In its first stage, the ESSENS project focused on the development of waveguides with subwavelength-grating (SWG) lateral claddings in the silicon nitride–on-insulator platform, as these are the fundamental building blocks for embedding the targeted superconducting nanowire single-photon detectors (SNSPDs). By using a finite-element-method (FEM) solver and 3D finite-difference time-domain (FDTD) simulations, the dimensions of the lateral SWG waveguides and homogeneous-to-SWG waveguide transitions were determined. Further fabrication, based on electron-beam lithography, and experimental characterization confirmed the suitability of the designed structures.
The supported mode of a homogenized lateral SWG waveguide with a nm-thin NbTiN nanowire on top was estimated by FEM simulations, revealing an absorption of ~5% per nanowire, in agreement with reports found in the literature. The fabrication process of the nanowires on top of the SWG waveguides was carefully applied, comprising the following steps: (1) sputtering of a thin NbTiN film on a silicon nitride wafer, (2) lithographic definition of gold electrical pads, (3) deposition of gold and lift-off, (4) lithographic definition of superconducting nanowires and etching, (5) lithographic definition of nanophotonic components, including SWG waveguides, and etching. The experimental characterization of the nanowires on the SWG waveguides yielded an absorption of ~5% at room temperature, in excellent agreement with the simulations.
Finally, taking advantage of the knowledge acquired throughout the previous work, a photonic integrated chip including waveguide-integrated cavity-based SWG-engineered SNSPDs was developed and characterized at cryogenic temperature. The attained results overcome the performance of state-of-the-art cavity-based NbTiN SNSPDs.
