Second- and third-order non-linear optical properties of multilayered structures and composites of C60 with electron donors

Second- and third-order non-linear optical properties of C60 based multilayered charge transfer structures with TPP, TPD and TPN molecules, acting as electron donors, are studied by the transverse optical second and third harmonic generation techniques. The studied structures are of ABABAB… and ABCABC… type, where layer A is made from electron accepting molecules (C60), layer B from electron donating (TPN, TPD or TPP) and layer C from neutral molecules (MgF2). The typical thickness of individual layers is about 2 nm. A significant increase of SHG is observed in ABCABC… type structures, where an effective charge transfer is expected with creation of a non-centrosymmetry in the perpendicular direction to the layers. The largest χ(2) (−2ω;ω,ω) susceptibility is obtained with multilayers containing rubrene (TPN). The SHG generation experiments performed on thin films with different thicknesses show that the non-linear optical response comes from the bulk material and not from the interfaces. A quadratic dependence of SHG intensity on input power is also observed, showing that the charge transfer takes place at the ground state. No increase of third-order NLO response is observed, as compared to the net materials.