4-Pixel NbN Hot-Electron Bolometer Integrated in a Si$_3$N$_4$ Planar Optical Waveguide with On-Chip Fiber-Alignment Trench
In this work, we design and characterize a 4-pixel superconducting hot-electron bolometer (HEB) based on niobium nitride (NbN), integrated with individual planar silicon nitride (Si$_3$N$_4$) waveguides. The implemented architecture enables simultaneous detection of an optical signal in four independent channels. To efficiently couple optical radiation under cryogenic conditions, we employ an edge (end-fire) coupling approach using dedicated U-shaped grooves that provide accurate and stable positioning of an optical fiber with respect to the on-chip waveguide facet. The device responsivity is measured as a function of the HEB operating point. The measured voltage responsivity reaches $3800~\mathrm{V/W}$ at a modulation frequency of $3~\mathrm{GHz}$. We demonstrate detection of optically modulated signals in the gigahertz range. The developed fabrication route is promising for compact integrated receiver systems and low-noise cryogenic microwave transducers, including superconducting nanowire single-photon detectors (SNSPDs).