Comprehensive Analysis on the Z-Scan Response of Thermally Evaporated CuTPP Thin Films in Terms of H Aggregation and Charge Transfer Dynamics

Nonlinear absorption (NLA) and nonlinear refraction (NLR) properties of the Cu(II) 5, 10, 15, 20-tetraphenyl-21H, 23H-porphine (CuTPP) thin film at 532 nm are investigated by using Z-scan technique with a laser of pulse width, tau = 6 ns and 30 ps. Powder XRD study of the CuTPP thin film reveals the nanocrystalline structure of the material embedded in an amorphous background. AFM and FESEM images depict a uniform, densely packed thin film with an RMS value of 14.69 nm. A characteristic broad blue-shifted Soret absorption band with davydov splitting in the UV-visible spectrum and stacking of particles illustrated in the AFM image highlights the formation of H-aggregates. Emission peaks at 651 and 711 nm are attributed to de-excitation from T-2(1) and T-4(1) tripmultiplet states to the ground state. In the 30 ps pulse regime, the NLA dynamics is influenced by saturation of tripmultiplet states and the band filling effect, resulting in the nonlinear saturation absorption phenomenon. However, NLA at 6 ns pulse duration is influenced by the excited-state absorption of triplet states, resulting in the reverse saturable absorption phenomenon. Closed-aperture Z-scan depicts self-focusing nonlinearity (Delta n > 0) in the picosecond regime. In the nanosecond regime, the CuTPP thin film depicts a positive NLR at a lower intensity, and at a higher intensity, it shows a self-defocusing (Delta n < 0) NLR nature. The presence of the Cu2+ ion in the porphyrin structure results in inception of the charge transfer (CT) state and leads to distinctive ultrafast relaxation dynamics from the S-2(2) state, thereby resulting in exclusive nonlinear optical (NLO) properties. Van der Waals interaction through p-stacking along with high electron exchange between Cu and porphyrin results in enhanced NLO coefficient values. This highlights the prominence of the CuTPP thin-film derivative for new-generation optical devices.