High-performance Superconducting Transition- edge Single-photon Detectors(Invited)

Superconducting Transition- edge Sensor (TES) is a thermal detector that measures the deposited energy by changing the superconducting film's resistance. Superconducting TES- based singlephoton detectors with high detection efficiency,low dark count rate,photon-number and arriving-time resolving capability, are ideal detectors for faint and rapidly varying sources,playing a unique role in studying pulsars,neutron stars,white dwarfs and exoplanets. The critical temperature (TC) is the key parameter that determines the energy resolution (.EFWHM) of superconducting TES single-photon detectors,which can be finely tuned by baking the superconducting film or fabricated devices in the air. We studied the characteristics of titanium and titanium/gold bilayer films and the dependence of T C on baking time (t(baking)) and baking temperature (T (baking)). It was found that the T (C) is logarithmically decreased with t (baking) for a fixed T (baking),while it is exponentially decreased with T (baking) for a fixed t(baking). By treating the phaseslip parameters as variables, we extended the two-fluid model to extract the key parameters of superconducting TESs including temperature and current sensitivity coefficients from the measured currentvoltage curves at different bath temperatures. By adding an M factor to take the excess noise into account, the simulated Delta E (FWHM) is consistent with the measured one. We then calculated the T C dependence of Delta E (FWHM) for a 20 mu mx20 mu m Ti TES device and found that superconducting TESs with a T C of less than 170 mK can easily discriminate the photon numbers with a good Delta E-FWHM. Finally, we designed and fabricated titanium- based superconducting TES single-photon detectors, which were embedded in an optical cavity to improve the absorption efficiency. The optical cavity is composed of a dielectric mirror with 8 periods of Ta2O5/SiO2 layers and an anti- reflection coating with 2 periods of Ta2O5/SiO2 layers. The refractive index of Ta2O5,SiO2 and titanium film were measured with an ellipsometer,thus the calculated absorption efficiency of the optical cavity at 1 550 nm is nearly 100%,which is in agreement with the measured value with an IR spectrometer. The fabricated superconducting TES single- photon detector with an active area of 20 mu mx20 mu m is aligned to a single-mode fiber with the help of an IR microscope and a two-dimensional moving stage,which reached a coupling efficiency of nearly 100%. A 1 550 nm pulsed light source is used to measure the optical response. To measure the system detection efficiency(eta(sys)),we used a calibrated power meter and two precision attenuators to determine the average photon numbered incident (mu(in)) on the superconducting TES. The detected photon number (mu(out)) was obtained from the height histogram with a large count of pulse responses. Then eta(sys) is the ratio of mu (out) to mu(in), eta(sys)= mu(out)/ mu(in). We realized high-performance superconducting TES single- photon detectors with a. sys of higher than 90% and an Delta E (FWHM) of 0.5 eV at 1 550 nm. In addition,we developed a superconducting TES single-photon detector to detect 850 nm photons. The thickness of titanium film was chosen to be 53 nm and its T C is beyond 400 mK,which makes possible to cool it with just a He- 3 sorption cooler instead of a dilution refrigerator. While working at 300 mK,the superconducting TES single- photon detector with a eta(sys) of 13% and an. E FWHM of 0.75 eV can still resolve 850 nm photon numbers. Such superconducting TES single photon detectors with high eta(sys),good Delta E (FWHM) and short recovery time pave the way for the detection of rapidly varying sources.