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Superconductor Science and Technology

NbN superconducting nanowire single-photon detector fabricated on MgF 2 substrate

Junjie Wu, Lixing You, L Zhang, Weijun Zhang, Hao Li, Xinlei Liu, Hui Zhou, Zehua Wang, X. M. Xie, Y X Xu, W Fang, Limin Tong

Published Apr 22, 2016• 7 citations
Publisher

Abstract

Abstract The performance of superconducting nanowire single-photon detectors (SNSPDs) relies on substrate materials. Magnesium fluoride (MgF 2 ) exhibits outstanding optical properties, such as large optical transmission range and low refractive index ( n = 1.38), making it an attractive substrate. We present the fabrication and the performance of SNSPDs made of a 4.5 nm thick NbN thin film deposited on MgF 2 substrate for the wavelength of 1550 nm. The front-side illuminated SNSPDs without an optical cavity showed a maximal detection efficiency of 12.8% at a system dark count rate (DCR) of 100 Hz, while the backside illuminated SNSPDs with a SiO 2 /Au optical cavity atop displayed a maximal detection efficiency of 33% at a DCR of 100 Hz.

DOI
10.1088/0953-2048/29/6/065011

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Abstract The performance of superconducting nanowire single-photon detectors (SNSPDs) relies on substrate materials. Magnesium fluoride (MgF 2 ) exhibits outstanding optical properties, such as large optical transmission range and low refractive index ( n = 1.38), making it an attractive substrate. We present the fabrication and the performance of SNSPDs made of a 4.5 nm thick NbN thin film deposited on MgF 2 substrate for the wavelength of 1550 nm. The front-side illuminated SNSPDs without an optical cavity showed a maximal detection efficiency of 12.8% at a system dark count rate (DCR) of 100 Hz, while the backside illuminated SNSPDs with a SiO 2 /Au optical cavity atop displayed a maximal detection efficiency of 33% at a DCR of 100 Hz.
Superconducting nanowire single photon detectors with an optical resonant cavity structure are fabricated directly on the single crystal substrate. In order to enhance the photons absorption of incident light and improve system detection efficiency (SDE) efficiently, the structure of SNSPD was designed and optimized using the finite-difference time-domain analysis for different kinds of substrates. The NbN-SNSPDs fabricated on different substrates are measured and compared. The SDE of NbN-SNSPD on MgO substrate reaches 66 % at the wavelength of 940nm at a dark count rate of 100 cps by illuminating from the back of the device through single mode optical fiber.
We have developed a practical superconducting nanowire single photon detector (SNSPD) system with our superconducting thin films and devices fabrication, optical coupling packaging, and cryogenic technology. The SNSPD devices were fabricated from high-quality niobium nitride (NbN) ultra-thin films that were epitaxially grown on single-crystal MgO substrates, and optical cavity structure were applied for improvement of photo absorption efficiency. The SNSPD system was developed based on a compact Gifford-McMahon (GM) cryocooler, in which SNSPD can operate continuously at temperature of 2.9 K +/- 10 mK with low power consumption without any liquid cryogens. The system detection efficiencies at 1550 nm wavelength (at a 100 Hz dark count rate) were found to be 2.6% for device without optical cavity and 21% for device with optical cavity. The response speed of devices showed maximally 200 MHz with 5 × 5 mm square device, and timing jitter of 100 ps full width at a half maximum (FWHM) was observed.
On-chip single photon detection is crucial for implementing on-chip quantum communication, quantum simulation, and calculation. Superconducting nanowire single-photon detectors (SNSPDs) have become one of the essential techniques to achieve high-efficiency, on-chip, single-photon detection at scale due to their high detection efficiency, low dark count rate, and low jitter. Silicon carbide (SiC) has emerged as a promising integrated photonics platform due to its nonlinear optical processing capabilities, compatibility with CMOS technology, and outstanding quantum properties as a device for single photon sources. However, achieving high-efficiency superconducting nanowire single-photon detection on SiC substrates has yet to be demonstrated. In this study, we deposited polycrystalline NbN thin films onto 4H-SiC substrates. We also ensured that the deposited NbN thin film had a flat surface with a roughness less than 1 nm on the C-side 4H-SiC substrate through optimized chemical mechanical polishing. The NbN-SNSPD achieved a saturated quantum efficiency covering the color center emission bandwidth wavelength range (from 861 to 1550 nm) of the 4H-SiC material. These results offer a promising solution for high-efficiency single-photon detection on fully integrated quantum optical chips on 4H-SiC substrates.
Superconducting nanowire single photon detectors (SNSPD) offer excellent performance for infrared single photon detection, combining high efficiency, low timing jitter, low dark count rates and high photon counting rates. Promising application areas for SNSPDs include quantum key distribution, space-to-ground communications and single photon remote sensing [1]. SNSPDs are typically made with ultrathin niobium nitride (NbN) films with thickness 4 nm and a superconducting transition temperature above 9 K. NbN offers high performance in the near infrared but their sensitivity drops at wavelengths beyond 2 um. There is growing interest in potential photon counting applications in the mid infrared domain (for example remote sensing of greenhouse gases in the atmosphere [2]). One way to overcome the wavelength limit in NbN SNSPDs is to use films with a lower superconducting energy gap [3]. Here we report on the study of SNSPDs fabricated with thin films of titanium nitride (TiN). We compare TiN films deposited by atomic layer deposition (ALD) and by magnetron sputtering. The TiN films range in thickness from 5 to 60 nm, with superconducting transition temperatures from ~1 K to 3.5 K. We have analyzed the films via transmission electron microscopy and variable angle spectroscopic ellipsometry. We characterize TiN SNSPDs performance from near to mid-infrared at wavelengths (1-4 um) with fast optical parametric oscillator (OPO) source. We compare the performance of TiN SNSPDs to devices based on other lower gap materials: MoSi, NbTiN, WSi. [1] Natarajan et al Superconductor Science and Technology 25 063001 (2012) [2] Abshire et al Laser Applications to Chemical, Security and Environmental Analysis, (Optical Society of America, 2008) paper LMA4 [3] Verma et al Applied Physics Letters 105 022602 (2014)
We demonstrate high-performance nanowire superconducting single photon detectors (SSPDs) on bN thin films grown at a temperature compatible with monolithic integration. NbN films ranging from 150 nm to 3 nm in thickness were deposited by dc magnetron sputtering on MgO substrates at 400 degrees C SSPDs were fabricated on high quality NbN films of different thickness (7 to 3 nm) deposited under optimal conditions. Electrical and optical characterizations were performed on the SSPDs. The highest QE value measured at 4.2K is 20% at 1300 nm.
We report on the enhancement of system detection efficiency in a superconducting nanowire single-photon detector (SNSPD) by applying the optical cavity structure. The nanowire was made using 4-nm-thick NbN thin films and covered with an SiO cavity and Au mirror designed for 1300-1600nm wavelengths. The device is mounted into fiber-coupled packages, and installed in a practical multichannel system based on GM cryocoolers. System detection efficiency depends on the absorptance of cavity structure, and reached 28% and 40% at 1550nm and 1310nm wavelengths, respectively. These values were considerably higher than an SNSPD without optical cavity.