Ultrafast linear kinetic inductive photoresponse of YBa2Cu3O7−δ meander-line structures by photoimpedance measurements

Abstract

We report the experimental demonstration of the linear kinetic-inductive photoresponse of thin-film YBa 2 Cu 3 O 7−δ (YBCO) meander-line structures, where the photoresponse amplitude, full-width-halfmaximum (FWHM), and rise-time are bilinear in the incident optical power and bias current. This bilinear behavior reveals a trade-off between obtaining high responsivity and high speed photodetection. We also report a rise-time as short as 29ps in our photoimpedance measurements.The interaction of light with superconducting samples is long known to perturb superconductivity 1-4 , which can be used as a probing mechanism for optoelectronic applications 5-13 . In general, photons of energy greater than the Cooper pair binding energy (2∆) can initiate a chain of pair-breaking events resulting in a deviation of the quasiparticle and pair densities from their equilibrium values. Typically, these distributions depend on temperature, optical power and wavelength, thermal boundary conditions, and material properties such as electron-electron and electron-phonon interactions times, electron density, coherence length, penetration depth, and geometry 6,14 . While determining the spatial and temporal distribution of quasiparticles and pairs under a time-varying optical illumination is a profound problem in non-equilibrium superconductivity, many of the important concepts of such an interaction for device applications can be captured by means of a much simpler and more phenomenological approach, namely the kinetic inductance model 13 . Within the kinetic inductance model the presence of the superconductive condensate, at a macroscopic level, can be adequately modeled by an additional inductive channel for charge transport. The optically initiated pair breaking mechanism, within this framework, should be interpreted as the spatial and temporal variations of the kinetic inductance and the normal resistance of the superconducting specimen.Many researchers have experimentally studied the kinetic inductive photoresponse of superconducting thin films through photoimpedance measurements 6,15 . In photoimpedance measurements, light induced changes in the microwave impedance of the superconducting structure are measured by an external high-frequency circuit. In its simplest form, the specimen is externally biased with a dc current and connected to a fast oscilloscope in series with a high bandwidth amplifier; absorption of optical photons then changes the impedance of the sample and produces a transient voltage response. A number a) Author to whom correspondence should be addressed. Electronic address: haatikia@maxwell.uwaterloo.ca of previous works have reported photoimpedance measurements on different superconductors mainly concluding that: 1) the resistive photoresponse dominates at temperatures well below the critical temperature (T c ), whereas the kinetic inductive response becomes the main mechanism of photoresponse close to T c ; 2) In the kinetic inductive regime the photoresponse could be very fast, with a rise time as low as 50...

Photoresponse of a La1.85Sr0.15CuO4nanostrip

“…We need high quality ultrathin films for SSPDs; with the thickness of about 5nm, a sharp superconducting transition, and a smooth surface. There have been several reports towards the realization of SSPDs using YBa 2 Cu 3 O 7- films with T c about 90 K [11][12][13][14][15]. However, as the thickness decreases to 5 nm, the quality of the films strongly deteriorates [14].…”