G. Aichmayr

Polarization control of the nonlinear emission of semiconductor microcavities.

The degree of circular polarization ( Weierstrass p ) of the nonlinear emission in semiconductor microcavities is controlled by changing the exciton-cavity detuning. The polariton relaxation towards K approximately 0 cavitylike states is governed by final-state stimulated scattering. The helicity of the emission is selected due to the lifting of the degeneracy of the +/-1 spin levels at K approximately 0. At short times after a pulsed excitation Weierstrass p reaches very large values, either positive or negative, as a result of stimulated scattering to the spin level of lowest energy (+1/-1 spin for positive/negative detuning).

Carrier and light trapping in graded quantum-well laser structures

We investigated the carrier and light trapping in GaInAs/AlGaAs single-quantum-well laser structures by means of time-resolved photoluminescence and Raman spectroscopy. The influence of the shape and depth of the confinement potential and of the cavity geometry was studied by using different AlGaAs/GaAs short-period superlattices as barriers. Our results show that grading the optical cavity improves considerably both carrier and light trapping in the quantum well, and that the trapping efficiency is enhanced by increasing the graded confining potential.

Electric-Field Tuning of Spin-Dependent Exciton-Exciton Interactions in Coupled Quantum Wells

We have shown experimentally that an electric field decreases the energy separation between the two components of a dense spin-polarized exciton gas in a coupled double quantum well, from a maximum splitting of 4 meV to zero, at a field of 35 kVcm. This decrease, due to the fieldinduced deformation of the exciton wave function, is explained by an existing calculation of the change in the spin-dependent exciton-exciton interaction with the electron-hole separation. However, a new theory that considers the modification of screening with that separation is needed to account for the observed dependence on excitation power of the individual energies of the two exciton components.

The role of spin in interacting excitonic gases

The spin-dependent exciton-exciton interactions in GaAs quantum well (QW) systems have been studied, under resonant and non-resonant excitation conditions at liquid helium temperatures. In the former case, a pure exciton gas is created, whereas in the latter the excitons are surrounded by a charged electron-hole plasma. In a double QW structure, the distance between electrons and holes comprising the excitons could be externally tuned by applying an electric field perpendicular to the well planes. The interactions were studied in terms of their dependence on excitation intensity, excess energy, spin polarization and electron-hole separation using time- and polarization-resolved photoluminescence up-conversion spectroscopy.

Capture and confinement of light and carriers in graded-index quantum well laser structures

Abstract We investigated graded-index separate-confinement heterostructure lasers and compared their performance with a conventional quantum well laser by using Raman and time-resolved photoluminescence spectroscopy. We found that grading the index of the Ga 1− x Al x As/GaAs waveguide and increasing the Al content improves considerably both, the light and carrier trapping and reduces carrier escape from the active region.

Spin dynamics and spin-dependent interactions in semiconductor heterostructures

Abstract We present new aspects on the spin dynamics of carriers and excitons in semiconductor heterostructures. In doped wells, we show that the two spin components of an optically created two-dimensional electron gas are well described by Fermi-Dirac distributions with a common temperature but different chemical potentials. In intrinsic wells, where excitonic effects dominate the optical properties, we show that exciton–exciton interaction produces a breaking of the spin degeneracy in two-dimensional semiconductors, which can be tuned by the application of an external electric field.

Detuning dependence of polariton spin dynamics

We have investigated the polariton recombination and spin dynamics in the nonlinear emission regime as a function of the cavity–exciton detuning (δ). The relaxation to K ~ 0 states, after non-resonant excitation, is governed by polariton final-state stimulated scattering. The time evolution of the degree of polarization of the photoluminescence () shows a very rich and novel behaviour, never observed in the excitonic emission in bare quantum wells. attains its maximum at a finite time after the excitation with a light pulse instead of monotonically decaying to zero as in the case of bare excitons. Furthermore, the sign of is strongly dependent on the detuning: it is positive for δ > 0 and negative for δ < 0, showing an oscillatory behaviour for δ ~ 0. The change in the sign of with the detuning is correlated with an energy splitting between the σ+- and σ−-polarized components of the photoluminescence.

Spin-Dependent Exciton–Exciton Interaction in Quantum Wells under an Electric Field

We give conclusive experimental evidence that spin-dependent exciton±exciton interaction processes can be altered by directly modifying the spatial exciton structure applying an electric field. The previously observed energy splitting in spin-polarized, dense exciton gases can be varied by the electric field from a maximum of 4 meV to 0 meV. On the basis of an earlier developed theoretical model it is seen that inter-exciton scattering processes dominating at zero field are quenched and others are enhanced on increasing the field strength.