Photon Number Resolution with an Iridium Optical Transition Edge Sensor at a Telecommunication Wavelength

We report the photon number resolution at a telecommunication wavelength using a fabricated iridium optical transition edge sensor (TES). Iridium is a chemically stable material, and hence, the iridium TES is expected to exhibit long-term stable device characteristics. Because of the material stability, iridium TES can be formed in relatively simple single-layer structure, which would exhibit uniform device characteristics if large-arrays are constructed in a future. An iridium TES with a sensitive area of 8 μm × 8 μm was fabricated via radio frequency magnetron sputtering, photolithography, and a lift-off technique. The device was cooled in a dilution refrigerator, and its characteristics such as current-to-voltage curve, power-to-voltage curve, and power-to-bath-temperature curve were investigated. The TES exhibited a transition temperature of 355 mK. The TES was irradiated with a pulsed laser source with a wavelength of 1528 nm. A fast response speed was obtained using the TES, and the dominant decay time constant was 761 ns. The photon number resolution was successfully performed, and the energy resolution was 0.464 eV in full width at half maximum.