Four-Photon Process Induced Upconversion Lasing

It is measured that upconversion fluorescent intensity of a new molecule is proportional to the quartic of excitation density. This demonstrates that it is a four-photon absorption process. Furthermore, at high-density ultrashort pulse laser pumping, an emission appears to satisfy four criteria of lasing, including the highly directional emission, the short emission duration, the spectral narrowing, and changing to linear output/input relationship above a threshold. The experimental results demonstrate that the four-photon-process-induced amplified spontaneous emission occurs.