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Conference on Lasers and Electro-Optics Pacific Rim (CLEO-PR)

High efficiency single photon detection with optimized SNSPD and compressed beam

L. Zhang, Lin Kang, Jianhui Chen, Peiheng Wu

Published Aug 1, 2015• 0 citations
Publisher

Abstract

The efficiency is one of the most important parameter of single photon detector (SPD). The ideal SPD is expected to have 100% efficiency without false counts (always called dark counts). Improving the efficiency and reducing dark counts are interesting work for both scientific research and practical applications. Superconducting nanowire single photon detector (SNSPD) was intensively developed for its merit of low dark count, less than 1 count per second. However, its efficiency was limited by the optical absorption of superconductor nanowire. In this work, we analyzed the optical absorption of superconductor nanowire of SNSPD in theory. Then, we designed a SNSPD considering film growth and microfabrication process by optimizing the device structure, including the filling factor, films thickness, substrate index and cavity. With the SNSPD, a high absorption of 97% was calculated by FDTD method and a detector efficiency of 90% was achieved in experiments assisted by beam compressing settings.

DOI
10.1109/cleopr.2015.7376007

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The efficiency is one of the most important parameter of single photon detector (SPD). The ideal SPD is expected to have 100% efficiency without false counts (always called dark counts). Improving the efficiency and reducing dark counts are interesting work for both scientific research and practical applications. Superconducting nanowire single photon detector (SNSPD) was intensively developed for its merit of low dark count, less than 1 count per second. However, its efficiency was limited by the optical absorption of superconductor nanowire. In this work, we analyzed the optical absorption of superconductor nanowire of SNSPD in theory. Then, we designed a SNSPD considering film growth and microfabrication process by optimizing the device structure, including the filling factor, films thickness, substrate index and cavity. With the SNSPD, a high absorption of 97% was calculated by FDTD method and a detector efficiency of 90% was achieved in experiments assisted by beam compressing settings.
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.
Implementations of quantum information require single-photon detectors (SPDs) with high detection efficiency (DE) at a wavelength of 940 nm, which is a challenge for the available semiconducting SPDs. Superconducting nanowire SPDs (SNSPDs) are capable of detecting visible and near-infrared single photons with high DE. However, these detection capabilities place stringent design requirements on the cavity and nanowire geometry structures. We design, fabricate, and measure SNSPDs with high DE optimized for the 940-nm wavelength. The NbN SNSPDs were fabricated on 1-D photonic crystals for high optical absorptance. By tuning the filling factor of the nanowire through numerical simulations and experiments, we were able to obtain an SNSPD (7 nm thick, 125 nm width, and 0.57 filling factor, as well as active area of18∗18 μm18\ast 18\ \mu\text{m}18∗18 μm) with a saturated system DE of 83.6$\pm3.73.7%, at a dark count rate of 10 Hz, and a low polarization dependence of 1.173.7\pm$0.02. To our best knowledge, this is the highest value reported for NbN SNSPDs at 940-nm wavelength. The availability of an SNSPD with high system DE at 940 nm may have a profound impact in the field of photonic quantum technologies, such as multiphoton entanglement.
Superconducting nanowire single-photon detectors (SNSPDs) with a composite optical structure composed of phase-grating and optical cavity structures are designed to enhance system detection efficiency and count rates. Numerical simulation by finite-difference time-domain method shows that the photon absorption capacity of SNSPDs with a composite optical structure can be enhanced significantly by adjusting the parameters of the phase-grating and optical cavity structures. The absorption capacity of the superconducting nanowires reached 69.8% at the wavelength of 850 nm with 0.3 filling factor. When the filling factor was reduced to only 0.08, the absorption capacity is still 48.52%. It greatly decreased the kinetic inductance of SNSPDs, and improved the count rates.
Superconducting nanowire single photon detector (SNSPD) has been widely used in many fields such as quantum communication due to its extremely high detection efficiency, low dark count rate, high count rate, and low timing jitter. Compared with conventional single-photon detectors with planar structure, SNSPD is typically made a periodical meandering structure consisting of parallel straight nanowires. However, owing to its unique linear structure, the detection efficiency of SNSPD is dependent on the polarization state of incident light, thus limiting SNSPD’s applications in unconventional fiber links or other incoherent light detection. In this paper, a polarization-insensitive SNSPD with high detection efficiency is proposed based on the traditional meandering nanowire structure. A thin silicon film with a high refractive index is introduced as a cladding layer of nanowires to reduce the dielectric mismatch between the nanowire and its surroundings, thereby improving the optical absorption efficiency of nanowires to the transverse-magnetic (TM) polarized incident light. The cladding layer is designed as a sinusoidal-shaped grating structure to minimize the difference in optical absorption efficiency between the transverse electric (TE) polarized incident light and the TM polarized incident light in a wide wavelength range. In addition, the twin-layer nanowire structure and the dielectric mirror are used to improve the optical absorption efficiency of the device. Our simulation results show that with the optimal parameters, the optical absorption efficiency of nanowires to both of the TE polarized incident light and TM polarized incident light has a maximum of over 90% at 1550 nm, and the corresponding polarization extinction ratio is less than 1.22. The fabricated device possesses a maximum detection efficiency of 87% at 1605 nm and a polarization extinction ratio of 1.06. The measured detection efficiency exceeds 50% with a polarization extinction ratio less than 1.2 in a wavelength range from 1505 nm to 1630 nm. This work provides a reference for high-efficiency polarization-insensitive SNSPD in the future.
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.
In this paper, we theoretically show that a multi-layer superconducting nanowire single-photon detector (SNSPD) is capable of approaching characteristics of an ideal SNSPD in terms of the quantum efficiency, dark count, and band-width. A multi-layer structure improves the performance in two ways. First, the potential barrier for thermally activated vortex crossing, which is the major source of dark counts and the reduction of the critical current in SNSPDs is elevated. In a multi-layer SNSPD, a vortex is made of 2D-pancake vortices that form a stack. It will be shown that the stack of pancake vortices effectively experiences a larger potential barrier compared to a vortex in a single-layer SNSPD. This leads to an increase in the experimental critical current as well as significant decrease in the dark count rate. In consequence, an increase in the quantum efficiency for photons of the same energy or an increase in the sensitivity to photons of lower energy is achieved. Second, a multi-layer structure improves the efficiency of single-photon absorption by increasing the effective optical thickness without compromising the single-photon sensitivity.