Cryogenic Measurement Set-Up for Characterization of Superconducting Nano Structures for Single-Photon Detection Applications

We discuss the design and development of a cryogenic set-up down to ~1.8 K for carrying out measurements on superconducting nanowire single-photon detectors. The set-up consists of two separate low-temperature inserts, one for electrical characterization and the other for optical measurements and characterization of single-photon detectors. A sample holder with necessary arrangements for precise alignment of laser light with the active area of the device to enhance optical coupling efficiency has been designed for the optical probe. The invar alloy has been used for the sample holder to ensure that the alignment is not disturbed at low temperature. Single-mode fibres due to their high transmission rate, minimum attenuation and least distortion have been used to shine light on the samples. The sample holder with 20-pin LCC socket and matching chip carrier provides convenient and fast sample mounting in the electrical insert. Mumetal has been employed to cover the sample space in both the inserts in order to attenuate any electromagnetic interference. Temperature stability with the passage of time has been monitored. It has been found that variation in temperature is less than 10 mK at the lowest operating temperature. Another important advantage of the system is its low enough liquid helium loss rate (~100 ml/h) with all inserts, which allows uninterrupted measurements for several days without any refilling of liquid helium.