  Low-loss photonic integrated circuits (PICs) are the key elements in future
quantum technologies, nonlinear photonics and neural networks. The low-loss
photonic circuits technology targeting C-band application is well established
across multi-project wafer (MPW) fabs, whereas near-infrared (NIR) PICs
suitable for the state-of-the-art single-photon sources are still
underdeveloped. Here, we report the labs-scale process optimization and optical
characterization of low-loss tunable photonic integrated circuits for
single-photon applications. We demonstrate the lowest propagation losses to the
date (as low as 0.55 dB/cm at 925 nm wavelength) in single-mode silicon nitride
submicron waveguides (220x550 nm). This performance is achieved due to advanced
e-beam lithography and inductively coupled plasma reactive ion etching steps
which yields waveguides vertical sidewalls with down to 0.85 nm sidewall
roughness. These results provide a chip-scale low-loss PIC platform that could
be even further improved with high quality SiO2 cladding, chemical-mechanical
polishing and multistep annealing for extra-strict single-photon applications.
