Controlling Thermal Emission and Light Matter Interaction using Phonon-Induced Epsilon-Near-Zero Materials in the Mid-Infrared Region

Abstract

This thesis explores phonon-induced epsilon-near-zero (ENZ) materials as a platform for tailoring light-matter interactions in the mid-infrared (mid-IR) spectral regime. After introducing the fundamental concepts of thermal emission, light-matter interaction in subwavelength structures, and the unique electromagnetic properties of polar dielectrics near their optical phonon resonances, we show how ENZ conditions can (i) enable spectrally selective, temporally improved thermal emission and (ii) support strong coupling between gap-plasmon resonances and ENZ phonon-polaritons.

The first research project of my thesis investigates ultra-thin silicon dioxide (SiO₂) films, which support phonon-polariton resonances near their longitudinal optical (LO) phonon frequency. By leveraging the dispersive behavior of SiO₂ in the ENZ regime, we demonstrate spectrally selective and temporally coherent thermal emission from structures with thicknesses much smaller than the emission wavelength. Full-wave electromagnetic simulations, supported by angle- and polarization-resolved emissivity measurements, confirm the emergence of partially coherent thermal radiation, offering a platform for narrowband mid-IR thermal sources.

In the second part of the thesis, we study the strong coupling between gap-plasmon resonances - supported by a metallic-dielectric-metal antenna array - and the ENZ phonon-polariton mode of an ultra-thin aluminum oxide (Al₂O₃) film. This interaction gives rise to hybrid polaritonic states characterized by modified dispersion and strong field confinement. Numerical and experimental investigations confirm that the system operates in the strong coupling regime, with tunable mode positions achieved by varying structural parameters such as antenna dimensions and ENZ film thickness.

Finally, we present a numerical study of field enhancement in the coupled gap-plasmon-ENZ system. Simulations reveal that decreasing the ENZ film thickness leads to significant field concentration within the structure, with electric field magnitudes approaching two orders of magnitude above the incident field. These findings underscore the potential of ENZ-coupled plasmonic systems for enhancing nonlinear optical effects and advancing mid-IR photonic applications.