Photothermal resistivity alignment of optical fibers to SNSPD

TL;DR

The paper addresses the challenge of reliably aligning optical fibers to SNSPDs by introducing a photothermal resistivity method that uses modulated IR heating to map the nanowire’s absorption and coupling efficiency. The approach scans the fiber over the NbTiN nanowire and detects the resulting resistance change with a lock-in amplifier, locating the alignment that maximizes the photothermal response. They demonstrate a peak signal of $23~\mu\mathrm{V}$ RMS with an SNR of about 20 and achieve sub-micron alignment precision, with XY and height resolutions on the order of hundreds of nanometers. This technique is fabrication-friendly, robust to misalignment in height and angle, and can complement or substitute traditional back-reflection-based alignment, with potential for in-situ coupling assessment during cooldown and broader applicability to nanoscale heat-transfer studies.

Abstract

We demonstrate a straightforward optoelectronic fiber alignment technique for superconducting nanowire single-photon detectors (SNSPDs) that exploits the temperature-dependent resistance of the nanowire under optical absorption. The target nanowire is illuminated via the fiber, and the local absorption of light heats the wire, causing a change in its resistivity. Scanning the fiber over the nanowire, the change in its resistivity is monitored by lock-in amplifier, mapping the spatial photothermal response correlated to absorption and coupling efficiency. The maximum of the response corresponds to optimal fiber-SNSPD alignment. This method allows for aligning the fiber to the center of the meander with sub-micron precision. The response is robust to variations in the angle and height of the fiber, providing an alternative or complement to fiber-to-chip alignment methods based on the back-reflection or transmission measurement.

Figures (10)

• Figure 1: Scheme of experimental apparatus for photothermal resistivity alignment and optical reflectivity alignment. Optical fibers are grey-colored. The power meter with optical circulator is utilized for back-reflection measurement for comparison to the resistivity measurement.
• Figure 2: (a) SEM image of the NbTiN nanowire. (b) Transport measurement of the NbTiN film. The inset shows the detail of the resistivity dependence in the vicinity of room temperature, where the alignment was carried out. The extracted temperature coefficient ratio at room temperature is $\alpha=-2.8\times10^{-4}$.
• Figure 3: (a) Optical conductivity determined from spectroscopic ellipsometry and DC conductivity from four-point measurement (circle). (b) transmission, reflectivity, and absorbance spectra calculated from conductivity using Fresnel equations at 0° angle of incidence. Red vertical line is guideline for $\lambda=1550$ nm. Red dashed line represents absorption assuming DC sheet resistance of the sample.
• Figure 4: Pseudocolored photo of fiber (purple) suspended over meander (red). Bonding pad of the meander (green) is aluminum wire-bonded (blue). Grounding wire-bonds are not visible in the photo.
• Figure 5: Equivalent scheme for the readout circuit.