Study Of Coherent Control On Molecular Ionization And Rotational Dynamic Using Shaped Femtosecond Laser Pulses

Ever since the invention of femtosecond laser pulse-shaping technology,researches of coherent control in molecular dynamic processes have been greatly promoted.Conventional pulse-shaping technology modulates the optical electric field as a scalar field,which couldn't control dynamic processes of three-dimensional polyatomic molecules effectively.The polarization pulse-shaping technology can produce time-dependent polarized shaped laser pulse trains In this work,a polarization pulse-shaping system based on 4f-geometry has been build,pulse sequences with varying polarization are generated and later be used to control the ionization of N₂O molecule and the rotational state population of N₂ in the atmosphere.The research works mainly include the following aspects:?1?Employing the dual liquid crystal scheme,a polarization pulse-shaping system based on 4f-geometry was built and calibrated.In the experiment,sinusoidal phase,pphase,slope phase and quadratic phase are loaded respectively onto the modulator.The shaped pulses are simulated in theory and characterized with SFG-Cross-Correlation technique.The results show that a linear polarization pulse sequence with varying polarization is generated,when the sinusoidal phase is loaded onto the liquid crystal spatial light modulator,whose polarization angle,interval between each sub-pulse and the numbers of sub-pulses can be modulated.While loadingpphase onto the liquid crystal spatial light modulator,there generates the linear polarization pulse with a valley in the pulse center.If we load slope phase onto the liquid crystal spatial light modulator,the laser pulses come into being a pair of sub-pulses with polarizations orthographic to each other,the interval between each sub-pulses can be modulated continuously.When we load quadratic phase onto the modulator,it will produce a linearly polarization chirped pulse whose polarization rotates over time.The experimental and theoretical results coincide with each other,which prove that we generate the time-dependent polarized shaped laser pulses in the experiment.?2?This work performed a study of strong-field nonresonant coherent control on ionization of N₂O using sinusoidal phase modulated laser pulses.The control on the total ionization yield was achieved by the variation of phase parameters.Results induced by constant polarization and varying polarization shaped pulses were contrasted to prove the modulation of the total ionization yield not to be directly related to the polarization changing of the sub-pulses.We measured the total ionization yield and kinetic energy distribution of ionized electrons in different parameters,and no peculiar ionization channel was found.We point out that the modulation process is mainly caused by the two-photon interference of multiple frequencies within the wide bandwidth of laser pulse.We operated apphase scan on ionization of N₂O,the result agrees with two-photon absorption model.We demonstrate that different ionization channels are first excited to the different Rydberg states incoherently,and then experience the nonresonant two-photon excitation modulated by the laser phase to ionize.The control is accomplished by the interferences of the nonresonant two-photon excitation induced by the shaped laser pulses.The experiment of SHG induced by the shaped laser field and theoretical simulation were performed,results prove that two-photon interference plays a dominant role in the modulation experiment.?3?A polarization pulse-shaping technique combined with balanced weak field polarization scheme is proposed to control the molecular rotational state population.We measured the N₂ revival signal and obtained rotational state population using Fourier transform methods.The rotational state population is controlled by the variation of the phase parameters.In the experiment of modulating N₂ rotational state population with slope phase,the revival structure reversion is observed in the specified conditions.We attribute this reversion to the counter-rotating linear polarization pulse generated by the overlapping of sub-pulse pairs.The experimental results also show that when the slope phase parameter A<0.16,adjusting slope phase parameters will lead phase interference in frequency domain,which can modulate Raman transition probability thus,the N₂ rotational transition from high population state to low population state can be controlled.When the slope phase parameter A>0.21,the interval between the sub-pulse pairs is larger than 1 ps.The two orthogonal polarized sub-pulses prepare two rotational wave packets respectively.The superposition of two rotational wave packets on variation delays results in the enhancement of the certain rotational state population.While the slope phase parameter A is between 0.16 and 0.21,the modulation is in transition of the two processes above,which not only shows the enhancement of some certain rotation states population but also the modulation of the rotational transition from high population state to low population state.This is the concurrent result of phase interference in frequency domain and superposition of two rotational wave packets.While in the experiment of modulating N₂ rotational state population with quadratic phase,no inversion of revival structure is found.The results show that with the increase of the quadratic phase parameter A,the shaped pulse duration gradually increase,the rotational process transits from non-adiabatic to adiabatic and the whole rotational state population decline.Nevertheless,the transition probability of some individual rotational states within the Raman transition frequency can still be enhanced through the proper phase parameter condition,which results in the enhancement of corresponding rotational state population.The scheme concerns not only the control of the overall odd?even?rotational state population,but also the control of a targeted rotational state population.The polarization pulse-shaping technique combined with balanced weak field polarization scheme is capable to control the N₂ rotational state population,and the scheme is proved to be feasible and practical.