We propose an analytical framework to design actively tunable narrowband thermal emitters at infrared frequencies. We exemplify the proposed design rules using phase-change materials (PCMs), considering dielectric-to-dielectric PCMs (e.g., ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Se}}_4{\mathrm{Te}}_1$) and dielectric-to-metal PCMs (e.g., ${\mathrm{VO}}_2$). Based on these, we numerically illustrate near-unity on-off switching and arbitrarily large spectral shifting between two emission wavelengths, respectively. The proposed systems are lithography-free and consist of one or several thin emitter layers, a spacer layer, which includes the PCM, and a back reflector. Our model applies to normal incidence, though we show that the behavior is essentially angle independent. The presented formalism is general and can be extended to any mechanism that modifies the optical properties of a material, such as electrostatic gating or thermo-optical modulation.

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• Nanophotonics
• Optoelectronics
• Phase-change materials
• Phonon polariton
• Thermal photovoltaics
• Multilayer thin films
• Optical sources & detectors
• Infrared techniques