Four-wave mixing from individual excitons: Intensity dependence and imaging



We use heterodyne spectral interferometry (HSI) to investigate the transient four-wave mixing field (FWM) from an individual localized excitonic transition. FWM intensity and spectrum as well as the spatial resolution in FWM hyperspectral imaging are measured as function of excitation intensity. A third-order non-linear response of the FWM followed by saturation and the onset of a Rabi-flop are observed with increasing intensity. The intensity dependence of the spectral response depends on the excitation pulse duration. For short pulses (∼0.2 ps), having a broad spectrum, a weak broadening of the emission line, similar to excitation induced dephasing (EID), is found. For long pulses (∼0.5 ps) instead, no signatures of EID are found, but individual multi-excitonic transitions are observed. When using the FWM intensity as image contrast by spatially scanning a region of the sample surface, a spatial resolution beyond the diffraction limit is measured in the third-order regime, while for higher intensities the FWM saturation deteriorates the spatial resolution. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)