Femtosecond Spectroscopy of Magneto-Excitons

Recently, quasi-2D excitonic (X) effects have been investigated extensively in semiconductor quantum well (QW), resulting in fundamental advances in our understanding of the effects of dimensionality reduction from 3D to 2D on the optical response of semiconductors, and have lead to numerous new applications in optoelectronic [1]. Efforts to make new semiconductor structures where electronic states are further confined in quasi-0 or quasi-1 dimensions, have had only very limited success because of the difficulty of obtaining narrow size-distribution of defect free samples. However, electronic states strongly confined in all dimensions, in material of excellent uniformity and optical quality, can be obtained by immersing a high quality QW well in a perpendicular magnetic field [2,3]. In such a situation, the crystal band discontinuities confine the electronic states in the direction perpendicular to the QW. In the QW plane, in addition to the Coulomb potential, -e2/εor, the e-h pairs experience the confinement of the quadratic-potential, (eHr)2/8mc2, imposed by the magnetic field, H. The relative strength of two confining potentials is measured by the dimensionless parameter, λ = (ao/lc)2, where ao is the Bohr radius and lc = (c/eH)1/2 is the cyclotron radius. The e-h pairs then form magneto-excitons (MX), which extrapolate continuously between quasi-2D excitons (λ = 0) and quasi-0D Landau levels (λ → ∞).