Controlling Photoassociation Reaction Of Ultracold Atoms And Orientation Of Cold Molecules Using Ultrashort Laser Fields

In this dissertation, with the time-dependent quantum wavepacket theory, we study the control of photoassociation reaction of ultracold atoms and orientation of cold molecules using ultrashort laser fields. The main works are summarized as follows:(1) We use a modulated two-color laser field to control photoassociation and prepare ultracold85Rb2molecules. Comparing the laser field modulated by two Gaussian pulses with that by two chirped pulses, we discuss the influence of linear chirp rate, frequency difference and pulse phase on photoassociation process, and explain the variation trend of population using the time-and frequency-resolved spectrum. The calculation proves that the two-color laser field modulated by two positively chirped pulse can not only improve the photoassociation efficiency, but also weaken the influence of phase on the photoassociation efficiency. We can efficiently control the photassociation reaction by regulating the frequency difference and phase.(2) We propose a two-step photoassociation scheme to enhance the associated molecule yield. Taking ultracold cesium atomic gas for example, the "compression effect" of wavefunc-tion of the ground electronic state will significantly improve the photoassociation probability of colliding atomic pairs in the short distance region. Consequently, the molecules with larger binding energy in the excited electronic state can be efficiently prepared using the two-step scheme. We investigate the influence of pulse duration, electronic amplitude and linear chirp rate on photoassociation process. Moreover, the thermal average photoassociation efficiency considering Boltzmann distribution is calculated.(3) We present a three-step excitation scheme to realize the field-free orientation of CO molecules. Using a femtosecond pulse and two single-cycle THz pulses to act on the CO molecules with J=0in sequence, an efficient molecular orientation is achieved by the Raman excitation and subsequent resonant excitation. The calculation shows that the key to enhance molecular orientation is regulating the population distribution on rotational states and decreasing the reduction of the orientation induced by the offset phase. Moreover, the three-step excitation scheme is robust against on the rotational temperature. The degree of orientation<<cosθ>>nm<-0.5can be achieved until T=22.8K. (4) We investigate the control of CO molecular orientation based on a single-cycle THz pulse train. Comparing the molecular orientation in three cases:(I) a single-cycle THz pulse,(Ⅱ) a single-cycle THz pulse train, and (Ⅲ) the combination of an off-resonant femtosecond pulse and a single-cycle THz pulse train, we find that the single-cycle THz pulse train with general intensity is enough to realize the efficient molecular orientation. The case (Ⅲ) is suitable to control molecular orientation in low rotational temperature, while the case (Ⅱ) is proposed to steer molecular orientation in the higher rotational temperature.