| In this thesis I present photoexcitation experiments on YBa2Cu 3O7−δ (YBCO) and niobium nitride (NbN) in order to demonstrate the viability of superconducting materials as optical detectors having the ultimate sensitivity, ultrafast response times, and broadband responsivity.; In the case of YBCO, we designed and implemented a comprehensive study of epitaxial YBCO thin films patterned into a microbridge structure so as to extract their intrinsic photo-induced carrier dynamics, in addition to demonstrating their excellent properties as photodetectors. Depending on if the microbridge was biased into the resistive or the superconducting state, a resistive electron heating or a kinetic-inductive photoresponse, respectively, was observed. Our cryogenic, 200-fs temporal resolution, and sub-millivolt sensitivity electro-optic sampling system was used to record the photoresponses. The two-temperature model was successfully fitted to the resistive electron heating photoresponses which allowed for the extraction of the electron-phonon and phonon-electron interaction time constants, while the Cooper pair breaking and quasiparticle recombination time parameters were obtained by successfully fitting the Rothwarf-Taylor model to the kinetic-inductive photoresponses. Our experiments also showed the photoresponses to be spectrally insensitive over the tested wavelength range of 0.4 μm to 0.81 μm. Further analysis also showed YBCO to have an effective quantum yield of ∼450, showing the promise of being a very sensitive detector.; To demonstrate a detector that posses single-photon detectivity, fast response times, broadband responsivity, good quantum efficiency, and high signal-to-noise ratio, we patterned a ultrathin NbN film into a sub-micron-wide strip and biased it close to the critical current value at a temperature far below its transition temperature. In this case a large, easily measurable voltage transient was generated when a photon was absorbed into the NbN strip. The response mechanism is due to a supercurrent-assisted hotspot formation that lead to a resistive barrier across the width of the strip and the generation of a voltage that is proportional to the resistance of the barrier. Our elegantly simple single-photon detector has already been found applicable in VLSI CMOS testing and to various quantum computational and quantum communications systems. |
