Nanophotonics 2020; 9(4): 855–863 Research article Yida Liua, Jinlin Songa, Weixian Zhao, Xuecheng Ren, Qiang Cheng, Xiaobing Luo, Nicholas Xuanlai Fang* and Run Hu* Dynamic thermal camouflage via a liquid-crystal- based radiative metasurface https://doi.org/10.1515/nanoph-2019-0485 and a uniform temperature field is observed in the infra- Received November 28, 2019; revised February 7, 2020; accepted red camera instead, demonstrating the very good dynamic February 25, 2020 thermal camouflage functionality. The present MLCM- based radiative metasurface may open avenues for high- Abstract: Thermal camouflage, which is used to con- resolution emissivity engineering to realize novel thermal ceal objects in the infrared vision for confrontation with functionality and develop new applications for thermal infrared detection in civilian or military applications, metamaterials and meta-devices. has garnered increasing attraction and interest recently. Compared with conductive thermal camouflage, that is to Keywords: thermal camouflage; mid-infrared; metasur- tune heat conduction to achieve equivalent temperature face; liquid crystal; nanophotonics; magnetic polariton. fields, radiative thermal camouflage, based on emissivity engineering, is more promising and shows much supe- riority in the pursuit of dynamic camouflage technology when resorting to stimuli-responsive materials. In this 1 Introduction paper, we demonstrate the emissivity-engineered radia- The dynamic structural colors in the skin of chameleons tive metasurface to realize dynamic thermal camouflage and cephalopods enable them to blend into the background functionality via a flying laser heat source on the metal- environment adaptively, thus they are known as the cam- liquid-crystal-metal (MLCM) platform. We employ a rig- ouflage masters in the natural world [1–3]. The active color- orous coupled-wave algorithm to calculate the surface changing feat stems from the chromatophore pigment cells emissivity of Au/LC/Au microstructures, where the LC- and reflective cells which can operate under mechanical orientation angle distribution is quantified by minimizing actuation of radial muscle and function as spectral filters the emitted thermal energy standard deviation through- to absorb and reflect visible light. The sophisticated archi- out the whole plate. Emissivity engineering on the MCLM tecture of the dynamic color-changing system has inspired platform is attributed to multiple magnetic polariton reso- the engineering of various adaptive artificial materials nance, and agrees well with the equivalent electric circuit and devices, like optoelectronic displays, soft robots, and analysis. Through this electrical modulation strategy, the camouflage systems, and their working spectra have been moving hot spot in the original temperature field is erased extended beyond the visible light with many civilian and military applications [1–5]. Among them, thermal cam- aYida Liu and Jinlin Song: These authors contributed equally to this ouflage, with the aim of concealing objects from infrared work. (IR) imaging, has attracted increasing attention, and can *Corresponding authors: Nicholas Xuanlai Fang, Department of be used to incapacitate IR detection [6–12]. Note that any Mechanical Engineering, Massachusetts Institute of Technology, object will radiate thermal energy whenever and wherever, Cambridge, Massachusetts 02139, USA, thus concealing target thermal radiation for thermal cam- e-mail: [email protected]. https://orcid.org/0000-0001-5713-629X; and Run Hu, State Key Laboratory of Coal Combustion, School of ouflage is tremendously challenging. Energy and Power Engineering, Huazhong University of Science and According to the Stefan-Boltzmann law, there are two Technology, Wuhan 430074, China, e-mail: [email protected]. ways to tune the thermal radiation energy for thermal cam- https://orcid.org/0000-0003-0274-9982 ouflage. One way is to change the target temperature as Yida Liu, Jinlin Song, Weixian Zhao, Xuecheng Ren, Qiang Cheng close as possible to approximate the background tempera- and Xiaobing Luo: State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science ture when their surface emissivities are comparable, and and Technology, Wuhan 430074, China. https://orcid.org/0000- the other way is to change the target surface emissivity to 0002-4370-2120 (Q. Cheng) generate the same amount of emitted thermal energy as the Open Access. © 2020 Nicholas Xuanlai Fang, Run Hu et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 Public License. 