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IEEE Transactions on Applied Superconductivity

Evaluation of NbTiN Superconducting Strip Photon Detectors With 0.1–30 µm Strip Widths

Tomohiro Haneishi, Masahiro Yabuno, Hiroki Kutsuma, Shigehito Miki, Taro Yamashita

Published Dec 25, 2024• 4 citations
Publisher

Abstract

We report the niobium titanium nitride (NbTiN) superconducting strip photon detectors (SSPDs) with strip widths ranging from 0.1 to 30 µm. The bias current dependences of the detection efficiencies at the 1550 nm wavelength and the dark count rates were systematically evaluated in the wide range of the strip widths. Single-photon detection was confirmed and a saturation trend in the bias current dependence of detection efficiency was also observed for all strip width SSPDs. In addition, we measured the physical properties of the NbTiN films and found the switching current of SSPDs gradually deviated lower from the estimated depairing current with increasing the strip width. Furthermore, the dark counts occurred at the lower bias current relative to the depairing current as the strip width increased. These results suggest that the bias current is unevenly distributed as the strip width increases and the concentrated bias current may induce the dark counts. Our results would provide valuable insights into a better understanding of SSPDs with a wide range of strip widths, the origin of dark counts, and the mechanism of photon detection in superconducting strip.

DOI
10.1109/tasc.2024.3521901

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We report the niobium titanium nitride (NbTiN) superconducting strip photon detectors (SSPDs) with strip widths ranging from 0.1 to 30 µm. The bias current dependences of the detection efficiencies at the 1550 nm wavelength and the dark count rates were systematically evaluated in the wide range of the strip widths. Single-photon detection was confirmed and a saturation trend in the bias current dependence of detection efficiency was also observed for all strip width SSPDs. In addition, we measured the physical properties of the NbTiN films and found the switching current of SSPDs gradually deviated lower from the estimated depairing current with increasing the strip width. Furthermore, the dark counts occurred at the lower bias current relative to the depairing current as the strip width increased. These results suggest that the bias current is unevenly distributed as the strip width increases and the concentrated bias current may induce the dark counts. Our results would provide valuable insights into a better understanding of SSPDs with a wide range of strip widths, the origin of dark counts, and the mechanism of photon detection in superconducting strip.
Superconducting single-photon detector (SSPD) is patterned from 4-nm-thick NbN film deposited on sapphire substrate as a 100-nm-wide strip. Due to its high detection efficiency, low dark counts, and picosecond timing jitter SSPD has become a competitor to the InGaAs avalanche photodiodes at 1550 nm and longer wavelengths. Although the SSPD is operated at liquid helium temperature its efficient single-mode fibre coupling enabled its usage in many applications ranging from single-photon sources research to quantum cryptography. In our strive to increase the detection efficiency at 1550 nm and longer wavelengths we developed and fabricated SSPD with the strip almost twice narrower compared to the standard 100 nm. To increase the voltage response of the device we utilized cascade switching mechanism: we connected 50-nm-wide and 10-μm-long strips in parallel covering the area of 10 μmx10 μm. Absorption of a photon breaks the superconductivity in a strip leading to the bias current redistribution between other strips followed their cascade switching. As the total current of all the strips about is 1 mA by the order of magnitude the response voltage of such an SSPD is several times higher compared to the traditional meander-shaped SSPDs. In middle infrared (about 3 μm wavelength) these devices have the detection efficiency several times higher compared to the traditional SSPDs.
