Generation and amplification of microwave signals via planar waveguides with embedded paramagnetic color centers

A concept for microwave (MW) amplification and maser generation in compact planar waveguides embedding a thin layer of paramagnetic color centers is presented. The ground state of the color centers can be optically or electrically spin-polarized, while population inversion is attained by Zeeman shifting of one spin sublevel below the populated state. A signal traveling along the waveguide, if resonant with the spin transition, can be amplified, or a self-sustained MW oscillation can be generated. A general analysis of the conditions for amplification and self-oscillation is developed for nitrogen-vacancy-rich diamond films, together with practical examples of impedance matching and resonator design. The proposed architecture combines near-unity filling factors with relatively low electromagnetic quality factors, enabling operation in strongly coupled configurations and broad frequency tunability through the external magnetic field. The same formalism naturally extends to compact lumped-element resonators operating at a few gigahertz while preserving millimeter-scale dimensions. Compared with the present state of the art of diamond masers, the approach offers reduced size, simplified integration with external electronics, and relaxed requirements on cavity quality factor.