[Paper Review] Single-photon detection in the mid-infrared up to 10 micron wavelength using tungsten silicide superconducting nanowire detectors

Varun B. Verma, Boris Korzh+16 more|arXiv (Cornell University)|Dec 17, 2020

Photonic and Optical Devices6 citations

TL;DR

This paper demonstrates tungsten silicide (WSi) superconducting nanowire single-photon detectors (SNSPDs) with saturated internal detection efficiency up to 10 µm wavelength, achieved by tuning film stoichiometry to increase resistivity and enhance energy sensitivity. The devices show high performance in the mid-infrared, enabling applications in exoplanet spectroscopy, LIDAR, and dark matter detection with ultra-low dark counts and high gain stability.

ABSTRACT

We developed superconducting nanowire single-photon detectors (SNSPDs) based on tungsten silicide (WSi) that show saturated internal detection efficiency up to a wavelength of 10 um. These detectors are promising for applications in the mid-infrared requiring ultra-high gain stability, low dark counts, and high efficiency such as chemical sensing, LIDAR, dark matter searches and exoplanet spectroscopy.

Motivation & Objective

• To extend high-efficiency single-photon detection into the mid-infrared beyond 3 µm, where molecular absorption signatures are prominent.
• To overcome the limitations of conventional NbN-based SNSPDs, which saturate at ~3 µm, by exploring alternative superconducting materials.
• To improve energy sensitivity in SNSPDs through material engineering, specifically increasing resistivity via tuning WSi film stoichiometry.
• To demonstrate saturated internal detection efficiency at wavelengths up to 10 µm, a critical step toward high system detection efficiency.
• To enable practical applications in photon-starved environments such as exoplanet atmosphere spectroscopy and quantum sensing.

Proposed method

• Fabricated WSi SNSPDs using reactive sputtering with variable sputtering power on silicon and tungsten targets to tune film stoichiometry and resistivity.
• Used nanowire widths of 50 nm and 70 nm, with lengths of 10 µm, to investigate the impact of geometry on detection efficiency.
• Measured photoresponse count rate (PCR) vs. bias current curves at 4.8, 7.4, and 9.9 µm using a quantum cascade laser (QCL) source in a cryostat.
• Employed background count rate measurements to assess dark count levels and device stability.
• Utilized secondary ion mass spectrometry (SIMS) to analyze film composition and correlate silicon content with superconducting transition temperature (Tc).
• Optimized device performance by increasing sputtering power on the silicon target from 180 W to 320 W, resulting in higher resistivity and improved energy sensitivity.

Experimental results

Research questions

• RQ1 Can WSi-based SNSPDs achieve saturated internal detection efficiency at mid-infrared wavelengths up to 10 µm?
• RQ2 How does increasing the resistivity of WSi films through stoichiometric tuning affect energy sensitivity and detection efficiency?
• RQ3 What is the impact of nanowire width (50 nm vs. 70 nm) on the saturation of internal detection efficiency at long wavelengths?
• RQ4 Can high internal detection efficiency be maintained at 10 µm despite challenges in photon absorption and thermalization?
• RQ5 What role does film thickness and superconducting gap energy play in enabling long-wavelength single-photon detection?

Key findings

• Saturated internal detection efficiency was achieved at 9.9 µm for 70 nm-wide WSi nanowires, indicating successful energy sensitivity extension to 10 µm.
• The 70 nm-wide nanowire showed a plateau in photoresponse at 9.9 µm, while narrower 50 nm wires exhibited non-saturating behavior, likely due to fabrication defects.
• Increasing sputtering power on the silicon target from 180 W to 320 W raised silicon content to 48 ± 10%, increased resistivity, and improved saturation of internal efficiency across all tested wavelengths.
• The film thickness decreased slightly to 2.61 nm, and the superconducting transition temperature dropped to 2.8 K from 3.1 K, indicating a more resistive, less superconducting film.
• The 50 nm-wide nanowire showed distorted PCR curves at low bias currents due to a 200 nA readout threshold, but still demonstrated improved saturation at higher currents.
• The results confirm that resistivity engineering via film stoichiometry is a viable path to extending SNSPD performance into the mid-infrared beyond the limits of NbN-based devices.