Jose Ordonez-Miranda*

Sorbonne Université, CNRS, Institut des Nanosciences de Paris, INSP, F-75005 Paris, France

*Email: jose.ordonez@cnrs.fr

In this plenary talk, I’ll present our recent experimental results obtained for the rectification, amplification, and memory of radiative thermal currents driven by the hysteretic insulator-metal transition (IMT) of VO₂.

In the first part, I’ll experimentally demonstrate the thermal rectification of the radiative heat flux between a VO₂ film and a heat fluxmeter, when their temperature difference is reversed. By testing three VO₂ films deposited on r-sapphire, c-sapphire, and silicon, the highest rectification factor of 61% is obtained for the first film. This value is consistent with the theoretical predictions of an analytical model and shows that the rectification can be enhanced by increasing not only the emissivity variations between the insulating and metallic phases of VO₂ films, but also by decreasing their emissivity in the metallic phase.

In the second part, I’ll present the experimental realization of a radiative thermal transistor capable of amplifying far-field heat currents. This three-terminal device exploits the IMT of VO₂ thin films deposited on both surfaces of a substrate using pulsed laser deposition (PLD). This phase transition induces a sharp variation of the infrared emissivity of the VO₂/substrate/VO₂ system, acting as the transistor base placed between two heat flux sensors playing the roles of an emitter and collector. We consider substrates of r-cut and c-cut sapphire, and Si/SiO₂ to optimize the thermal performance of the developed thermal transistor. By measuring the heat fluxes emitter-base and base-collector, we find that the thermal transistor implemented with a VO₂/r-sapphire base yields the highest amplification factor of 126. This record figure of merit underscores the critical role of the VO₂ substrate selection and demonstrate the potential of radiative thermal transistors for advanced thermal management applications.

In the third part, I’ll present the laboratory operation of a radiative thermal memristor made up of a VO₂ nanofilm exchanging heat by far-field thermal radiation with a heat-flux sensor. This two-terminal device exploits both the IMT and thermal hysteresis of VO₂ grown on a silicon substrate by PLD. This phase transition induces large emissivity variations and a hysteresis width of 5 K around the mean temperature T₀ = 338.7 K. Under a low-frequency sinusoidal modulation of the VO₂ temperature around T₀, we observe a pinched, infinity-shaped Lissajous curve for the heat flux as a function of the temperature difference. From this characteristic curve we extract a history-dependent thermal memristance with clear ON and OFF states defined by the emissivity contrast of VO₂ between its insulating and metallic phases. These memristive states remain robust over multiple cycles, enabling the encoding of binary information in a purely radiative configuration.

Our experimental results thus demonstrate the operation of contactless thermal diodes, transistors and memristors, which represent the fundamental elements of thermotronics, by analogy to electronics.

References:

[1] I. Y. Forero-Sandoval et al., VO₂ substrate effect on the thermal rectification of a far-field radiative diode, Phys. Rev. Applied 14, 034023 (2020).

[2] I. Alonzo-Zapata et al., Vanadium dioxide radiative thermal transistor achieves hundredfold amplification of far-field heat current, Phys. Rev. Applied 24, L031001 (2025).

[3] J. A. Chan Espinoza et al., Experimental development of a radiative thermal memristor, doi: 10.13140/RG.2.2.15183.75682