Development of Superconducting Nanostrip X-Ray Detector for High-Resolution Resonant Inelastic Soft X-Ray Scattering (RIXS)

To achieve a resolving power greater than 100 000, we are developing a superconducting nanostrip single photon detector (SSPD) for an ultrahigh-resolution RIXS spectrometer in the Taiwan Photon Source. To achieve the target resolving power, the SSPD, combined with a microstripline (MSL) structure, which acts as a delay line, should have a line pitch of 0.5 μm. This study uses numerical simulations to design SSPD. It consists of a meander of a 50-nm-thick Nb nanostrip and a 45-nm-thick SiO2 underlayer. The width of the line and the gap of the meander are 400 and 100 nm, respectively. The estimated velocity of the transmitted pulses due to X-ray absorption in the Nb nanostrip of the above SSPD is ~5% of the speed of light in free space. A detection area of 16 mm × 5 mm is sufficient to allow a time difference of ~1 ns between neighboring nanostrips to be distinguishable using conventional time to digital converters (TDCs). We fabricated a trial SSPD imager with dimensions of 4 mm × 0.1215 mm, and measured signal pulse characteristics for ions simulating soft X-ray photons. During an experiment, the maximum time difference of 7.5 ns between output pulses from both ends of the nanostrip successfully exhibited the proper imaging operation.

I. Introduction

Superconducting nanostrip single photon detectors (SSPDs) have been developed to detect near-infrared and optical single photons. SSPDs exhibit remarkable recovery times and timing precisions that are orders of magnitude shorter than existing single photon detectors based on semiconductors and superconductors. [1]–[3] SSPDs consist of superconducting lines with sub-μm widths and several-nm to tens-nm thickness. Since SSPDs usually have meandering shapes, they can exhibit a one-dimensional spatial resolving power if the single strip of the meander where a single photon hits can be identified. The spatial resolution is equal to the line pitch of the meandering pattern, which is normally less than 1 μm. Hence SSPDs must be fabricated with a delay line structure inside to determine the position of the single strip line absorbing a single photon. [4] Fig. 1 illustrates the principle of the position determination. A photon striking a nanostrip generates an electric signal traveling towards both ends with opposite polarities. The signals arrive at both ends with different timings. A photon event position is determined by the difference in the arrival times (Δt = t₁- t₂). Fig. 1.

Image of a superconducting nanostrip photon detector. Velocity (V) in a nanostrip is determined by the inductance (L) and the capacitance (C) per unit length of the transmission line. t₁ and t₂ are the arrival time of signals 1 and 2, respectively. X₁ and X₂ are the length from a hot spot position to either end.