Radiative heat transfer at nanoscale: experimental trends and challenges (2110.09781v1)

Abstract: Energy transport theories are being revisited at the nanoscale, as macroscopic laws known since a century are broken at dimensions smaller than those associated with energy carriers. For thermal radiation, where the typical dimension is provided by Wien's wavelength, Planck's law and associated concepts describing surface-to-surface radiative transfer have to be replaced by a full electromagnetic framework capturing near-field radiative heat transfer (photon tunnelling between close bodies), interference effects and sub-wavelength thermal emission (emitting body of small size). It is only during the last decade that nanotechnology has allowed for many experimental verifications-with a recent boom-of the large increase of radiative heat transfer at nanoscale. In this minireview, we highlight the parameter space that has been investigated until now, showing that it is limited in terms of inter-body distance, temperature and object size, and provide clues about possible thermal-energy harvesting, sensing and management applications. We also provide an outlook on open topics, underlining some difficulties in applying single-wavelength approaches to broadband thermal emitters while acknowledging the promises of thermal nanophotonics and observing that molecular/chemical viewpoints have been hardly addressed. Context: revisiting thermal radiative energy transport and conversion at the nanoscale

• The paper demonstrates significant near-field radiative heat transfer enhancements, achieving conductance increases by up to two orders of magnitude compared to far-field cases.
• It employs detailed experimental setups, including plane-plane, microsphere-plane, and nanotip-plane configurations to analyze photon tunneling and sub-wavelength emission phenomena.
• The study highlights potential applications in thermal-energy harvesting and near-field thermophotovoltaic conversion while addressing complex theoretical and technical challenges.

Radiative Heat Transfer at the Nanoscale: Experimental Progress and Challenges

The minireview "Radiative Heat Transfer at Nanoscale: Experimental Trends and Challenges" authored by Lucchesi et al. provides a detailed examination of the evolving field of nanoscale thermal radiation, emphasizing the experimental parameters explored thus far and identifying opportunities for future exploration. The paper underscores the inadequacy of classical macroscopic thermodynamic laws like Planck's law when applied at nanoscale dimensions, necessitating a reconceptualization using a comprehensive electromagnetic framework. This scrutiny addresses nuances such as photon tunneling, interference effects, and sub-wavelength emission phenomena.

Key Experimental Trends

The authors delineate the landscape of experimental endeavors, illustrating that the parameter space explored extends across inter-body distances, temperature differentials, and object sizes. Nevertheless, the experimental investigations remain confined regarding the extent of these parameters. The paper identifies three predominant geometries studied: plane-plane, microsphere-plane, and nanotip-plane configurations. Each setup reveals distinct thermal exchange characteristics. Recent experiments have begun testing larger temperature differentials, though systematic exploration remains limited.

A salient observation in the paper is the strong dependency of the power exchanged on object size, with experiments prominently classified by temperature, power density, and physical dimensions. Current demonstrations highlight significant near-field radiative heat transfer enhancements, achieving conductance increases by up to two orders of magnitude relative to far-field counterparts. Notably, these observations are contingent upon overcoming practical challenges associated with creating and maintaining nanoscale gaps necessary for near-field interactions.

Implications and Prospective Applications

Nanoscale thermal radiation holds substantial promise for various applications, including thermal-energy harvesting, sensing, and management. The paper identifies clear implications for technologies such as near-field thermophotovoltaic (TPV) conversion, where increased infrared radiation transfer facilitates advanced energy harvesting mechanisms. Similarly, thermal management applications, particularly in densely packaged electronic circuits, could benefit from targeted heat dissipation strategies in the near field.

Additionally, the work underscores potential advancements in spectroscopic applications, which historically necessitate bulky Fourier-transform infrared spectroscopes. With finer architectural nanostructuring, smaller-scale spectroscopic devices might become feasible. The ongoing surge in Internet of Things (IoT) technologies also poses an opportunity for integrating these nanoscale energy converters, driven by their capability to operate autonomously with minimal energy inputs.

Theoretical and Technical Challenges

The pathway to realizing the full potential of nanoscale radiative heat transfer technologies is fraught with theoretical and technical challenges. Fundamentally, the necessity to tailor radiative spectra while ensuring substantial power transfer is complex, given the constraints imposed by thermal photon Bose-Einstein distributions. Approaching practical barriers—such as achieving larger temperature differences with minimal energy losses, along with refined spatial and spectral resolution—demands rigorous engineering solutions.

The authors suggest embracing novel concepts from nanophotonics, such as metamaterials and 2D-materials, for elevating heat transfer efficiency and spectrum control. Prospective studies utilizing nanoparticles and single-atom thin films may unravel new possibilities, although initial applications have been impeded by power transfer limitations at such scales.

Conclusion

The minireview elucidates the experimental milestones achieved in the field of nanoscale radiative heat transfer and ventures into upcoming experimental and theoretical frontiers promising technological advancements. Despite inherent challenges, the fusion of energy manipulation techniques from nanophotonics and conventional thermodynamics paves the way for innovative solutions in energy management and conversion. With continued exploration, nanoscale thermal radiation research is poised to significantly influence numerous sectors, including energy, electronics, and material science.