Nonlinear Optical Properties of Quasi-One Dimensional Magneto-Excitons
Application of a strong magnetic field to a bulk semiconductor reduces the dimensionality of the electronic excitations from 3D to 1D. This gives the opportunity to explore the nonlinear optical properties of one dimensional systems in high quality material, without size fluctuation while avoiding the fabrication of delicate samples.
In this talk we report the first investigations of ultrafast nonlinear optical spectroscopy of GaAs under magnetic field up to 12 Tesla.
We find that even the linear absorption is strongly modified by application of a magnetic field. The lowest energy magneto-excitons remain Lorentzian under magnetic field. However, the high energy magneto-excitons that appear at the onset of the Landau level transitions, become Fano-resonances due to quantum interference with the underlying 1D continua.
The nonlinear optical properties of the two species of excitons were investigated by ultrafast four wave mixing (FWM). The Time resolved amplitude (TR-FWM) and power spectra (PS) were measured as well as the time integrated amplitude (TI-FWM). It was found that the two types of excitons exhibit very contrasted nonlinear optical properties.
For the Lorentzian excitons, the magnetic field enables us to tune the strength of Pauli Blocking relative to that of Coulomb interaction. Signature of exciton-exciton interaction as well as quantum beats were observed. Surprisingly the spectrally resolved FWM exhibit extra resonances that are not seen in the linear spectra. They may be related to excited states of the excitons that, in the linear absorption spectra, are masked by broadening in the absorption edge.
The Fano resonances exhibit the most unusual temporal behavior either in coherent wave mixing or in their quantum beats with the Lorentzian excitons. While the TR-FWM and PS correspond to the same dephasing time, the TI-FWM is instantaneous! This reveals a new type of quantum interference which most likely originates from the fact that Coulomb interaction governs both the appearance of Fano resonance and their nonlinear response. This behavior has even more striking consequences on the quantum beats between Lorentzian and Fano magneto-excitons. Just a few percent of contribution associated with the latter can quench most of the emission of the former! These experimental results are new and so far not explained.