Engineering room-temperature quantum magnetic sensors with fibre-cavity diamond lasers

This thesis studies the engineering of quantum magnetic sensors based on diamond lasers containing NV^- colour centres. Specifically, the thesis experimentally and theoretically investigates two different scenarios for realizing diamond lasers in mechanically tunable fibre-based Fabry-Perot cavities. 

In the first scenario, we experimentally explore a diamond-loaded open tunable fibercavity system as a contender for realizing lasing with the negatively charged nitrogen-vacancy (NV^-) centre in diamond as the gain medium. We measure the transmission characteristics of a cavity-resonant laser beam at a wavelength of 721 nm, close to the maximum of emission of the NV^- phonon sideband, both with and without a green pump laser at 532 nm. For moderate green pump powers, we observe an amplification of the resonant red laser light and at the same time a reduction of the spontaneously emitted background photons. When increasing the pump power further, the amplification saturates and at the same time we see an increase in spontaneous photon emission. A qualitative model including stimulated emission and charge state switching of the NV^- centre captures the dynamics in the experiment very well. The results allow conclusions to be drawn about the material challenges of realizing an NV^- laser in diamond. 

In the second scenario, we theoretically study Raman lasing from a diamond crystal containing NV^- centres. The intra-cavity absorption by the NV^- centres modifies the Raman laser, and two interesting regimes are identified based on the position of the Raman and pump laser wavelengths with respect to the zero-phonon line (637nm) of the NV^- centre. When both the pump and Raman wavelengths are below the zero-phonon line we identify a magnetically tunable bistability. When only the pump wavelength is below the zero-phonon line, we predict a shift in the laser threshold dependent on the NV^- spin centres. Furthermore, we propose that such a system can be used as a technically demanding magnetic field sensor with a predicted DC sensitivity of the order of pT/ √flz.