Coherent Quantum Control Of Two-photon Absorption And Polymerization In ITX By Shaped Ultrashort Laser Pulses

With the development of femtosecond laser, photo-initiated polymerization via two-photon absorption (TPA) has attracted enormous research interests. It has been widely used in informatics, communications, microelectronics, materials and biology. Coherent control based on femtosecond laser pulse-shaping is one of the most efficient tool for the control of multiphoton absorption process in atomic, molecular and nanostructure in semiconductor systems. In this thesis, we focused on the studies of the coherent control on the multiphoton absorption in polymers initiatively, and the polymerization process by using the shaped ultrashort laser pulses:The research development of micro/nano-fabrication by using femtosecond laser, quantum coherent control of multi-photon absorption and the pulse-shaping technology were introduced respectively.(1) We introduced the basic setup and the physical theory for Fourier transform optical pulse shaping with programmable modulators. We calculated the influences of the phase functions in the temporal profile and the spectrum of the nth-order laser electric field. We calculated the TPA probability as a function of the π-phase step positions with changing the central frequencies and the bandwidths of the absorption spectrum. The influence of central frequencies of the excited shaped laser on the TPA probability was also studied.(2) We experimentally used a π-phase scan of800nm femtosecond laser pulses, and controlled effectively the TPA in polymer with a photo initiator isopropyl thioxanthone (ITX) and the photoresist of SU-8. The TPA probability and photoluminescence intensity can be decreased by78%and85%while the laser pulse energy was kept as a constant. (3) We experimentally used the shaped pulses in femtosecond laser fabrication. The experimental results demonstrated that the polymerization can be efficiently controlled by shaped ultrashort laser pulses. By using this method, the volume of fabricated microrod decreases to1/125compared with that induced by Fourier transform limit laser pulses, and it is less than1/25of the diffraction limit.(4) We calculated the (2+1) three-photon absorption and TPA probabilities of the polymeric molecular system ITX irradiated simultaneously by two femtosecond laser beams with central wavelengths of800nm and532nm, respectively. By phase-shaping the two pulses, the (2+1) three-photon absorption probability of ITX is continuously tuned from1%to250%, and the TPA probability can be coherent control simultaneously.