Mechanism of Light Detection in Current-Biased Superconducting MgB2 Microbridges

We report on time-resolved, transient photoimpedance measurements performed on current-biased microbridges excited by 100-fs wide, 800-nm wavelength, and 76-MHz repetition-rate optical pulses. Our bridges had dimensions of 20 mum by 40 mum and were patterned by conventional photolithography and ion milling from 150-nm-thick, epitaxial MgB₂ films grown by a hybrid physical-chemical vapor-deposition technique. For high-speed detection experiments, the microbridges were embedded into coplanar-strip transmission lines and connected to a 20-GHz-bandwidth sampling oscilloscope via a semirigid coaxial cable. The test structures were placed inside an optical cryostat and studies were performed in the temperature range between 15 K and 40 K. At temperatures, e.g., 20 K, far below the bridge critical temperature and under low optical excitation, large and fast (130 ps in duration) photoresponse transients were observed, which we associate with the kinetic-inductive (Cooper-pair breaking) mechanism. At temperatures approaching the MgB₂ critical temperature and/or under intense light illumination, an additional slow component of the photoresponse with a nanosecond decay time was measured. We interpret this second photoimpedance component as a resistive response, resulting from the hot-electron heating effect. The observed picosecond kinetic photoresponse makes our MgB₂ bridges attractive candidates for fast and efficient optical detectors and photon counters.