık, Halil; Ozbay, Ekmel
Monotonous thermal radiation emitted from an ordinary object can be brought into a dynamic and versatile form that can be shaped according to the application area with the ingenious design of the surface coatings. Building the coatings with phase change materials provides exceptional and surprising properties in terms of tunability, adaptability and multifunctionality. In this paper, we investigate the thermal radiation properties in the MWIR band through comprehensive thermographic measurements and theoretical methods while a thin (similar to 90 nm thick) vanadium dioxide (VO2) layer on the sapphire substrate (VO2 thin film) is placed on different ordinary objects under heating/cooling conditions. It is indicated that the emission of the metal object (low emittance) can be boosted and the emission of the blackbody-like object (high emittance) can be suppressed at the relevant temperatures. The thermal emission of the objects covered with thin VO2 film at high temperatures (>75 degrees C) is determined by only the VO2 thin film, since the VO2 layer is completely metallized and the MWIR radiation of the underlying object is masked. When the actual temperature of the object behaving like a blackbody rises up to 95 degrees C, the temperature detected in the MWIR thermal camera is reduced by more than 20% to approx. 75 degrees C due to the VO2 thin film on this object, providing thermal camouflage. It is experimentally and theoretically revealed that the underlying physical mechanism on these strange results is associated with the drastic change in the infrared optical parameters of the VO2 as a result of the applied temperature. (C) 2020 Elsevier Ltd. All rights reserved.
Open Access
Graphene-Based Adaptive Thermal Camouflage
(American Chemical Society, 2018) Salihoğlu, Ömer; Uzlu, H. B.; Yakar, Ozan; Aas, Shahnaz; Balci, Osman; Kakenov, Nurbek; Balci, S.; Olcum, S.; Süzer, Şefik; Kocabas, Coşkun
In nature, adaptive coloration has been effectively utilized for concealment and signaling. Various biological mechanisms have evolved to tune the reflectivity for visible and ultraviolet light. These examples inspire many artificial systems for mimicking adaptive coloration to match the visual appearance to their surroundings. Thermal camouflage, however, has been an outstanding challenge which requires an ability to control the emitted thermal radiation from the surface. Here we report a new class of active thermal surfaces capable of efficient real-time electrical-control of thermal emission over the full infrared (IR) spectrum without changing the temperature of the surface. Our approach relies on electro-modulation of IR absorptivity and emissivity of multilayer graphene via reversible intercalation of nonvolatile ionic liquids. The demonstrated devices are light (30 g/m2), thin (<50 μm), and ultraflexible, which can conformably coat their environment. In addition, by combining active thermal surfaces with a feedback mechanism, we demonstrate realization of an adaptive thermal camouflage system which can reconfigure its thermal appearance and blend itself with the varying thermal background in a few seconds. Furthermore, we show that these devices can disguise hot objects as cold and cold ones as hot in a thermal imaging system. We anticipate that, the electrical control of thermal radiation would impact on a variety of new technologies ranging from adaptive IR optics to heat management for outer space applications.
Open Access
Invisible thin-film patterns with strong infrared emission as an optical security feature
(Wiley-VCH Verlag, 2018) Bakan, G.; Ayas S.; Serhatlioglu, M.; Elbuken, Çağlar; Dana, A.
Spectrally selective thermal emission is in high demand for thermophotovoltaics, radiative cooling, and infrared sensing applications. Spectral control of the emissivity is historically achieved by choosing the material with suitable infrared properties. The recent advancements in nanofabrication techniques that lead to enhanced light-matter interactions enable optical properties like infrared emissivity that are not naturally available. In this study, thermal emitters based on nanometer-thick dielectrics on field-enhancement surfaces as optical security features are proposed. Such a function is achieved by generating patterns by ultrathin dielectrics that are transparent in the visible and exhibit strong infrared absorption in the spectral range of thermal cameras. The invisible patterns are then revealed by thermal imaging. The field-enhancement surfaces enhance the emissivity of the patterns, in turn reduce the minimum temperature to detect the thermal emission down to ≈30 °C from >150 °C to exploit ubiquitous heat sources like the human body. The study provides a framework for the use of thermal emitters as optical security features and demonstrates applications on rigid and flexible substrates.
Open Access
Nearly perfect resonant absorption and coherent thermal emission by hBN-based photonic crystals
(Optical Society of America, 2017) Hajian, H.; Ghobadi, A.; Butun, B.; Özbay, Ekmel
In this paper, we numerically demonstrate mid-IR nearly perfect resonant absorption and coherent thermal emission for both polarizations and wide angular region using multilayer designs of unpatterned films of hexagonal boron nitride (hBN). In these optimized structures, the films of hBN are transferred onto a Ge spacer layer on top of a one-dimensional photonic crystal (1D PC) composed of alternating layers of KBr and Ge. According to the perfect agreements between our analytical and numerical results, we discover that the mentioned optical characteristic of the hBN-based 1D PCs is due to a strong coupling between localized photonic modes supported by the PC and the phononic modes of hBN films. These coupled modes are referred as Tamm phonons. Moreover, our findings prove that the resonant absorptions can be red- or blue-shifted by changing the thickness of hBN and the spacer layer. The obtained results in this paper are beneficial for designing coherent thermal sources, light absorbers, and sensors operating within 6.2 μm to 7.3 μm in a wide angular range and both polarizations. The planar and lithography free nature of this multilayer design is a prominent factor that makes it a large scale compatible design. © 2017 Optical Society of America.