Quantifying Mid-IR Radiation from a Single Nano-Object—The Next Generation Thermal Engineering Using Phonon Polaritons

Rigorous thermal control at nanoscales has become essential with the miniaturization of electronic devices. Thermal management in nano-devices has been challenged by the diffusive nature of heat conduction by classical heat carriers, e.g., phonons and electrons, due to their short mean-free-path. Furthermore, low-dimensional systems enlarge the boundary scattering of the heat carriers, which suppresses the thermal conductivity of specimens. It leads to substantial challenges in thermal management by making heat conduction diffusive owing to the incoherence of heat carriers. If one could engineer the transport of thermal energy, arguably the most ubiquitous form of energy, with similar degree of controllability as electrical and optical energy, a variety of energy transport and conversion technologies can be improved.<br/><br/>In this talk, I will introduce a thermo-photonic engineering approach to manipulate nanoscale heat transport by using surface phonon polaritons (SPhP), coupled energy carriers of optical phonons and mid-IR photons within the Reststrahlen band of polar dielectrics (e.g. SiO₂). In a recent decade, nonlinear heat transfer mediated by SPhPs has brought increased attention in various forms of heat transfer, including near- and far-field radiation and conduction. I will firstly discuss how the SPhP can be utilized to tailor thermal radiation properties, especially to achieve a coherent, near-monochromatic far-field thermal emission, which is a big departure from the incandescent behaviour in the classic textbook as described by the Planck’s law. The key feature of the design is to utilize nanoscale emitters whose dimension is comparable to or smaller than the thermal wavelength, a regime when the Planckian energy distribution no longer holds (as Planck himself originally noted). I will show my experimental and theoretical work to quantify the far-field thermal radiation from these rationally-designed single nano-emitters. Anisotropic SiO₂ nanoribbons were designed to enable independent control of the incoherent and coherent behaviors by exploiting the large disparity in the skin depth and wavelength of surface phonon polaritons. A thermometry platform was devised to extract the thermal emissivity from such polar dielectric nanoemitters with nanowatt-level emitting power.<br/><br/>Furthermore, new approaches of exploring polaritonic heat conduction will be introduced. Theoretical predictions suggest that these distinctive heat carriers exhibit anomalously long propagation lengths over 100s μm. The theoretical prediction of the thermal conductivity has been achieved by assuming the infinite length of samples in which Rossland approximation is valid up to ~250 W/m-K of thermal conductivity in a 50 nm thick SiO₂ film due to the long propagation lengths. While the past theoretical models for heat conduction treated polaritons as phonons described by the kinetic theory with an abnormally long propagation length, the existing understandings failed to account for the radiative behaviours of SPhPs which involve evanescent waves confined along the surface, at the same time, vastly dispersed in empty surroundings. The exploit of polaritonic heat conduction and radiation shares the same origin of coupled energy of mid-IR photons and optical phonons within the Reststrahlen band in polar dielectric surfaces. However, the implications of SPhPs have been studied separately and limited to specific heat transfer mechanisms, leading to a discontinuous understanding of polaritonic heat transfer along the heat channel that converts radiation to conduction. I will introduce our experimental observations of quasi-ballistic heat conduction by SPhPs to exploit a new mode of heat conduction which is dispersive in radiative nature, yet directional guided along the surface.