Study On Coherent Control Of Atom And Molecular Dynamics By Pulse Shaping

Coherent control based on femtosecond laser pulse shaping has been proven to be an efficient control technique to investigate the nonlinear laser-material interaction, which has been applied in Physics, Chemistry and Biology. In this thesis, we focus on studying the coherent control of non-resonant two-photon transition, single-photon fluorescence, coherent anti-Stokes Raman scattering, resonant mediate multiphoton absorption and resonant enhanced multiphoton ionization by pulse shaping.(1) The development of quantum coherent control and the physical basis of pulse shaping were systematically reviewed.(2) We experimentally and theoretically studied the coherent control of non-resonant two-photon transition in molecular system. We demonstrate that, by shaping the femtosecond pulses with simple phase patterns of cosinusoidal andπphase step, the non-resonant two-photon transition probability in a molecular system can be reduced but not completely eliminated because of the broad absorption line and complicated energy level. Then, we investigated the influence of the absorption bandwidth and line shape on the coherent control of non-resonant two-photon transitions. If the absorption bandwidth is less than twice of the laser spectral bandwidth, a coherent feature with a coherent peak and two coherent dips can be achieved. The absorption bandwidth decides the coherent peak or dip intensity, and the absorption line shape affects the symmetry of the two coherent dips. Furthermore, we investigated the coherent enhancement of the non-resonant two-photon fluorescence (TPF) by shaping the femtosecond pulse with the phase jump, which could be attributed to the wave-packet constructive interference in the excited states.(3) Coherent enhancement of single-photon fluorescence in IR125 and IR144 were achieved by polarization modulation and spectral phase modulation, respectively. We demonstrated that the enhancement of single-photon fluorescence results from the non-resonant two-photon absorption of a higher excited state.(4) We showed that, by tailoring the probe pulse, the CARS signal of neighboring Raman energy levels can be distinguished from their indistinguishable spectrum, and then their selective excitation and suppression can be realized by supplementally modulating the pump (or Stokes) pulse withπphase step. Furthermore, we theoretically and experimentally showed that, by shaping both the pump and probe pulses with theπphase step, the CARS signal from one quantum system can be enhanced and simultaneously that from the other quantum system is effectively suppressed. Comparing with shaping only the probe pulse, our scheme can greatly improve the selectivity.(5) We theoretically investigated the coherent enhancement of resonance-mediated (2+2) four-photon absorption. It was found that, by shaping the spectral phase with aπphase step, the resonance-mediated (2+2) four-photon transition probability can be enhanced. Furthermore, the dependences of coherent enhancement on the detuning between the two non-resonant two-photon absorptions, laser spectral bandwidth and laser centre frequency were explicitly discussed and analyzed.(6) We theoretically studied the coherent phase control of typical (2+1) resonantly enhanced multiphoton ionization photoelectron spectroscopy (REMPI-PES) in an atomic system. By properly designing the spectral phase of a femtosecond pulse, we realized the manipulation of the photoelectron energy, the photoelectron spectral bandwidth and the photoelectron intensity at a certain kinetic energy. Moreover, for a complicated quantum system, we can also obtain a high-resolution photoelectron spectrum and fine energy-level structure of the excited states in spite of the broad-width spectrum of the femtosecond pulse.