Nanoscale radiative heat transfer

Near-field heat engines are devices that convert the evanescent thermal field supported by a primary source into usable mechanical energy. By considering the balance of both energy and entropy fluxes, as described in [1] for two bodies and extended to three-body systems in [2], the thermodynamic performance of these heat engines can be analyzed for applications such as energy harvesting. The work flux that can be obtained from the thermal radiation is considerably higher in the near-field regime as compared with the more common blackbody case [1]. An upper bound for the extracted work or thermodynamic availability can be obtained by disregarding the entropy production in the conversion process, thus giving the maximum useful work which is a measure of the device performance. Furthermore, theoretical limits for energy and entropy fluxes in three-body systems were discussed in [2] and compared with their corresponding two-body counterparts. Such considerations confirm that the thermodynamic availability in energy-conversion processes driven by three-body photon tunneling can exceed the thermodynamic availability in two-body systems.

By increasing the number of involved objects, the physics of the system typically becomes richer due to additional degrees of freedom arising from many-body interactions. As a generalization of the three-body case, a general theory of photon-mediated energy and momentum transfer in N-body planar systems out of thermal equilibrium was introduced in [3]. The framework is based on the combination of the scattering theory and the fluctuational-electrodynamics approach in many-body systems, describing both radiative heat transfer and nonequilibrium Casimir-Lifshitz forces. By making a Landauer-like formulation of the heat transfer problem, explicit formulas for the energy transmission coefficients of the system are derived and expressed in terms of the reflection and transmission coefficients of the single bodies. An analogous formulation is introduced to quantify momentum transfer coefficients describing Casimir-Lifshitz forces out of thermal equilibrium. The framework developed in [3], for instance, allow us to study temperature distributions in many-body structures (see the figure).

The near-field radiative heat transfer between objects can also be controlled by the action of external fields. An interesting example is what happens with magneto-optical materials like InSb, where dipolar resonances can show a significant dependence on an applied magnetic field. Hence, the heat flux emitted by a magneto-optical particle can be dramatically altered by tuning this applied field. On account of this fact, a new thermomagnetic effect was predicted in [4]: a magnetoresistance for the heat flux carried by thermal photons. As shown in the accompanying figure, thermal resistance variations of about 50% along chains of InSb-Ag nanoparticles were anticipated for fields of a magnitude of about 500 mT. Moreover, for these chains at room temperature, the resistance can be increased by almost a factor of 2 with magnetic fields of 2 teslas. As discussed in [4], this thermal magnetoresistance results from a strong spectral shift of localized surface waves supported by the particles under the action of the magnetic field. This effect is promising for practical applications, especially in the field of thermal management at the nanoscale as well as for magnetic sensing with temperature or heat flux measurements.

In reference [5], we explored the temporal evolution of radiative heat transfer between two bodies across a separation gap by modulating one of the intensive quantities which are responsible for heat exchanges. By studying the heat transfer between two dielectrics with an oscillating temperature difference and between a semiconductor and a dielectric with an oscillating photon chemical potential difference, we proved the existence of a radiative heat shuttling, a supplementary flux superimposed to the one produced by the mean gradient. This effect is intrinsically related to the nonlinearity of radiative exchanges. When the optical properties of the media do not change during the time evolution of either the temperature or the chemical potential, the shuttling amplifies the transfer between the two bodies. On the contrary, we demonstrated the possibility of thermal insulating when the radiative channels display a negative differential thermal resistance. These results pave the way for a novel strategy for an active management of radiative heat exchanges in nonequilibrium systems.

In the close vicinity of a hot solid, at distances smaller than the thermal wavelength, a strong electromagnetic energy density exists because of the presence of the evanescent field. In [6] we introduced a many-body conversion principle to harvest this energy using graphene-based pyroelectric conversion devices. The converter consists of an active layer encapsulated between two graphene field-effect transistors which are deposited on the source and on the cold sink. By tuning the bias voltage applied to the gates of these transistors, the thermal state and the spontaneous polarization of the active layer can be controlled at kHz frequencies. We theoretically demonstrated that the power density generated by these conversion systems notoriously surpass the current production capacity of near-field thermophotovoltaic conversion devices with low grade heat sources and relatively small temperature differences. In addition, the power required to modulate the temperature of the active layer is much smaller than the delivered power, opening so a new avenue for high-frequency pyroelectric energy harvesting from stationary thermal sources. See more details in [6].