NbN superconducting nanowire single-photon detector fabricated on MgF ₂ substrate

The performance of superconducting nanowire single-photon detectors (SNSPDs) relies on substrate materials. Magnesium fluoride (MgF 2 ) exhibits outstanding optical properties, such as large optical transmission range and low refractive index ( n = 1.38), making it an attractive substrate. We present the fabrication and the performance of SNSPDs made of a 4.5 nm thick NbN thin film deposited on MgF 2 substrate for the wavelength of 1550 nm. The front-side illuminated SNSPDs without an optical cavity showed a maximal detection efficiency of 12.8% at a system dark count rate (DCR) of 100 Hz, while the backside illuminated SNSPDs with a SiO 2 /Au optical cavity atop displayed a maximal detection efficiency of 33% at a DCR of 100 Hz.