Two-photon Pumped Laser Based On Ultrafast Energy Transfer

Frequency up-converted lasing pumped by the process of multi-photon absorption is promising for the myriad applications in biomedical optics. It has significant advantages over the conventional method of sum-frequency generation for the applications where the phase-matching condition cannot be fulfilled. The pump source in near infrared wavelength for multi-photon-pumped laser can penetrate deeply into tissues, making it feasible to do in vivo operation. The multi-photon excitation has a better spatial confinement, which is also instrumental for targeted treatment.Nevertheless, there are still critical challenges remaining towards practical applications of multi-photon-pumped lasers. Ideal gain medium for up-converted lasing should hold the ease of establishing population inversion with efficient multi-photon absorption, which makes most of the conventional laser materials unsuitable for the application. Therefore in order to solve the problem, it is necessary to search the gain material with the potential for low threshold multi-photon-pumped laser.In this thesis, I introduce our exploration to address this issue with a new material design scheme of energy transfer. We try to design a hybrid gain medium with efficient two-photon absorbing donors and high gain acceptors. The strong coupling ensures an efficient energy transfer from the donor to acceptor that can be used to realized multi-photon-pumped lasers with low excitation threshold.This scheme of energy-transfer pumping is verified in this thesis with polymer gain medium of MEH-PPV as acceptor and two-photon-absorbing dye as donor. More specifically, we obtained the results including:(1) effective two-photon absorption is enhanced in the hybrid polymer/dye medium as measured by the method of z-scan;(2) Optical gain is observed at the up-converted frequency domain by the femtosecond transient absorption spectroscopy. The process of energy transfer is confirmed to be responsible for the gain.(3) The optical gain coefficient is measured to25cm-1with the technique of variable stripe length test in the optimally-doped hybrid system, which is comparable to the value in the neat polymer under one-photon excitation.(4) The excitation threshold (0.13mJ/cm2) for amplified spontaneous emission in the optimally-doped hybrid system is obtained to be over one-order lower than the magnitude in neat films, which can be incorporated onto distributed feedback structures to demonstrate low threshold two-photon-pumped lasers.