Abstract
Maximizing photon absorption into thin active structures can be the limiting factor for photodetector efficiency. In this work, a fiber-coupled tunable cavity is demonstrated, designed to achieve close to unity absorption of photons into a thin film superconducting nanowire single photon detector (SNSPD). A technique for defining a stable cavity between the end of a telecommunications optical fiber and a reflective substrate is described and realized. Cavity resonances are demonstrated both through the tuning of input wavelength and cavity length. The resulting optical cavity can tune the resonant absorption in situ over a wavelength range of 100 nm. This technique is used to maximize the single photon absorption into both a back-side-coupled Au mirror SNSPD and a front-side-coupled distributed Bragg reflector cavity SNSPD. The system detection efficiency (SDE) is limited by imperfections in the thin films, but in both cases we demonstrate an improvement of the SDE by 40% over bare fiber illumination.
Highlights
A limiting factor for superconducting nanowire single photon detector (SNSPDs)[1] to achieve unity system detection efficiency (SDE) has been achieving high absorption of incident light into the nanowire.[2]
The resulting optical cavity can tune the resonant absorption in situ over a wavelength range of 100 nm
The system detection efficiency (SDE) is limited by imperfections in the thin films, but in both cases we demonstrate an improvement of the SDE by 40% over bare fiber illumination
Summary
A limiting factor for superconducting nanowire single photon detector (SNSPDs)[1] to achieve unity system detection efficiency (SDE) has been achieving high absorption of incident light into the nanowire.[2] In previous work demonstrating the principles of an optical-cavity embedded SNSPD, a radiation absorptance for NbN of only 10%–21% depending on incident polarization at a wavelength of 1550 nm was calculated.[3] Improving this is key to achieving an efficient detection technology. One of the simplest routes to improve absorption is to reflect the unabsorbed light back through the detector by positioning a reflector behind the SNSPD. This can be thought of as allowing the photon a second pass of the absorber.[4]. By replacing the simple reflector with a distributed Bragg reflector (DBR), further enhancement for a given wavelength can be obtained,[5] with simulations showing above 83% absorption.[6]
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