We demonstrate by the means of two dimensional two photon k-space spectroscopy that two photon absorption in CdSe nanoplatelets is highly directional. 85% of the concatenated absorption dipoles are in-plane oriented. We show that this strong anisotropy is related to competing two photon pathways with one inter- and one intra-band transition or one inter- and one inter-valence-band transition, as based on the crystal symmetry inter-, intra- and inter-valence- band transitions exhibit different transition dipole orientations. Based on the identified relevant transitions we explain the observed, up to now puzzling, quadratic volume scaling of two photon absorption cross sections in CdSe nanoplatelets, leading to record TPA cross sections. It is further shown, how excitonic correlation and coherence in these colloidal quantum wells lead to size tunable, directional, unprecedented TPA cross sections, opening up new perspectives for their usage e.g. in confocal two photon microscopy, orientation sensing or imaging.
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