High dynamic range cationic two-photon photopolymerization

Cationic-induced two-photon photopolymerization is demonstrated at 710 nm, using an isopropylthioxanthone / diarylidonium salt initiating system for the cationic polymerization of an epoxide. The polymerization threshold J2th is found to be approximately 1 GW/cm, with a dynamic range of > 100, i.e. the material can be fully polymerized at intensities > 100 times the threshold level without damage. The polymerization rate R is found to be proportional to the m=1.7 power of the intensity, or R = [C (J-J2th)] m = [C (J-J2th)] , which implies a significantly stronger localization of the photochemical response than that of free radical photoinitiators. R and J2th significantly improve when the concentration z of the initiator (onium salt) increases.