Tunable Narrowband Thermal Emitters using Phase-Change Materials: Design Rules for Simple Devices

The ability to control the spectrum, the direction, and the polarization of thermal emission is critical for numerous applications, including infrared (IR) sources, thermal camouflage, radiative cooling, molecular sensing, and energy conversion. For many of these applications, narrowband IR emission is desirable. However, simple, lithography-free structures tend to have spectrally broadband emissivity. It is also valuable to achieve active control of thermal emission, enabling switching between <i>on</i> and <i>off </i>emission states or shifting the peak emission wavelength. In the mid-IR region, the spectral range of interest for thermal emission near room temperature, a common approach for active tuning has been phase-change materials (PCMs). Tunable narrowband sources have previously been achieved, albeit with relatively complex structures and a performance that is not ideal over the whole operation range. Furthermore, these devices have limited spectral tunability as they rely on the temperature dependence of the materials' resonances.<br/>This work presents an analytical framework to design actively tunable narrowband thermal emitters at IR frequencies [1]. The proposed systems are lithography-free and consist of one or several thin emitter layers, a spacer layer that includes the PCM, and a back-side reflector. Numerically, we show near-unity <i>on</i>-<i>off </i>switching and arbitrarily large spectral shifting between two emission wavelengths using actual materials. Although our theoretical formalism assumes normal incidence, the performance remains near-ideal up to large angles. The presented framework is general and applies to <i>any </i>mechanism that modifies the optical properties of a material, such as electrical gating or optical modulation. Finally, we are currently working on an experimental demonstration using In3SbTe2, a non-volatile PCM that can be reversibly switched between a dielectric amorphous and a metallic crystalline phase in the entire infrared spectral range [2].<br/>References:<br/>Giteau, M., Sarkar, M., Ayala, M. P., Enders, M. T. & Papadakis, G. T., Design Rules for Active Control of Narrowband Thermal Emission Using Phase-Change Materials, <i>Phys. Rev. Appl.</i> <b>19,</b> L051002 (2023).<br/>Heßler, A., Wahl, S., Leuteritz, T., Antonopoulos, A., Stergianou, C., Schön, C.-F., Naumann, L., Eicker, N., Lewin, M., Maß, T. W. W., Wuttig, M., Linden, S. & Taubner, T., In3SbTe2 as a programmable nanophotonics material platform for the infrared, <i>Nat Commun</i> <b>12,</b> 924 (2021).