Groundbreaking discoveries in the fields of light-matter interactions and thermoelectrics in the past two decades have profoundly shaped our understanding of how photons, electrons, and phonons interact. Increased control over the quality of engineered systems, novel measurement techniques, and quantitative improvements in theory are the driving force behind modern, record-high values of light-matter coupling strength and thermoelectric performance. In this work, I bring together experimental and theoretical techniques to study the interplay between magnons and spins in ErFeO3, photons and plasmons in Fischer nanostructures, and electrons and phonons in thermoelectric active cooling materials. Specifically, I perform terahertz time-domain magneto-spectroscopy measurements on the rare-earth orthoferrite ErFeO3 as a function of temperature and magnetic field, and we propose a novel protocol that uses this material as a solid-state quantum simulator of an extended Dicke model. Then, I conduct aperture-based scanning near-field optical microscopy measurements on Fischer nanostructures, and observe field enhancement and localization with resolution beyond the diffraction limit. Lastly, I study active cooling under arbitrary external thermal resistances, and map out the regions where active cooling is advantageous compared to Carnot-limit refrigeration. These results lead to a deeper understanding of fundamental interactions in magnetic, semiconducting, and low-dimensional materials, and further motivate translating research into engineering solutions.