Immersion of Nanodiamonds into Three-Dimensional Direct-Laser-Written Waveguides

Waveguide-coupled sensors have several applications such as magnetometry, electrometry or thermometry, harnessing the resolution of nano-sized probes as well as tight light control in macroscopic waveguide networks. We present our approach to incorporate nanodiamonds into direct-laser-written (DLW) three-dimensional photonic structures. These nanodiamonds house ensembles of nitrogen vacancy (NV) centers, acting as probes that can be read-out optically. Guided by the waveguide structure, detection of the optical signal from the nanodiamond does not require direct optical access. In fact, the waveguides written feature extended planar sections laid onto the substrate as well as three-dimensional structures. The latter enables simultaneous addressing and imaging of waveguide inputs and outputs through the glass substrate using a single microscope objective. The NV center offers an accurately controllable spin in a solid-state system, serving as a sensitive probe of, e.g., magnetic fields. Additionally these defect centers are photostable and compatible with DLW. We present details of the the integration process and show optically detected magnetic resonance spectra in waveguide-embedded nanodiamonds and compare their performance with free-space emission. This approach opens the way for on-chip three-dimensional structures for optically integrated spin-based sensing.