Coherent dynamics of electrons after the interband photoexcitation with an ultrashort laser pulse has been studied in past years in various systems with tunneling between the two-dimensional 2D states. Both superlattices1, 2 and double quantum wells DQWs3–5 are under investigation. Direct measurements of the coherent submillimeter-wave emission caused by the interwell dipole moment oscillations, and the optical pump-probe measurements of phase coherence of such oscillations have been carried out. Characteristic features of coherent dynamics in such systems are connected with the existence of the tunneling frequency, and the coherent dynamics takes place when this frequency is higher than the relaxation rate. The features of the transient process during the photoexcitation depend on the optical pumping characteristics frequency and duration of the pulse as well as on the tunneling frequency. 6 For this reason, it is interesting to investigate triple tunnel-coupled quantum wells TQWs, where tunneling frequencies between the left (l) and central (c) wells, and between the central and right (r) wells may be different. Due to this difference, the transient evolution of the dipole moment demonstrates different behavior: harmonic oscillations, beats, and nonperiodic oscillations. Motivation of this article consists in determination of conditions for transition from the harmonic coherent regime which is realized in DQWs to the nonharmonic and even nonperiodic transient processes.

Theoretical analysis of the ultrafast excitation processes in DQWs has been done in a few articles. A model of three discrete levels without longitudinal motion has been used for explanation of the pump-probe experiments. 4 Numerical calculations of the DQWs dipole moment dynamics, which take into account the longitudinal motion of the photoexcited carriers and Coulomb interaction between them have been reported. 7 Transient evolution of the photoexcited electrons concentration and DQWs dipole moment, and its dependence on the light frequency, pulse duration p and splitting of