Selective thermal emitters emit thermal radiation in a narrow frequency range enabling applications in sensing, waste heat energy conversion, and radiative cooling. Waste heat energy recovery through thermophotovoltaics requires high performance selective thermal emitters. To date, the achieved conversion efficiency values fall well below thermodynamic limits. The primary factors limiting device performance arise from material limitations of commonly used optical materials. In this dissertation, I will demonstrate the need for alternative material platforms and show how these new platforms enable unconventional thermal light sources using the principles of phase, symmetry, and topology. First, I will discuss physical modeling to predict the optical properties of doped semiconductors at high temperatures. This analysis will demonstrate the role loss engineering plays in designing selective thermal emitters. Next, I will present experimental results of loss engineering in hybrid plasmonic-photonic resonators resulting in passive parity-time (PT) symmetry in thermal emission. Using the principles of non-Hermitian physics in such a loss asymmetric system provides a pathway for overcoming the trade-off between spectral linewidth and peak emissivity. Furthermore, controlling the coupling between horizontal and vertical modes in such a hybrid system allows for the observation of higher-order non-Hermitian phenomena. This control permits the creation of exceptional concentric rings and thermal emitters with non-trivial topology. Additionally, I will present an experimental demonstration of iron pyrite (FeS2) as an ultrahigh index dielectric material for mid-infrared metamaterials. Iron pyrite has a very large refractive index, up to 4.4, with an optical band gap close to 1 eV far surpassing performance estimates using the Moss rule's common form. Finally, I will conclude with an experimental demonstration of a hyperbolic metamaterial using aligned films of single-walled carbon nanotubes. The optical anisotropy of the aligned films facilitates the creation of ultra-small thermal emitters with volumes below ~λ3700.