Efficient Optical Coupling of Color Center Ensembles in Diamond

Abstract

Nitrogen-Vacancy (NV) centers in diamond have emerged as a leading platform for quantum sensing, particularly in magnetic field detection. Efficient interrogation of large NV ensembles is crucial for achieving high sensitivity. This paper presents a comprehensive analysis of the optical efficiencies that govern the performance of NV-based sensing systems, focusing on excitation, conversion, and collection, leading to a novel collection design improving upon state-of-the-art results. To optimize various excitation schemes, we developed numerical tools capable of accurately calculating excitation efficiencies for arbitrary power density distributions. Using these tools, we investigated several techniques, including reflective coatings and corner excitation, and achieved excitation efficiencies exceeding 0.7, compared to the$\sim 0.4$efficiency achieved by standard methods. We also measured the conversion efficiency of NV centers, quantifying the ratio of absorbed green excitation vs. fluorescence, in various diamond samples. This value is usually missing in the literature, and therefore important to quantify in different regimes. We obtained a value of ηv=0.22 ± 0.03, which is consistent with theoretical expectations. Furthermore, we designed and experimentally validated a novel Compound Trapezoid Collector (CTC) for enhanced photon collection. The CTC offers performance comparable to that of traditional Compound Parabolic Collectors (CPCs), but with a significantly smaller form factor and simplified manufacturing. Our prototype CTC achieved a collection efficiency of 0.33, with the potential to reach approximately$\sim 0.8$. This work provides insights into optimizing the optical efficiencies of NV-based sensing systems and contributes to the development of practical devices.