In this paper, we present a large-area niobium titanium nitride (NbTiN) superconducting microstrip single-photon detector (SMSPD) fabricated using a photolithography process. We fabricated a 1-μm-wide NbTiN-SMSPD covering 420 × 420 μm2active area with a filling factor of 20% using an i-line stepper. The detector is cooled down to approximately 2.2 K, and its performance is evaluated at the wavelengths from visible to near-infrared. The detector operates stably without a shunt resistor and shows single-photon sensitivity for photons with a wavelength of 1550 nm. The bias current dependence of the detection count rate is measured at the wavelengths from 406 to 1550 nm. The saturation of the detection count rate with increasing the bias current was successfully observed at wavelengths below 850 nm, suggesting an internal detection efficiency approaching 100%. Furthermore, we evaluated the timing jitter performance using the femtosecond pulse laser with a wavelength of 1550 nm. Consequently, we achieved a system timing jitter of 85 ps for a light spot diameter of 100 μm.
Superconducting nanowire single photon detectors (SNSPD) have unique characteristics of ultra low dark counts and wide spectrum sensitivity. These natures are indispensable for the evaluation of ultra-broadband parametric fluorescence, which are used for the quantum optical coherence tomography and novel optical non-linear experiments. Here we report the spectral dependence of the detection efficiency of a meander type SNSPD device, having reduced strip width of 50 nm, over a wide spectrum range up to near infra-red wavelength. The fiber coupled, meander type device was fabricated using 6 nm thick Niobium nitride (NbN) nanowires of reduced strip width, 50 nm, patterned over a MgO substrate with active area of 10 x 10 μm2. A maximum efficiency of 32% at 500 nm, 30% at 600 nm, 16% at 800 nm, 10% at 1000 nm, and 1% at 1550 nm with the normalized bias current of 0.95 (bias 37 μA ) was observed at 4.2 K. The salient feature of the device is, it exhibits a very low dark count rate (DCR) of only 2 Hz at the standard operating bias of 37 μA and ultra low DCR of 0.01Hz at 34 μA. Moreover, at this reduced bias with 0.01Hz DCR, the detection efficiency is not appreciably decreased in the visible region (32% at 500 nm and 30% at 600 nm) and an order decrease is observed (0.1%) at 1550 nm. The noise equivalent power (NEP) is of the order 10-19 WHz-1/2 in the visible region and 10-17 WHz- 1/2 in the near IR region. Ultra-broad band parametric fluorescence of band width from 791 nm to 1610 nm generated by a quasi-phase matched (QPM) device was successfully detected with this SSPD.
We present high performance fiber-coupled niobium titanium nitride superconducting nanowire single photon detectors fabricated on thermally oxidized silicon substrates. By applying a double sided cavity structure, simulated absorptance in the nanowire meander reached 97% at 1550 nm wavelength. The fabricated device showed a system detection efficiency (DE) of 74% at 1550 nm, dark count rate of 100 c/s, and a full width at half maximum timing jitter of 68 ps under a bias current of 18.0 μA with a practical Gifford-McMahon cryocooler system.
We report the fabrication and characterization of a niobium titanium nitride (NbTiN)-based superconducting nanowire single-photon detector (SSPD or SNSPD) on the Si waveguide toward the realization of waveguide-coupled SSPD. The device consists of a silicon optical waveguide fabricated using a silicon-on-insulator (SOI) substrate and a 100-nm-wide NbTiN nanowire constructed on the waveguide, with an inductor to prevent latch operation. For the fabricated device, the observed superconducting critical temperature and critical current were 7.85 K and 34.4 μA at 0.3 K, respectively. We investigated the optical response of the fabricated device and found that the device exhibited a single-photon response by measuring of the input power dependence of the output count rate. Furthermore, the fabricated device showed a clear bias current dependence of the output counts similar to conventional SSPDs, and the pulse-generation probability is estimated to be 90.1% based on two assumptions: a sigmoidal model to the detection efficiency and the device has a 100% pulse-generation probability at the saturated region.
We have fabricated and characterized a niobium titanium nitride (NbTiN)-based superconducting nanowire single- photon detector (SSPD or SNSPD) on the silicon (Si) waveguide toward the realization of waveguide-coupled SSPD. The superconducting critical temperature and critical current were 7.85 K and 34.4 μA at 0.3 K, respectively. We investigated the optical response of the fabricated device and found that the device exhibited a single-photon response. Furthermore, the fabricated device showed a clear bias current dependence of the output counts similar to conventional SSPDs.