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Applied Physics Letters

Superconducting nanowire single-photon detector in an optical cavity for front-side illumination

Burm Baek, Jeffrey A. Stern, Sae Woo Nam

Published Nov 9, 2009• 38 citations
Publisher

Abstract

We have integrated superconducting nanowire single-photon detectors (SNSPDs) with an optical cavity design for front-side illumination. Our optical cavity design increases the coupling efficiency of light to the nanowire detector and enables straightforward proximity fiber-coupling for a multichannel detector system. Using a confocal optical scanning technique, we measured a significantly enhanced optical absorptance of 73% in comparison with 20% in a typical bare nanowire device at 1550 nm and 3 K. Our method of fabrication of these devices on a silicon wafer and the local optical absorptance measurement are important steps toward developing next-generation SNSPD technology.

DOI
10.1063/1.3263715

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We have integrated superconducting nanowire single-photon detectors (SNSPDs) with an optical cavity design for front-side illumination. Our optical cavity design increases the coupling efficiency of light to the nanowire detector and enables straightforward proximity fiber-coupling for a multichannel detector system. Using a confocal optical scanning technique, we measured a significantly enhanced optical absorptance of 73% in comparison with 20% in a typical bare nanowire device at 1550 nm and 3 K. Our method of fabrication of these devices on a silicon wafer and the local optical absorptance measurement are important steps toward developing next-generation SNSPD technology.
Increasing the detection efficiency (DE) is a hot issue in the development of the superconducting nanowire single photon detector (SNSPD). In this work, a cavity-integrated structure coupled to the SNSPD is used to enhance the light absorption of nanowire. Ultra-thin Nb films are successfully prepared by magnetron sputtering, which are used to fabricate Nb/Al SNSPD with the curve of 100 nm and the square area of 4 × 4 μm 2 by sputtering and the lift-off method. To characterize the optical and electrical performance of the cavity-integrated SNSPD, a reliable cryogenic research system is built up based on a He 3 system. To satisfy the need of light coupling, a packaging structure with collimator is conducted. Both DE and the dark count rates increase with l b . It is also found that the DE of SNSPD with cavities can be up to 0.17% at the temperature of 0.7K under the infrared light of 1550 nm, which is obviously higher than that of the SNSPD directly fabricated upon silicon without any cavity structure.
We developed superconducting nanowire single-photon detectors with an optical cavity (OC-SNSPDs) for multichannel systems. For efficient coupling, the devices were installed in compact fiber-coupled packages after their substrate thickness was reduced from 400 to 45 microm. The measured detection efficiency (DE) measurement at different substrate thicknesses and the estimation of optical coupling efficiency indicated that approximately 98% of the input light beam could be radiated on a 15 x 15 microm2 nanowire area from behind the substrate. The DEs of a NbN OC-SNSPD system were observed to be 9.5% and 25% at 1550 nm and 1310 nm, respectively (dark-count rate: 100 c/s).
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.
Single photon detection advanced rapidly in the last decade. The superconducting nanowire single-photon detector (SNSPD) is an emerging technology, which offers low dark count rates, broadband sensitivity from visible to mid-infrared wavelengths and very high quantum efficiency. All these features are appealing for future emerging applications in remote gas sensing, quantum key distribution and quantum information processing. By enclosing the detector in an optical cavity, tuned to the operating wavelength, the total system detection of the SNSPD is improved. Based on the cavity design, the detector absorption is boosted at the desired experimental wavelength. Devices have been fabricated and tested, in this thesis, with enhanced detection efficiency at near-infrared, infrared and mid-infrared wavelengths. NbTiN detectors fabricated on Ta2O5 / SiO2 distributed Bragg reflectors (DBR) for front-side coupling exhibit a system detection efficiency of ≈ 90% at λ = 940 nm for the best device measured. SNSPDs were also fabricated in NbTiN on metallic mirrors for back-side coupling for 1550 and 2333 nm experimental wavelengths. The first design exhibited a system detection efficiency of ≈ 84.5%, at λ = 1550 nm for the best device measured. The second exhibited low system detection efficiency since the device suffered from unexpectedly high dark count rate mainly due to blackbody radiation at 2.2 K. However, it was possible to measure the timing jitter of this device at 350 mK indicating a FWHM of 84 ps. Another technique to improve the total system detection efficiency of the SNSPD, is by fabricating a hairpin-shaped nanowire atop a single mode waveguide. In this design, photons travel through the waveguide and via evanescent coupling are absorbed in the nanowire. These SNSPD devices have been designed to thread along single mode Si ridge waveguides using a hairpin-shaped nanowire structure, which offers enhanced coupling and absorption efficiency via evanescent coupling. These ultra-thin superconducting MoSi detectors are precisely patterned on top of a single mode Si waveguide (600 nm) with alignment error less than 40 nm. Low temperature electrical and nano-optical characterization was carried out using a two-stage pulse tube (PT) coldhead, using an additional 4He / 3He sorption pump, low vibration characterization of SNSPD devices at 350 mK was carried out. Characterised devices exhibit low dark count rate <5 counts per second and timing jitter of ≈ 83 ps full width half maximum (FWHM) at wavelength (λ) = 1550 nm and at critical current ≈ 30 µA.
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 designed superconducting nanowire single-photon detectors (SNSPDs) integrated with silver optical antennae for free-space coupling and a dielectric waveguide for fiber coupling. According to our finite-element simulation, (1) for the free-space coupling, the absorptance of the NbN nanowire for TM-polarized photons at the wavelength of 1550 nm can be as high as 96% by adding silver optical antennae; (2) for the fiber coupling, the absorptance of the NbN nanowire for TE-like-polarized photons can reach 76% including coupling efficiency at the wavelength of 1550 nm by adding a silicon nitride waveguide and an inverse-taper coupler.