Transition from Phononic to Geometrical Mie Modes Measured in Single Subwavelength Polar Dielectric Spheres

Spherical dielectric resonators are highly attractive for light manipulation, thanks to their intrinsic electric and magnetic resonances. Here, we present measurements of the mid-infrared far-field thermal radiation of single subwavelength dielectric spheres deposited on a gold substrate, of radii ranging from 1 to 2.5 μm, which agree quantitatively with simulated absorption cross sections. For SiO₂microspheres, we evidence the excitation of both surface phonon-polariton (SPhP) modes and geometrical electric and magnetic Mie modes. The transition from a phonon-mode-dominated to a Mie-mode-dominated emission spectrum is observed, with a threshold radius of ∼1.5 μm. We also show that the presence of the metallic substrate augments the computed spheres absorption cross-section due to increased local field enhancement, arising from the near-field interaction of the spheres oscillating charges with their image in the metallic mirror. In contrast, measurements of single subwavelength SPhP-inactive PTFE spheres reveal that the mid-infrared response of such lossy spheres is dominated by their bulk absorption. Our results demonstrate how engineering the geometrical and dielectric properties of subwavelength scatterers can enable the control of thermal emission near room temperature, with exciting perspectives for applications such as radiative cooling.