High-order harmonic generation by a driven mesoscopic ring with a localized impurity

We investigate theoretically the electron dynamics in a single-channel mesoscopic ring with a localized impurity subjected to picosecond linearly polarized asymmetric electromagnetic pulses. A nonequilibrium coherent population of electronic states that possesses a time-dependent polarization is induced. The associated radiation emission decays on a time scale determined by the system relaxation and hence lasts long, after the pulses have perished. We derive analytically and confirm numerically that the presence of an impurity influences strongly the time-dependent charge polarization and allows the generation of higher harmonics in the terahertz range.