Femtosecond nonlinear-optical response of quantum wells in strong magnetic fields

Abstract

The near-band-edge nonlinear optical response of semiconductors is dominated by exciton-exciton interactions and anarmonicities in the exciton-photon and exciton-phonon couplings. These mechanisms depend critically on dimensionality, as is demonstrated by nonlinear optical studies of quantum wells.1 A magnetic field applied perpendicularly to a quantum well confines the electronic states within the plane of the layer, so that confinement is obtained along all three directions. For large magnetic fields the magnetic confinement length can be made smaller than the excitonic Bohr radius. The quasi-two-dimensional states of the quantum wells can thereby be continuously tuned to quasi-zero dimensions without introducing additional material inhomogeneities. The first effect of such confinement is a change of the optical nonlinearities associated with many-body interactions. These changes are manifested in altered oscillator strengths, altered transition energies, and broadened electronic resonances. The second effect is a modification of particle-particle scattering. Changes in these interactions affect the kinetics of relaxation by modifying the scattering channels available to a nonthermally excited system. We use femtosecond nonlinear optical techniques to measure the manybody and kinetic response of magnetically confined electronic states as the quasi-two-dimensional states are continuously tuned to quasi-zero dimensions.