Ultrafast third-order nonlinear optical properties of self-assembled PCPDTPhSO₃Na/C₇₀-(EDA)₈ periodically overlapping film

A set of ultrathin multilayer films with alternated layers containing Poly[4,4-bis(4-sulfobutyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl]-1,4-phenylene sodium sa (PCPDTPhSO₃Na) as donor and amine group modified fullerene derivative C₇₀-(ethylenediamine)₈ (C₇₀-(EDA)₈) as acceptor were prepared using layer-by-layer assembly technique on a quartz substrate. Driven by the electrostatic force, C₇₀-(EDA)₈ and PCPDTPhSO₃Na were alternately adsorbed on the substrate to form donor–acceptor periodically overlapping structure. The growth of the film was monitored by UV-Vis absorption spectra, showing regularly consecutive deposition process. The ultrafast third-order nonlinear optical properties of the films were investigated via a 4f coherent imaging system with phase object (NIT-PO) with 190 fs laser pulses at 400 nm. Experimental results demonstrate that the ultrathin films exhibit excellent self-focusing effect (n₂∼4.02 × 10⁻¹⁶m²/W) and reverse saturable absorption properties (β∼7.75 × 10⁻¹⁰m/W). Our studies indicate that the PCPDTPhSO₃Na/C₇₀-(EDA)₈ film has significant potential for applications in future nonlinear optical devices.