Enhanced ultrafast cubic nonlinearity in the carbazole–picric acid complex for potential applications in photonic devices: Kerr nonlinearity with two-photon absorption

Carbazole-based complexes have garnered attention in ultrafast nonlinear optics due to their distinct molecular structures, which facilitate robust nonlinear interactions for promising photonic devices. In this context, we have synthesized a donor-acceptor-based carbazole-picric acid (Cz-PA) complex. Herein, the structure of the complex was confirmed by X-ray crystallography and infrared (IR) spectroscopy. Ultraviolet-visible (UV-vis) spectroscopy was conducted to investigate the photophysical properties. A Z-scan experiment was performed to explore the cubic nonlinearity under femtosecond (fs) pulsed laser excitation. The optical nonlinear measurements were carried out with variations in the peak intensity of the excitation laser. The material exhibits electronic Kerr-induced positive refractive nonlinearity (<i>n</i>₂ > 0) and positive absorptive nonlinearity (<i>β</i> > 0) induced by the self-focusing effect and two-photon absorption (2PA) process, respectively. Furthermore, we estimated enhanced cubic nonlinear susceptibility (<i>χ</i>⁽³⁾) in the order of 10^-21 m² V^-2 with second-order hyperpolarizability (<i>γ</i>) in the order of 10^-57 C m⁴ V^-3, and they showed an increment with peak intensity. The present material possesses comparably enhanced optical nonlinearity among the carbazole-based organic compounds. Additionally, the acquired values of the 2PA cross-section and figures of merit reveal the potential applications of Cz-PA in photonic devices with ultrafast switching.