856 Y. Liu et al.: Dynamic thermal camouflage via a liquid-crystal-based radiative metasurface background. As a result, thermal camouflage can be sorted the nanostructured surfaces. To date, only a very limited into conductive camouflage and radiative camouflage. number of studies report dynamic thermal camouflage due Conductive thermal camouflage tunes the in-plane to the stringent requirement and challenging implementa- heat conduction along a predesigned plate with aniso- tion [1]. Fortunately, dynamic control of thermal radiation tropic thermal conductivities for achieving an equivalent can be achieved by introducing responsive materials that temperature profile as in a homogeneous plate outside can change their thermal radiation under external stimuli the devices. The transformation thermotics theory and the like electrics, magnetics, heat, mechanics, etc. [30–37]. scattering cancellation technique are frequently adopted to These stimuli-responsive materials enable us to achieve design such thermal metamaterials with stringent require- dynamic thermal camouflage to pursue the final goal like ments of the thermal properties, like thermal conductivity camouflage masters in nature. Compared to other external and specific heat [13–24]. For the fabrication of thermal stimuli, electrical modulation is more flexible, fast, and conductive metamaterials, compromise tactics are usually reliable, thus attracting more attention recently, and has employed by trading off the desired performance for the been successively applied onto quantum wells, graphene, fabrication feasibility, e.g. stacking the alternative-layered biomimetic materials for thermal camouflage applications. structures based on the effective medium approximation Among these possibilities, nematic liquid crystals (LCs), as theory [6–8], special design of bilayer structures [9, 22, a family of stimuli-responsive materials, also show great 23], or engineering phonon/defect-phonon scattering in potential for electrically controlling thermal radiation due the microscale material system by defect engineering [24], to their inherently high optical anisotropy, low power con- holey engineering [25], or phonon engineering [26–28]. sumption, easy controllability of LC particle orientation, Moreover, the in-plane heat conduction is easy to control, sub-millisecond response time, and high compatibility with but sometimes hard to use in practice. For instance, though almost all optoelectronic materials [44–48]. LCs can also be perfect thermal cloaking, camouflage, illusion functions integrated into metal-insulator-metal gratings, in which the have been achieved on plates in previous studies, we can insulator layer is composed of LCs, to construct a dynamic easily observe the targets from the out-of-plane direction platform that powerfully combines the physics flexibility of (z plane). Trying to fix this, a general illusion thermotics a metal-insulator-metal framework and precise tunability strategy has been proposed to realize conductive camou- of LCs, with applications like waveguides, filters, and reso- flage both in the x-y plane and the z plane, by maintain- nators [49, 50]. Nevertheless, such a dynamic platform has ing perfect external camouflage and creating internal never been explored for thermal camouflage feasibility. split illusions [6, 7]. Based on general illusion thermotics, In this paper, we propose a general strategy to dynam- the concept of encrypted thermal printing was proposed ically tune the thermal emission from a metal-liquid- recently and shows the possibility to achieve a dynamic crystal-metal (MLCM) radiative metasurface for dynamic thermal illusion [8]. Conductive camouflage devices are thermal camouflage. The rigorous coupled-wave algorithm usually excessively large to warp the target, which further (RCWA) method is employed to calculate the surface emis- limits their applications. Another key defect of most con- sivity with varying the director-axis orientation angle of ductive camouflage is the static characteristic that once LCs. The MCLM unit cells can be controlled independently the structure and materials are designed and manufac- to achieve different amounts of thermal radiation energy, tured, the pursued thermal camouflage performances no and the orientation angles and emissivities of the whole longer function or are maintained when the target moves metasurface are screened by achieving the same amount or the ambient environment changes. of thermal radiation energy for the same detected temper- In contrast, radiative camouflage is more promising ature, though at different real temperatures. A more vivid for practical purposes since the radiative properties mainly demonstration is presented on the MCLM metasurface depend on surface characteristics with broad applicabil- when a flying laser heat source is moving on the bottom ity, whether the surfaces are flattened or