Study Of The Ultrafast Optical Control In Multiphoton Absorption

Multi-photon absorption is a common nonlinear physical phenomenon in the interaction between the light and material, which is very important for the research of the level structure in excited state, laser induced inflorescence, chemical reaction control, and other various nonlinear optical processes. Quantum coherent control, referred to steer a quantum system towards a desired outcome by inducing a constructive or destructive interference among different optical pathways, has been widely utilized in the research of the multi-photon absorption in the past decades and achieved great developments and results. In this thesis, we studied the ultra-fast optical coherent control of the multi-absorption in atomic, molecular and ionic system, and the details are presented as follows:1) We briefly introduced the the concept of the multi-absorption, the principles and methods of the quantum coherent control and the ultra-fast pulse shaping technique, and the main work of this thesis;2) On the quantum control of the multi-photon absorption in atomic system, we studied the influence of the laser spectral bandwidth on the coherent control of the resonance-enhanced multiphoton-ionization photoelectron spectroscopy. We observed that the photoelectron signals cannot be obtained by scanning the single π phase step position when the measured position of the kinetic energy is not at the peak value, which is affected by the laser spectral bandwidth. We proposed the new method of the double π phase step scanning to achieve high resolution photoelectron spectroscopy at any measured position of the kinetic energy with any laser spectral bandwidth.3) On the quantum control of the multi-photon absorption in molecular system, we first studied the coherent control in molecular alignment and orientation. We proposed that the field-free molecular alignment can be induced by V-style spectral phase modulation and showed that the molecular alignment degree and its temporal structure can be precisely controlled by properly designing the controlling parameters characterizing the V-style spectral phase modulation. Next we studied the enhancement of the molecular orientation degree induced by the combined electrostatic/four-color laser fields. We analyzed the reason of the enhancing effect and demonstrated the relations between the enhancement of the orientation degree and the laser parameters.4) Secondly, we theoretically and experimentally demonstrated the control of the intermediate state absorption in (n+m) resonance-mediated multi-photon absorption process by the polarization-modulated femtosecond laser pulse. We derived the analytic solutions of the intermediate state absorption by polarized modulation and analyzed the correlations between the laser intensity and the intermediate state absorption. The theoretical results are verified finally by the multi-photon absorption in Coumarin 480 and IR125 dyes.5) On the quantum control of the multi-photon absorption in ionic system, we studied the up-conversion fluorescence generation in Er3+-doped nanocrystals by the square phase-shaped femtosecond laser pulse. The studies showed that multiple subpulse formation by the square phase modulation can create different excitation pathways for various up-conversion fluorescence generations. By properly controlling these excitation pathways, the multicolor up-conversion fluorescence can be finely tuned.