Photo-thermal double-crosslinked second-order nonlinear optical materials with high orientation stability

Second-order nonlinear optical (NLO) materials have become the core of photonic devices. The stability of chromophore arrangement is a key factor limiting the service life of materials. In this study, we employed a novel photo-thermal double-crosslinking networks to enhance the orientation stability of chromophore for the first time by integrating photo and thermal crosslinking techniques. Benzocyclobutene (BCB) and double bonds were introduced as crosslinked units on the polymer and chromophore, respectively. Thermal crosslinking is achieved through the Diels-Alder (D-A) reaction between BCB, while photo crosslinking is accomplished via a thiol-ene click reaction. The UV–vis spectra results demonstrate that photo-thermal double-crosslinking can provide a certain level of protection to the chromophore against decomposition at elevated temperature. Maximum electro-optic coefficient ( r 33 ) of BCB-based double-crosslinking networks is 23.3 p.m. V −1 (at 1.3 μm, 25 wt%). Little difference is observed between the results of photo-thermal double-crosslinking polymer ( r 33 = 5.8 pm V −1 , 10 wt%) and host-guest polymer ( r 33 = 6.7 pm V −1 , 10 wt% ), illustrating that the double-crosslinking reaction does not impair r 33 . The analysis of the thermal simulated depolarization (TSD) curve indicates that the orientation stability of the double-crosslinked network structure (T peak  = 162 °C) is significantly enhanced compared to single thermal crosslinking (T peak  = 158 °C) or single photo crosslinking (T peak  = 144 °C), owing to the dual fixation of the chromophore through both photo and thermal crosslinking processes. It could be concluded that the enhancement of orientation stability does not come at the cost of the electro-optic coefficient. In a word, photo-thermal double-crosslinked second-order nonlinear materials are expected to be a promising option to research NLO optical applications.