Showing papers by "Khaled Karrai published in 2001"

Reflected image of a strongly focused spot

Abstract: In various experimental situations light is focused on or near an interface between dielectric media. For some techniques such as confocal microscopy and laser tweezers the focused light is incident from the optically denser medium. If the numerical aperture (NA) of the focusing lens is sufficiently large, some field components of the focused light undergo total internal ref lection (TIR) at the interface. Under these conditions the ref lected wave is phase shifted with respect to the incident wave and the apparent ref lection point appears to be displaced beyond the

Luminescence quenching in InAs quantum dots

Abstract: We report how photoluminescence from self-assembled InAs quantum dots depend on pumping power and vertical electric field. The InAs dots, which are embedded in a capacitor-like structure, act as efficient trapping centers for excitons. At a high enough electric field, however, the photoexcited electrons tunnel out of the dots fast enough to quench the emission. For samples with two adjacent layers of vertically aligned dots, we find that the threshold voltage for quenching depends very strongly on the optical pumping power. In total contrast to this, we find no comparable effect for samples grown with a single layer of dots. We explain this in terms of efficient storage of electrons and holes in the double-layer samples.

Optical emission from single, charge-tunable quantum rings

Abstract: We have succeeded in preparing excitons with a specific charge in single semiconductor quantum rings. Buried InAs quantum rings are loaded with electrons from a reservoir through a tunneling barrier and an additional electron–hole pair is generated by optical excitation. Single rings are addressed with nano-optical techniques. We observe abrupt shifts in the emission energy as electrons are added one by one. Furthermore, the experiments provide unique insights into the interaction of electrons in semiconductor nano-islands with their environment.

Low-temperature scanning probe microscopy of surface and subsurface charges

Abstract: The operation of a cryogenic scanning force microscope is demonstrated with a sensitivity of about 50 fN/Hz at 5 kHz modulation. This microscope is used as an electrometer in noncontact mode in order to map the local electrostatic forces and capacitance of several nanostructures at 4.2 K. Capacitance imaging of nanostructured surfaces with subatto-Farad resolution is demonstrated.

Activation of single domains in (111) PZT and (100) SBT

Abstract: (111)Lead Zirconate Titanate PZT films (fully poled) of the composition PbTi0.75Zr0.25O3 have been investigated by Tapping Mode Atomic Force Microscopy (TMAFM). As expected, the PZT crystallites were partitioned into slabs of 90° domains with domain walls oriented along planes. The polarization c-axis of the domains was inclined by ∼35° towards the surface and tilted alternately by +45° or -45° to the surface normal. Mechanical activation of the surface with plasma-sharpened supertips with a force of ∼5 nN allowed to monitor reversible mechanical switching of single domains involving 180° polarization reversal of entire domain slabs. The observed elastic changes in surface topography amounted to ∼1 nm. Similarly, single domains in rhombohedral a-side oriented SrBi2Ta2O9 (SBT) crystallites were switched mechanically which was observed by a change in surface corrugation of ∼10 nm.