Improving light-matter interaction through the exploration of engineered environments is pivotal for the development of high-performance quantum devices. Among these, masers -- analogous to lasers -- are crucial for scientific research and practical applications, offering unparalleled performance in spectroscopy, amplification, and timekeeping. Despite significant progress in the past decade in developing room-temperature solid-state masers using various magnetic quantum emitters, the study of their interaction environments, particularly microwave resonators, remains relatively unexplored. This doctoral thesis focuses on the innovative design of subwavelength resonators paired with a pentacene-based gain medium to enhance the performance, thermal stability, and functionality of room-temperature solid-state masers.

This thesis begins with a successful demonstration of a three-dimensional polarization vortex maser, endowed with first-order orbital angular momenta, using a compact strontium titanate (SrTiO$_3$) cubic resonator. This configuration enables the generation of volumetric vortex maser beams and facilitates dynamic manipulation of the beam's polarization states through the reconfigurability mechanism of the resonator.

Furthermore, this work addresses the inherent sensitivity of conventional maser resonators, which are based on high-index dielectrics, to thermal fluctuations that occur during optical excitation or read-out of quantum states. To mitigate these issues, we investigate two potential dielectric-free, all-metallic resonators based on defect cavities in locally resonant metamaterials and split-ring resonators (SRR) with enhanced magnetic Purcell effect at subwavelength scales.

Moreover, we constructed a pentacene-based maser using a monolithic all-metallic toroidal SRR cluster of subwavelength size ($\leq\lambda_m/17$), which exhibits a fundamental magnetic mode at approximately 1.45 GHz ($\lambda_m\sim 207$ mm), and a remarkably low mode volume ($8.1\times10^{-6} \lambda_m^3$). Through experimental investigations, we demonstrate that the proposed resonator exhibits a high Purcell factor ($5\times10^{6}$). We also evidence the remarkable stability of the output pulse to thermal heating induced by thousands of consecutive optical excitations over an extended period of five hours, significantly surpassing that of masers based on dielectric resonators.

Overall, these findings underscore the potential for further advancements in maser technologies, offering insights into the improvement of the interaction between quantum emitters and microwaves for diverse applications in quantum sensing, deep-space telecommunication and radio astronomy, etc.