| dc.description.abstract | Due to the long free-space wavelengths associated with radiation in the infrared (IR), the realization of flat and sub-diffractional optical components promises advances in imaging, communications, sensing, and thermal emitters. One of the approaches to realize this objective is through the excitation of polaritons, i.e., quasiparticles resulting from strong coupling of electromagnetic waves with materials, to compress light into deeply subdiffraction scales. My dissertation focuses on manipulating and designing those polaritons in both the spatial domain, i.e., polariton propagation characteristics, and the spectral domain, i.e., spectral manipulation.
The platform I used to engineer the polariton propagation is hyperbolic phonon polaritons (HPhPs), which can be supported in materials where the real parts of their permittivities along different directions are opposite in sign. Compared to surface polaritons, HPhPs offer further confinement of long-wavelength light to deeply subdiffractional scales, and volume propagation that enables control of the polariton wavevector by changing the underlying medium. This allows for greater control of polaritonic resonators and near-field polariton propagation without deleterious etching of hyperbolic materials. We demonstrate HPhPs can be significantly tuned and modulated by the underlying substrate dielectric functions. With this concept, we also demonstrated that HPhPs could be guided by the underlying silicon waveguide structure for guiding in both mid- and near-infrared light in the same device.
Polaritons can also be excited and designed in the spectral domain. For Tamm plasmon polaritons (TPP) supported by distributed Bragg reflectors (DBRs) on metals, high quality-factor absorption resonances can be realized, which can be used as narrow-band light sources in the mid-infrared when the device is working at elevated temperatures, e.g., 200 °C. However, to design multiple resonances is challenging as numerous structural parameters must be optimized. I developed an algorithm based on stochastic gradient descent (SGD) to optimize TPP-thermal emitters (TPP-EMs) composed of an aperiodic DBR deposited on doped cadmium oxide (CdO), where layer thicknesses and carrier density are inversely designed. The combination of the aperiodic DBR with the designable plasma frequency of CdO enables multiple TPP-EM modes to be simultaneously designed with arbitrary spectral control not accessible with metal-based TPPs. | |
