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Optics Express

Polarization-insensitive fiber-coupled superconducting-nanowire single photon detector using a high-index dielectric capping layer

Anna Mukhtarova, Luca Redaelli, Dibyendu Hazra, H. Machhadani, S. Lequien, M. Hofheinz, Jean-Luc Thomassin, F. Gustavo, Julien Zichi, Val Zwiller, Eva Monroy, Jean‐Michel Gérard

Published Jun 22, 2018• 19 citations
Publisher

Abstract

Superconducting-nanowire single photon detectors (SNSPDs) are able to reach near-unity detection efficiency in the infrared spectral range. However, due to the intrinsic asymmetry of nanowires, SNSPDs are usually very sensitive to the polarization of the incident radiation, their responsivity being maximum for light polarized parallel to the nanowire length (transverse-electric (TE) polarization). Here, we report on the reduction of the polarization sensitivity obtained by capping NbN-based SNSPDs with a high-index SiNx dielectric layer, which reduces the permittivity mismatch between the NbN wire and the surrounding area. Experimentally, a polarization sensitivity below 0.1 is obtained both at 1.31 and 1.55 µm, in excellent agreement with simulations.

DOI
10.1364/oe.26.017697

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Superconducting-nanowire single photon detectors (SNSPDs) are able to reach near-unity detection efficiency in the infrared spectral range. However, due to the intrinsic asymmetry of nanowires, SNSPDs are usually very sensitive to the polarization of the incident radiation, their responsivity being maximum for light polarized parallel to the nanowire length (transverse-electric (TE) polarization). Here, we report on the reduction of the polarization sensitivity obtained by capping NbN-based SNSPDs with a high-index SiNx dielectric layer, which reduces the permittivity mismatch between the NbN wire and the surrounding area. Experimentally, a polarization sensitivity below 0.1 is obtained both at 1.31 and 1.55 µm, in excellent agreement with simulations.
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.
Abstract In this paper, the design of superconducting-nanowire single-photon detectors which are insensitive to the polarization of the incident light is investigated. By using high-refractive-index dielectrics, the index mismatch between the nanowire and the surrounding media is reduced. This enhances the absorption of light with electric field vector perpendicular to the nanowire segments, which is generally hindered in these kind of detectors. Building on this principle and focusing on NbTiN nanowire devices, we present several easy-to-realize cavity architectures which allow high absorption efficiency (in excess of 90%) and polarization insensitivity simultaneously. Designs based on ultranarrow nanowires, for which the polarization sensitivity is much more marked, are also presented. Finally, we briefly discuss the specific advantages of this approach in the case of WSi or MoSi nanowires.
Superconducting nanowire single photon detectors (SNSPDs) deliver superior performance over their competitors in the near-infrared regime. However, these detectors have an intrinsic polarization dependence on the incident wave because of their one-dimensional meander structure. In this paper, we propose an approach to eliminate the polarization sensitivity of SNSPDs by using near-field optics to increase the absorption of SNSPDs under transverse magnetic (TM) illumination. In addition, an optical cavity is added to our SNSPD to obtain nearly perfect absorption of the incident wave. Numerical simulations show that the maximum absorption of a designed SNSPD can reach 96% at 1550 nm, and indicate that the absorption difference between transverse electric (TE) and TM polarization is less than 0.5% across a wavelength window of 300 nm. Our work provides the first demonstration of the possibility of designing a polarization-insensitive and highly efficient SNSPD without performing device symmetry improvements.
Superconducting nanowire single-photon detectors (SNSPDs) struggle to balance high system detection efficiency (SDE) and low polarization dependence. This work proposes a co-design of the NbN films, α -Si anti-reflection coating (ARC), and SiO 2 /Ta 2 O 5 distributed Bragg reflector (DBR) to reduce the polarization-dependent optical response of the nanowires. Simulations based on rigorous coupled-wave analysis (RCWA) and the finite-element method show absorptance above 99% for both transverse-electric (TE) and transverse-magnetic (TM) polarizations and absorptance above 90% over the wavelength range from 1530 to 1640 nm. Experimentally, the device achieves 97% SDE at 1530 nm (polarization sensitivity, PS = 1.1), PS < 1.3 over 1490–1640 nm, 39.5 ns recovery time, and 63.5 ps timing jitter. Retaining a conventional single-layer meander nanowire geometry, this design unifies high efficiency and polarization insensitivity, advancing SNSPD applications in quantum communications, free-space links, and single-photon imaging.
Polarization sensitivity is an intrinsic characteristic of the superconducting nanowire single-photon detector (SNSPD). It is caused by the periodical meander structure of the SNSPD, and may severely limit the scope of the SNSPD's applications. In this study, we have designed and fabricated a SNSPD with much reduced polarization sensitivity, based on a previously published theoretical result that involves high refractive index compensation materials. Using a home-built broadband automatic measurement system, the device detection efficiencies of the fabricated SNSPD have been measured for wavelengths ranging from 1200 to 1700 nm. The measurement results show that the polarization sensitivity of the fabricated device is greatly reduced, with the device detection efficiencies at 1550 nm being 61% and 56% for cases of parallel and perpendicular polarizations, respectively. The measurement results are in good agreement with the numerical simulations.
A highly efficient single-photon detector with strong polarization sensitivity is an important device in the field of quantum communication. Here, we present the design of a kind of superconducting-nanowire single-photon detector (SNSPD), in which the superconducting nanowire is embedded in a resonant cavity covered by a metasurface made of a coupled asymmetric split-ring resonator (CASRR). When illuminated by an incident light with different polarizations, CASRR will show totally different optical responses, which is used to obtain high polarization contrast in SNSPDs. Our simulation results show that an NbN nanowire meander with a filling factor of 0.2 can achieve an absorption efficiency of 84.3% at a wavelength of 1550 nm, when the electric field of the incident light is parallel to the NbN nanowire. Moreover, the polarization extinction ratio, i.e., the ratio of absorption rates between cases, where the electric fields are parallel and perpendicular to the NbN nanowire, is 522. Comparing with the results reported before, our SNSPD design demonstrates a much better performance.