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. The nanodiamonds house ensembles of 10^3 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, our waveguides combine extended planar sections laid onto the substrate on the one hand with three-dimensional coupling structures on the other hand. The latter effectively rotates the propagation direction of light signals from parallel to the substracte surface within the waveguide network to perpendicular to the substrate at the in- and outputs. This 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 the DLW process. We show optically detected magnetic resonance spectra together with Rabi oscillations on an effective two-level system in waveguide-embedded nanodiamonds. We compare their performance with free-space emission and complement our experimental studies by numerical simulations.
This approach opens the way for on-chip three-dimensional structures for optically integrated spin-based sensing.
Applications in, e.g., optical communication, light routing, and emerging optical quantum technologies on a chip call for waveguide networks featuring tight control over the photons used. Quantum simulators on a chip harness this high level of control to guide and manipulate entangled photon states in sophisticated networks to gain insight into the role of entanglement in interacting many-body systems.
We have recently shown direct laser written polymer waveguides fabricated from a low-fluorescent negative tone photoresist via two-photon lithography . These waveguides feature bend radii down to 40 µm and loss coefficients smaller than 0.81 dB/mm, facilitating networks with high integration density. For coupling control, a novel three-dimensional coupler design was shown, giving optical access to all in- and outputs of the waveguide network simultaneously via one microscope objective.
We present an in-depth analysis and optimization of these coupling structures in simulation and experiment.
 A. Landowski et al., APL Photonics 2, 106102 (2017)