Light manipulation at the nanoscale, which is known as nanophotonics, has attracted enormous attention. Particularly, metallic nano-resonators, which support localized surface plasmons, have been under intense study. The potential applications of metallic nano-resonators include surface-enhanced spectroscopy, plasmon-enhanced solar energy harvesting, ultra-thin optical components, etc. However, two issues that hinder their applications in real life have long been identified. On the one hand, the optical response lacks tunability once the nano-resonators are fabricated. On the other hand, the large Ohmic losses inherent in noble metals severely limit their performance. In this thesis, these two issues are investigated through both numerical and experimental studies. In the first part, phase-change material Ge2Sb2Te5 (GST) is introduced to add tunability to the nano-resonators. GST has a large refractive index contrast between its amorphous and crystalline phases. In this research, it is used as a tunable dielectric environment for the resonators, achieving significant shifts of the resonances. Practical applications, such as hotspot manipulation and phase-front engineering, are studied in detail. In the second part, nano-resonators made of polar crystals (4H-SiC and hexagonal-BN) that support phonon polaritons are investigated. Due to the absence of free charge carriers, Ohmic losses are greatly reduced in these materials. The field enhancements and quality factors of the corresponding resonators are therefore found to be much larger than their plasmonic counterparts. Further analyses of the near-field distributions reveal the natures of the resonances, providing fundamental knowledge for future nanophotonic designs based on phonon polaritons.