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Authors: A.W. Achtstein, J. Hennig, A. Prudnikau, M. Hardzei, M. Artemyev, U. Woggon Institute of Optics and Atomic Physics, Technical University of Berlin, Berlin, Germany Institute for Physico-Chemical Problems, Belarussian State University, Minsk, Belarus Semiconductor nanoparticles have unique properties due to spatial confinement and shape control affecting the exciton wave function in the linear and nonlinear optical regime. This work studies the optical nonlinearities in colloidal 0D and 1D II-VI semiconductor nanocrystals. To understand the nonlinear effects at a nanoscale, it is necessary to investigate the spatial confinement and its influence on the nonlinear optical properties. The expected strong geometry and size dependence of the two photon absorption (TPA) cross-section has been tested by measuring a size series of colloidal CdS dots and rods starting from a near 0D system to long 1D nanorods in a z-scan setup. A pronounced change of TPA cross sections at the transition from dots to elongated rods has been found. While the bulk TPA coefficient of CdS is only 17 GM/nm3, it grows to above 100 GM/nm3 for dots and random oriented rods, whereas oriented rods can reach more than 1000 GM/nm3. The contributions of spatial confinement and local field effects of the dielectric environment are evaluated separately to get a deeper insight in the pure confinement effect on TPA. Additional Two-Photon Luminescence Excitation (TPLE) measurements reveal the spectral dependence of the TPA coefficients in those nanosystems for both resonant and non resonant excitation and show good agreement with the results of TPA measurements. Author for correspondence: alexander.achtstein@tu-berlin.de |
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