Nonequilibrium kinetic inductive response of Y-Ba-Cu-O photodetectors

We present a comprehensive study of the light-induced transient nonequilibrium kinetic inductive response of high-quality, epitaxial Y-Ba-Cu-O (YBCO) thin films. The test structures consisted of 20-30 µm wide coplanar strip (CPS) transmission lines with 7-10 µm separation, patterned in 100 nm thick YBCO films grown on MgO and LaAlO₃ substrates. Each CPS structure contained a 5 µm wide, 7-10 µm long microbridge. The photoresponse of the microbridge, biased far below the film critical current and temperature, and excited by 100 fs laser pulses, was measured using a cryogenic subpicosecond electro-optic sampling system. The physical origin of the photoresponse is attributed to the nonequilibrium quasiparticle generation and recombination effect and fitted with the Rothwarf-Taylor model. Our measurements show 1.9 ps wide bipolar waveforms for the LaAlO₃-based samples and 2.2 ps wide waveforms for the MgO-based samples. We regard the measured microbridge response times and the corresponding material time constants to be the intrinsic dynamics of YBCO. Our results show that the quasiparticle recombination time is very weakly temperature dependent and there is no phonon-trapping effect in YBCO. The picosecond response of YBCO makes it a suitable material for THz digital and communication applications.