Optical Calculations for Enhancing the System Efficiency of Superconducting Nanowire Single-photon Detectors
- Category: Optics & Photonics
- Tags: karl berggren, kristen sunter
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Download as PDFOptical losses limit the system detection efficiency of systems using superconducting nanowire single-photon detectors (SNSPDs)[1]. SNSPDs are typically fabricated with a meandering structure of niobium nitride to cover a large area, but the nanowire thickness is limited due to the need to effectively cool the detector during operation to prevent latching in the non-superconducting state[2]. Thus, it is important to concentrate as much of the intensity of the incident light as possible in the NbN layer. Integrating SNSPDs into optical stacks optimized for absorption in the NbN layer can lead to better detection of low photon flux IR sources, such as are required for VLSI circuit evaluation.
Previous designs for optical stacks include adding a quarter-wave optical cavity above the detector and an anti-reflection coating to the bottom of the substrate for use with back-illumination of the device through the substrate[3]. The optical stack structure can be further optimized by optimizing the thickness of the cavity, adding an optical layer between the substrate and the SNSPD layer, and exploring options for the control of the refractive index. Both numerical calculations using the matrix transfer method and finite element analysis using COMSOL MultiPhysics 4.2 are used to design optimized structures. Figure 1 shows an example of a transfer matrix calculation that demonstrates the increase in the absorptance in the NbN layer with thickness and with the fill factor in the layer. Figure 2 shows the expected absorptance of a 50% fill factor, 4-nm-thick device on a silicon nitride/silicon substrate (red line), where the silicon nitride layer thickness has been optimized. The addition of an optical cavity can enhance the absorptance by approximately a factor of two (compared to the case without an optical cavity, shown as the blue line).
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- Figure 1: The calculated absorptance in an NbN SNSPD versus the NbN thickness for devices with different fill factors on a magnesium oxide substrate: 30% (blue), 40% (green), 50% (red), 60% (black).
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- Figure 2: The calculated absorptance in the in NbN device layer on a silicon nitride/silicon substrate with a SiOx cavity and gold mirror on top (red line) versus the cavity thickness. The blue line shows the absorptance for the NbN device on SiNx/Si without a cavity.
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