Manipulation of Near-Field Radiative Heat Flow via Mid-Infrared Plasmonic Coupling

With the continuous reduction in the size of electronic devices to the nanoscale, it becomes imperative to gain a deep understanding of heat transport mechanisms at these levels. In recent years, breakthroughs in nanotechnology and radiative engineering have introduced thermal radiation as a promising mechanism to tailor both the magnitude and direction of heat transfer. The use of metamaterials enhances our capability to manipulate radiative heat flow in nanoscale systems, particularly in the context of near-field radiation, which can significantly exceed the blackbody limit by multiple orders of magnitude. Heat transport in the near field of metamaterials depends on their geometry, optical properties of the constituent materials, and interfacial properties. By fine-tuning interfaces in or underneath a metamaterial we can manipulate heat flux, and facilitate an augmented flow of radiative heat from specific spectral modes into bulk heat carriers. One proposed way to tune this flow is by capitalizing on the hybridization of resonant modes in plasmonic materials. Highly doped cadmium oxide (CdO) is poised as an exceptional candidate for that purpose as it allows for high electron mobilities while maintaining dielectric properties mechanically. Also, sub-wavelength films of CdO support epsilon-near-zero (ENZ) modes in the mid-infrared range of the spectrum. Most importantly It has been shown that carefully constructed multilayers of CdO with varying carrier densities exhibit tunable hyperbolic plasmonic dispersion properties, allowing for carrying heat across interfaces at a significantly higher rate than conduction.<br/>In this research, we conduct experimental investigations into the interactions between hyperbolic plasmon-polaritons (HPPs) within a layered hyperbolic metamaterial (HMM) and ENZ modes within a doped CdO thin film. Our approach involves employing a unique ultrafast pump-probe technique that enables the precise detection of heat transfer modes with sub-picosecond temporal resolution. This technique relies on a wavelength-tunable mid-infrared probe pulse, allowing us to directly interface with plasmonic heat carriers, thereby providing a direct assessment of polariton dynamics within nanoscale material systems. By utilizing this method, we investigate the ultrafast thermally-induced near-field coupling of HPP and ENZ modes via far-field sensing. Furthermore, we explore the impact of a MgO dielectric gap positioned between the HMM and ENZ layers on the efficiency of near-field coupling between HPP and ENZ modes, and we demonstrate the capability of tuning the near-field radiative heat transfer by introducing a dielectric spacer in an HMM/ENZ structure. Our results provide a new path for novel designs of plasmonic devices with far-reaching applications in tailored thermal emission and thermal management of microelectronics.