All-metallic magnetic Purcell enhancement in a thermally stable room-temperature maser

Enhancing the spontaneous and stimulated emission rates of magnetic quantum emitters through the Purcell effect is essential for designing high-performance quantum devices based on high quality factor and small mode volume. At room temperature, structures constructed using high-index dielectric materials have been favored due to their ability to effectively confine electromagnetic fields. However, these dielectric resonators are plagued by an inherent sensitivity to thermal variations, which are unavoidable during the optical excitation or readout of the quantum states. Here, we propose to solve this issue with a dielectric-free, all-metallic toroidal split-ring resonator cluster of subwavelength size (≤λ_m/17), which exhibits a fundamental magnetic mode at approximately 1.45 GHz (λ_m ~ 207 mm) and a remarkably low mode volume ( $8.1\times 10^{-6}{\lambda }_m^3$ ). Through experimental investigations, we demonstrate that the proposed resonator exhibits a high Purcell factor (5 × 10⁶), and observe maser action when paired with a pentacene-based gain medium. We evidence the remarkable stability of the output pulse against thermal variations caused by thousands of consecutive optical excitations, by far surpassing that of masers based on dielectric resonators.