Pump-orientation-probe technique and its applications

The pump-orientation-probe technique is a recently-developed novel transient measurement technique, which has unique advantages in probing the ultrafast dynamics of charge separation in colloidal nanostructures. In this technique, the linearly-polarized pump pulse is applied to generating electron-hole pairs, and the circularly-polarized spin-orientation pulse is used to establish the electron spin polarization, whose dynamics is detected by monitoring the polarization change of the linearly-polarized probe pulse. Initially, the wavefunctions of the electron-hole pairs are spatially overlapped, and the lifetime of the electron spin is short because of the strong electron-hole exchange interaction. If the electrons or the holes are trapped by the surfaces of the colloidal nanostructures, the spatial separations between the electrons and the holes weaken the exchange effect, and thus the lifetime of the electron spin is largely lengthened. The evolutions of electrons and holes from their spatial overlap to separation can be revealed by monitoring the change of the electron spin dynamics. Based on the introduction of the conventional two-beam carrier pump-probe and spin pump-probe techniques, the features and optical layout of three-beam pump-orientation-probe technique are described in depth. The application to probing negative or positive photocharging in CdS colloidal quantum dots is taken for example and discussed in depth. Compared with the conventional time-resolved absorption or time-resolved fluorescence spectroscopy, the pump-orientation-probe technique can detect the dynamics of trapping electrons or holes and distinguish the type of charging state easily and directly, which has particular advantages under the high-power excitation condition. Further outlook of the three-beam pump-orientation-probe technique is also presented finally.