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An analytical investigation of resonant impurity and exciton states in a narrow quantum well ͑QW͒ is performed. We employ the adiabatic multisubband approximation assuming that the motions parallel and perpendicular to the heteroplanes separate adiabatically. The coupling between the Coulomb states associated with the different size-quantized subbands ͑N =1, 2, …͒ is taken into account. In the two-and three-subband approximation the spectrum of the complex energies of the impurity electron and the exciton optical absorption coefficient are derived in an explicit form. The spectrum comprises a sequence of series of quasi-Coulomb levels ͑n͒ where only the series belonging to the ground subband N = 1 is truly discrete while the excited series N ജ 2 consist of quasi-discrete energy levels possessing non-zero widths ⌫ Nn . Narrowing the QW leads to an increase of the binding energy and to a decrease of the resonant energy width ⌫ Nn and the resonant energy shift ⌬E Nn of the impurity electron. Displacing the impurity center from the midpoint of the QW causes the binding energy to decrease while the width ⌫ Nn and the corresponding shift ⌬E Nn both increase. A Lorentzian form is recovered for the exciton absorption profile. The absorption peak is narrowed and blue shifted for a narrowing of the quantum well. A successful comparison with existing numerical data is performed. For GaAs QW's it is shown that the resonant states analyzed here are sufficiently stable to be observed experimentally.

We demonstrate an instantaneous all-optical manipulation of optical absorption in InGaAs/GaAs quantum dots (QDs) via an electro-absorption effect induced by the electric field of an incident free-space terahertz signal. A terahertz signal with the full bandwidth of 3 THz was directly encoded onto an optical signal probing the absorption in QDs, resulting in the encoded temporal features as fast as 460 fs. The instantaneous nature of this effect enables femtosecond all-optical switching at very high repetition rates, suggesting applications in terahertz-range wireless communication systems with data rates of at least 0.5 Tbit/s.

In this work we describe the ultrafast excitation kinetics of a biased quantum well, arising from the optically induced dynamical screening of a bias electric field. The initial bias electric field inside the quantum well is screened by the optically excited polarized electron-hole pairs. This leads to a dynamical modification of the properties of the system within an excitation pulse duration. We calculate the excitation kinetics of a biased quantum well and the dependency of resulting electronic and optical properties on the excitation pulse fluence, quantum well width, and initial bias field strength. Our calculations, in particular, predict the strongly nonlinear dependency of the effective optical absorption coefficient on the excitation pulse fluence, and ultrabroadband terahertz emission. Our theoretical model is free of fitting parameters. Calculations performed for internally biased InGaN / GaN quantum wells are in good agreement with our experimental observations ͓Turchinovich et al., Phys. Rev. B 68, 241307͑R͒ ͑2003͔͒, as well as in perfect compliance with qualitative considerations.

We present an analytical investigation of quasi-one-dimensional excitons in thin uniform ͑single͒ and double nanoscaled cylindrical quantum wires ͑UQWR and DQWR͒ surrounded by a barrier of infinite height and exposed to external electric and strong magnetic fields. The DQWR is formed by inserting an impenetrable longitudinal barrier in a single-quantum wire. Both external fields are directed parallel to the quantum wire ͑QWR͒ axis. The radius of the QWRs and the magnetic length are taken to be much less than the exciton Bohr radius. For the dependencies of the positions and widths of the complex quasidiscrete energy levels of the indirect exciton in the DQWR, in which the carriers are separated by the insertion on the confinement, electric field strength and width of the interwire barrier are derived. The confinement ͑insertion͒ leads to an increase ͑decrease͒ of the exciton binding energy. The impact of the electric field ionization of the exciton is less pronounced for strongly confined and weakly separated carriers. The coefficient of the exciton absorption in the UQWR as a function of the confinement and electric field is calculated in an explicit form. The effect of the confinement and electric field on the exciton peak closely resembles that on the quasidiscrete level of the indirect exciton in the DQWR. Electron-hole attraction increases remarkably the optical Franz-Keldysh electroabsorption in the frequency region below the edge and distant from the exciton peaks. The coefficient of absorption reflecting the electric field ionization and autoionization caused by the coupling between the discrete and continuous exciton states adjacent to the different size-quantized or Landau levels is obtained analytically. A comparison of our analytical results with numerical data is performed. Estimates of the expected experimental values for the parameters of GaAs/GaAlAs QWR show that the autoionized exciton magnetostates in thin biased QWRs are sufficiently stable to be observed.