Fig. 1: Toroidal split-ring resonator (SRR) cluster for strong and stable Purcell enhancement, and its demonstration in a maser.

a Highly concentrated magnetic energy density distribution within the cross-section of the resonant mode supported by the clustered SRR. The white dashed lines depict the magnetic field contours. The inset is a photograph of the clustered SRR fabricated in silver using the lost-wax metal casting method. b Room-temperature maser experiment. A pentacene:p-terphenyl crystal is loaded into the clustered SRR. Both are housed inside a cylindrical copper cavity featuring an adjustable copper top, which enables adjusting the resonance frequency. The pentacene molecules are excited by a yellow pulsed laser via an optical fiber inserted into the crystal through the cavity bottom. A loop probe is utilized to extract the maser signals.