Characterizing the Diamond Waveguide Platform with High Density of NV− Centers for Quantum Sensing Applications*

The negatively charged nitrogen-vacancy center in diamond (NV) is an excellent candidate for nanoscale sensing and quantum information applications at room temperature. Efficient light-matter interaction is crucial for manipulating NV-center systems in diamond because it directly affects the contrast and, consequently, sensitivity. Laser-written waveguides enhance this coupling [1] by both guiding light and creating additional NV centers during the laser writing process. Thus, a comprehensive characterization of this platform is essential for its future use. In this study, we present an in-depth analysis of this platform using the ensemble of NV centers it contains, with insights into the diffusion of vacancies, waveguide translational symmetry, and spatial profiles of strain and changes in the refractive index.

The laser writing process is used to fabricate the waveguides, while simultaneously generating a high concentration of vacancies. Subsequently, the annealing process combines these vacancies with the high-density of native nitrogen impurities in the waveguide, producing a considerable number of NVs. We examine the vacancy diffusion profile to determine the diffusion constant. Next, we perform zero-field optically detected magnetic resonance (ODMR) measurements using a confocal microscope. The probed ODMR signal is sensitive to strain-driven changes to NV center spin eigenstates. We fit the ODMR data with a theoretical model that simulates the response from all NV center orientations under strain, assuming the waveguide's translational symmetry. As a result, we extract relevant strain components indirectly justifying this assumption. To gain further insights, we perform strain imaging within the structure. Based on this, we also determine the spatial variation of the refractive index, which directly affects the signal quality.