Strong nonlinear optical response in the visible spectral range with epsilon-near-zero organic thin films (Conference Presentation)

We demonstrate an efficient ENZ response in the visible spectral range using organic molecular ultrathin films possessing a Lorentz-type dispersion. For this purpose, two polymethine dyes: sodium [5,6-dichloro-2-[[5,6-dichloro-1-ethyl-3-(4-sulphobutyl)- benzimidazol-2-ylidene]-propenyl]-1-ethyl-3-(4-sulphobutyl)-benzimidazolium hydroxide] (TDBC), and [2,4-bis[8-hydroxy-1,1,7,7-tetramethyljulolidin-9-yl]squaraine] (HTJSq) were used in spin-coated polymer films at different doping concentrations. By varying the doping concentration in thin films, the real part of highly dispersive permittivity e1 can be manipulated and tuned such that the spectral width of ENZ region -1 < e1 <1 resides in the visible spectral range. These results are not only extremely relevant for applications requiring a custom-tailored ENZ region in the visible but also provide important novel information on how molecular aggregation affects the ENZ properties. In particular, based on our findings, we stress that J-aggregate is not always a mandatory molecular assembly for obtaining a strong ENZ response. Instead, molecular aggregates with the size of a few nanometers resulting in strong molecular interactions (i.e. Davydov splitting of the lowest transition in energy) are required to achieve a strong ENZ response. The ENZ-enhanced optical Kerr nonlinearity is then investigated in the optimum concentration films of TDBC and HTJSq. Both nonlinear refractive index and nonlinear absorption coefficient are found to be strongly enhanced in the ENZ region originating from the coupling of excitonic transition dipoles associated with large molecular aggregates.