Novel photo-controllable third-order nonlinear optical (NLO) switches based on azobenzene derivatives

Abstract

Photo-controllable third-order nonlinear optical (NLO) switches are of high interest for extensive and significant applications. In this work, three new azobenzene-based photo-controllable third-order NLO switches were designed and synthesized. Before UV light irradiation, the compounds exhibit strong reverse saturation absorption (RSA). After irradiated by 365 nm light for 1 min, strong self-focusing behaviors (positive refraction) are observed. The transformations are reversible with good fatigue resistance. From UV–vis absorption spectra and 1H NMR spectra we can find that the transformations are caused by the reversible trans ↔ cis interconversion in their azobenzene moieties. The results of time-resolved pump-probe with phase object (POPP) experiment show that the mechanism of RSA behaviors is two-photon induced excited state absorption, and self-focusing behaviors are originated from Kerr induced excited state refraction. The different electron-density of frontier orbitals and the natural charges possibly lead the transformation of the third-order NLO response from RSA to self-focusing behavior. The photoswitch behaviors with the response in high-speeds, good sensitivity and properties change exceed the reported materials. This work provides a new exploration for the design of photo-controllable third-order NLO switches.