Harnessing and controlling thermal radiation is key in various applications ranging from renewable energy, molecular sensing, to thermal circuitry and camouflage. In this talk, I will discuss how emerging low-dimensional, polar, and phase-change materials can be used to control the flow of heat via radiation. I will present recent results on directional as well as nonreciprocal thermal emission without the need of any lithographic process. I will further discuss a theoretical platform for inducing a strong degree of tunability in such thermal emitters, using phase-change materials. Finally, I will introduce the notion of the thermal near field, where the blackbody limit in the amount of heat exchanged between macroscopic bodies does not hold. I will demonstrate a fully analytical framework for its description, as well as its applications in renewable energy.

Thermal radiation is emitted by all objects that have a non-zero temperature. Near room-temperature, bodies emit at mid-infrared (IR) frequencies. The photonics community has recently focused significant on this spectral range. “Thermal photonics” is an emerging sub-field to nanophotonics aiming to harness and control thermal radiation via nanophotonic principles and nanostructures. In this talk, I will introduce the properties of relevant materials to the mid-IR, in particular several low-dimensional materials such as hexagonal boron nitride (hBN) and molybdenum trioxide (MoO3). I will also discuss the attractive properties of vanadium dioxide (VO2) as a platform for tunable thermal photonics. I will demonstrate how the strong polar resonances of materials such as hBN and a-MoO3 are favorable for gaining significant directional control over the azimuthal and zenith angle of thermal emission [1]. I will also demonstrate that the strong in-plane anisotropy of a-MoO3 can be leveraged for controlling the polarization of mid-IR light and present experimental results on a deeply subwavelength half-wave plate that is orders of magnitude thinner than standard polarizers. Furthermore, I will demonstrate that, using a lithography-free platform, one can even break Kirchhoff’s law of thermal emission with magneto-optical materials, leading to nonreciprocal devices that are highly relevant for energy applications.All aforementioned concepts can benefit by being actively tunable. I will briefly outline the landscape of tunable mid-IR photonics [2], and I will introduce a simple and fully analytical formalism for designing tunable thermal emitters, based on crystallographic phase-changes via temperature control [3]. 

Finally, I will introduce the thermal near field, where two objects exchange an amount of radiative heat that can exceed the blackbody limit, due to evanescent excitations that are excellent carriers of thermal radiation. Although near-field heat transfer has been described with fluctuational electrodynamics since the ‘70’s, the analytical description of the effect has remained a challenge. I will present a fully analytical approach that describes near-field heat transfer, based on which we have derived strict upper bounds to the effect [4], and will briefly outline applications where near-field radiative heat transfer becomes highly relevant [5, 6].

[1] Lithography-free directional control of thermal emission, M. Sarkar, M. Giteau, M. Enders, G. T. Papadakis, arXiv preprint arXiv:2210.01026 (2022)
[2] Dynamic modulation of thermal emission – a Tutorial, M. F. Picardi, K. N. Nimje, G. T. Papadakis, arXiv preprint arXiv:2219.01587 (2022)
[3] Active control of narroband thermal emission with phase-change materials, M. Giteau, M. Sarkar, M. P. Ayala, M. T. Enders, G. T. Papadakis, arXiv preprint arXiv:2210.02155 (2022)
[4] The role of optical loss and tight bounds in polariton-mediated near-field heat transfer, M. Pascale, G. T. Papadakis, arXiv preprint arXiv:2207.01386, accepted in Phys. Rev. Materials (2022)
[5] Thermodynamics of light management in near-field thermophotovoltaics, G. T. Papadakis, M. Orenstein, E. Yablonovitch, S. Fan, Phys. Rev. Applied 16 (6) 064063 (2021)
[6] Perspective on near-field radiative heat transfer, M. Pascale, M. Giteau, G. T. Papadakis, arXiv preprint arXiv:2210.00929, accepted in Applied Physics Letters (2022)

Funding
We acknowledge funding from “la Caixa” Foundation (ID 100010434), from the PID2021-125441OA-I00 project funded by MCIN /AEI /10.13039/501100011033/ FEDER, UE, and from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No 847648. The fellowship code is LCF/BQ/PI21/11830019. This work is part of the R&D project CEX2019-000910-S, funded by MCIN/ AEI/10.13039/501100011033/ , from Fundacio Cellex, Fundacio Mir-Puig, and from Generalitat de Catalunya through the CERCA program.