Mechanism of nonequilibrium optical response of high-temperature superconductors

This paper discusses the mechanism of the nonequilibrium optical response of high-temperature superconductors below ${\mathit{T}}_{\mathit{c}}$. Optical-response studies include pulsed-photoresponse measurements and modulated-reflectivity (transmission) measurements using femtosecond spectroscopy. A model is presented to explain the mechanism of the optical response based on the nonequilibrium dynamic transitions of electrons (quasiparticles and Cooper pairs) and phonons. These nonequilibrium transitions may cause flux motion due to activation by high-energy quasiparticles and phonons in the frame of BCS theory; moreover, these transitions may also change the kinetic inductance due to the reduction in the superconducting-electron density. Relaxation of the high-energy quasiparticles (generated by photons) through the electron-phonon and electron-electron scattering is rather fast: on the order of a picosecond, however, the speed limit of the photoresponse is governed by the phonon escape time. The results of the presented analysis suggest that further femtosecond-spectroscopy measurements may reveal more information about the superconducting anisotropic energy gap and band structure, pairing mechanism, quasiparticle-vortex interactions, and vortex energy structure in high-${\mathit{T}}_{\mathit{c}}$ superconductors. The results also strongly suggest that with proper optimization of device parameters (geometry and thermodynamic properties for fast heat removal, increasing pinning site density, critical current density, etc.), high speed (on the order of a ps response time), and sensitive detectors covering a broad electromagnetic spectrum (e.g., from ultraviolet to far infrared and beyond) can be developed.