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

We present a comprehensive study of the light-induced transient nonequilibr ium kinetic inductive response of high-quality, epitaxial Y-Ba-Cu-O (YBCO) thin films. The test structures consisted of 20-30 mu m wide coplanar strip (CPS) transmission lines with 7-10 mu m separation, patterned in 100 nm th ick YBCO films grown on MgO and LaAlO3 substrates. Each CPS structure conta ined a 5 mu m wide, 7-10 mu m long microbridge. The photoresponse of the mi crobridge, biased far below the film critical current and temperature, and excited by 100 fs laser pulses, was measured using a cryogenic subpicosecon d electro-optic sampling system. The physical origin of the photoresponse i s attributed to the nonequilibrium quasiparticle generation and recombinati on 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 res ponse times and the corresponding material time constants to be the intrins ic dynamics of YBCO. Our results show that the quasiparticle recombination time is very weakly temperature dependent and there is no phonon-trapping e ffect in YBCO. The picosecond response of YBCO makes it a suitable material for THz digital and communication applications.