Complex impedance of a transition-edge sensor with sub-μs time constant

Optical transition edge sensor (TES) detectors that can resolve the energy of a single optical photon have proven to be desirable in multicolor fluorescence microscopy. Here, detectors with a higher energy resolution can distinguish dyes having similar or closer emission wavelengths, thus enabling the observation of multiple kinds of dyes simultaneously. To improve energy resolution, it is necessary to know how different the measured energy resolution is from the limit determined by the temperature sensitivity αI and current sensitivity βI, as extracted from the complex impedance. Due to the very fast response of an optical TES (the time constant is shorter than 1 µs), the complex impedance must be measured up to frequencies larger than 10 MHz. However, at high frequencies, the parasitic impedance in the circuit and reflections of electrical signals caused by discontinuities in the characteristic impedance of the readout circuits become significant. To reduce these effects, twisted pair cables are replaced with coaxial ones in this work; thus, a cleaner transfer function of the readout at high frequencies is obtained. The measured impedance of the studied TES is consistent with that given by the single-block model.