Quantum theory of the diamond maser: Stimulated and superradiant emission

We present a quantum theory of diamond masers operating at any temperature using a cavity quantum electrodynamical framework. Special attention is paid to the recently demonstrated room-temperature solid-state masers based on nitrogen-vacancy (NV) defect centers in diamond, but the model can easily be modified for other photoexcited chromophores such as pentacene-doped paraterphenyl, vacancies in silicon-carbide or boron nitride. We show that the eight energy levels involved in the optically pumped NV center polarization process can be mapped to a simple pumped two-level-system. We then derive simple analytical expressions for the optical pump threshold condition for masing as well as the steady-state microwave output power which can be used to design and predict maser performance. Finally, we investigate second-order correlations and find that typical diamond masers operate in an intermediate regime between the good and bad cavity limits where photon emission is driven by both stimulated and superradiant processes.