An Open Microcavity for Diamond-based Photonics

In recent years, tunable Fabry-Perot microcavities have emerged as a compelling platform for enhancing the flux of coherent photons from single colour centres in solid-state hosts. A prominent example of one such colour centre is the nitrogen-vacancy (NV) centre in diamond. The NV centre has a highly coherent, optically addressable electron spin. Furthermore, the NV centre is a source of single photons. Advances in the creation of entangled spin-photon pairs allow for establishing remote spin-spin entanglement – a key building block in a quantum network. However, the scalability past a few network nodes is limited by modest entanglement rates, in turn limited by the detection efficiency of coherent photons. Limiting factors include the long radiative lifetime and the small branching ratio of “useful” photons into the zero-phonon line (ZPL). However, neither the radiative lifetime nor the branching ratio are rigid features of the NV centre – the flux of ZPL photons can be greatly accelerated in a resonant microcavity.
This thesis reports on the realisation of a high-quality tunable Fabry-Perot microcavity embedded with a diamond membrane. However, the diamond alters the cavity performance, rendering the cavity sensitive to surface related losses. Despite operating in a geometry where the standing wave inside the cavity possesses an anti-node at the diamond surface, quality ($\mathcal{Q}$) factors exceeding $100\,000$ were realised. The benefit of this geometry is the strong confinement of the vacuum electric-field to the diamond – the current cavity design allows for the realisation of Purcell factors exceeding 300, thus increasing the fraction of photons emitted into the ZPL from $3\,\%$ to $89\,\%$.
The versatile design of the microcavity was demonstrated further by enhancing the Raman transition from the single crystalline diamond. Compared to free-space measurements under likewise identical conditions, a 59-fold intensity enhancement was demonstrated. This enhancement factor encompasses the Purcell effect and the improved detection efficiency provided by the cavity. The Raman transition couples to all cavity modes, allowing for in situ optimising and benchmarking the cavity performance. Additionally, it facilitates coupling to the external single-mode detection optics. Further enhancement of the Raman intensity can be achieved by establishing a double resonant condition, with both the pump laser and the Raman transition being resonant. Resonant recirculation of the pump laser increases the power density inside the cavity, providing a platform with prospects of realising a Raman laser with sub-mW threshold pump power. Exploiting a small thickness gradient in the diamond enabled continuous tuning of the double resonance condition across a spectral window of $\sim1\,\textrm{THz}$. The tuning range is only limited by the travel range of the piezo – with an adequate travel range, continuous tuning is, at least in principle, possible across the entire reflective stopband.