Ultrafast coherent dynamics in optically driven BCS systems

Papenkort T, Kuhn T, Axt VM

Abstract

We present simulations of the nonlinear ultrafast dynamics of a BCS superconductor. Pump-probe measurements and a coherent control technique are employed to gain insight into the excitation dynamics of the superconductor. In pump-probe spectra the energy gap of the superconductor is visible as an area of low absorption followed by a sharp peak. Excitation with a short laser pulse lowers the gap and thereby shifts the peak. However, the numerical calculations reveal the emergence of a nonadiabatic regime on short time scales. A sufficiently short pump pulse causes a fast oscillation of the modulus of the BCS order parameter. Although this parameter can usually be identified with the energy gap, its oscillation remains hidden in pump-spectra, i.e., the gap observed in the spectra is independent of the delay time. Instead the gap in the spectra corresponds to the mean value of the oscillation. By making use of a coherent control technique involving a second pump pulse, this oscillation may be made visible in probe spectra. This is because the reaction of the BCS system to the second pump pulse depends strongly on its instantaneous state at the time of the impact of this pulse. In particular the gap observed in the spectra after the second pump pulse for varying pump-pump delay times mimics the oscillation of the order parameter after the first pump pulse. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim