1. Introduction
Transforming near infrared irradiation into visible or ultraviolet irradiation is required in many fields such as data storage, fluorescence imaging, optical sensor and so on [1], [2], [3] and [4]. So far, these investigations mainly focused on second harmonic, third harmonic, etc. However, relatively small nonlinear coefficients of current materials have deferred their practical applications. Another feasible method is multiphoton-absorption-induced upconversion fluorescence, by which large nonlinear absorption and emission can take place with small nonlinear coefficient. In the recent years, systematic investigation has been made on two-photon-absorption-induced upconversion fluorescence, and many important conclusions have been achieved. More recently, three-photon absorption (3PA) in organic molecules has attracted much interest in the scientific community because of their potential applications [5], [6] and [7].