Quantum Control Of Diatomic Molecules With Ultrashort Laser Pulses

This thesis develops theoretical model for quantum control of diatomic molecules with ultrashort laser pulses, which mainly involves the coherent control of quantum state and spatial degrees of freedom. The theoretical method is based on the nu-merical solution of the time-dependent Schrodinger equation including the vibrational and rotational degrees of freedom. The effect of rotational temperature on the quan-tum control is considered by statistically averaging over the solutions of Schrodinger equation for all possible initially rovibrational states weighed by a Boltzmann factor. For the quantum state manipulation, a model for controlling the population trans-fer between the electronic states is discussed, and the effects of the Stark shift and neighboring electronic states on the population transfer are considered. By utilizing the Stark shift to modulate the transition frequency and choosing the suitable laser frequency, the population can be adiabatically transferred from the initial electronic state to the target electronic state. By combining exactly wave packet calculations and the qualitative analysis of simple three-level approximation the coherent population transfer between molecular rovibrational states by a stimulated Raman adiabatic pas-sage (STIRAP) mechanism is also investigated to control the rovibrational quantum state in the molecular electronic states. The calculated results have shown that the population can be adiabatically transferred from one rovibrational state to another rovibrational state, and the initial rotational temperatures have evident influence on transfer process.For the control of spatial degrees of freedom, two strategies for generating an efficient field-free molecular orientation are proposed. Firstly, with LiH molecules as an example, a scenario used for controlling molecular orientation was suggested with an infrared laser pulse and a delayed half-cycle pulse. The infrared laser pulse ex-cites the molecules in a thermally initial state to a specific rovibrational state, and then the half-cycle pulse orients the molecules by rotational excitation. Numerical calculation shows that an efficient field-free time-dependent orientation can be real-ized even at room temperature. Secondly, a strategy for generating carrier-envelope phase-dependent field-free molecular orientation was proposed with the use of carrier- envelope phase (CEP) stabilization and asymmetric terahertz (THz) few-cycle laser pulses. An efficient field-free molecular orientation can be obtained even at higher temperatures. Moreover, a simple dependence of the field-free orientation on the CEP was demonstrated, which implies that the CEP becomes an important parameter for control of molecular orientation. More importantly, the realization of this scenario is appealing based on the fact that the intense few-cycle THz pulse with duration as short as a few optical cycles is available as a measrue tool. This thesis also investigates in detail the dynamics of field-free orientation driven by THz few-cycle pulses. Exact results by numerically solving the time-dependent Schrodinger equation including the vibrational and rotational degrees of freedom are compared to the rigid-rotor approx-imation as well as to the impulsive approximation. Two different molecules, LiH and LiCl, are considered. A delta-kicked rotor model is well demonstrated for understand-ing the dynamics of field-free molecular orientation with THz few-cycle pulses.For application of the field-free molecular orientation, an experimentally feasible approach was proposed to determine the CEP of a few-cycle pulse by observing the field-free molecular orientation. The degree of orientation sensitively depends on the CEP, providing a new route for measurement of the CEP without phase ambiguity. By taking advantage of revivals of the field-free molecular orientation, an important effect of the CEP drift caused by the dephasing of the generating medium on the accurate measurement of the CEP value is naturally eliminated.