Ultrafast optical nonlinearities and refraction enhanced optical limiting performance of azonaphthalene dyes

Abstract

Azo molecules represent a pivotal class of near-infrared dyes with extensive applications in energy storage/conversion and smart materials. While their nonlinear absorption (NLA) effects have garnered significant attention, the optical limiting (OL) mechanisms and performance of these molecules remain underexplored. This study investigates the near-infrared broadband nonlinear absorption and OL mechanisms of two azo derivatives (AZON-1 and AZON-2) using Z-scan measurement (620–900 nm) and transient absorption spectroscopy. The impact of molecular planarity differences on nonlinear optical responses is analyzed. Ultrafast dynamics are elucidated through transient absorption measurements. These measurements uncover photophysical mechanisms. Notably, AZON-2 exhibits intensity-dependent transitions from saturation absorption (SA) to reverse saturation absorption (RSA) at 620 nm, attributed to the interplay between SA and excited-state absorption (ESA). Both samples demonstrate robust NLA across 620–900 nm, primarily originating from ESA and two-photon absorption (TPA). The nonlinear responses under 650 nm laser irradiation in OL experiment are simulated. These results are validated via Z-scan techniques and correlated with DFT-derived molecular structure-property relationships. AZON-1 exhibits a low OL threshold of 11 mJ/cm² at 650 nm with an effective NLA coefficient of 2 × 10⁻¹³ m/W, basically consistent with the Z-scan results. For AZON-2, a synergistic SA-RSA fitting model reveals that cooperative SA-RSA effects enhance OL response speed. This work advances the understanding of nonlinear optical properties in azo derivatives, enriching experimental paradigms and providing design guidelines for organic materials with superior nonlinear optical performance.