Cyclic State Molecular Orientation Optimized By Designed Trains Of Half-Cycle Pulses

In the last decade laser-induced spatial manipulation of molecules, molecular alignment and orientation, has become a hot subject of growing interest ranging from chemical and photochemical processes, to high-order harmonic generation, molecular ionization and dissociation and quantum information, etc.Generally speaking, molecular alignment/orientation can be divided into two categories. When the external field is turned on/off slowly compared with the rotational period of the molecule, the molecule aligns/orients within the external field. In this case, once the external field becomes extinct, the alignment/orientation disappears. When the external field is turned on/off rapidly compared with the rotational period of the molecule, the molecule becomes aligned/oriented after the external field has died off. This is so called field-free alignment/orientation.In many applications of molecular alignment and orientation, it is required that the aligned/oriented molecules are free from any external field, because the latter will strongly disturb the system under investigation. Therefore, field-free alignment/orientation becomes a focus of intensive studies in recent years. It is also a main topic of the present thesis.Molecular orientation is more difficult to be realized than alignment as an additional spatial symmetry breaking scheme has to be involved. However, the rapid development of laser technology provides us with preponderant conditions to achieve the subject. In particular, the half-cycle pulse (HCP), which is highly asymmetric in its positive and negative field strengths, serves as a powerful tool to realize molecular orientation under field-free condition.In field-free molecular orientation researches, there are two main goals: in addition to the promotion of orientation degree, the extension of orientation duration becomes another primary goal. In this paper, we present a variational method to calculate the optimal population distribution in a chosen population range,which provide realistic object and direction to come true perfect orientation by further optimizing orientational external field. We are clearly aware that a single short HCP cannot provide perfect orientation, and its effect saturates with intensity. Under the action of optimized HCP train with a repetition period, persistent molecular orientation can be obtained when the initial state of molecular evolution is prepared as one of the optimal cyclic state of the system. The cyclic state, which is a particular superposition of the field-free rotational eigenstates, is found by the generalized Floquet theorem.However, we notice that a single train of HCP (THCP) and the relevant optimal control scheme no longer work well, when the repetition period becomes longer. In this paper, we present a scheme using two trains of HCPs with different amplitudes irradiating alternately on polar molecules to prolong the effective orientation sustainable time.Under directing of theoretical results of the variational method, in this paper, we devise a train of half-cycle pulse clusters (THCPC) to achieve an effeicent and long-lived field-free molecular orientation for a longer repetition period. Substituting THCPC for THCP provides other two adjustable parameters, the pulse number m and the pulse separationΔt in a cluster, besides the interaction parameterωbetween molecule and external field. It is well known that there are two factors of realizing optimal orientation effect: (1) achieving an entire phase matching at the midpoint between every two adjacent HCP (or HCP clusters), and (2) making the structure of the cyclic wavepacket approach the theoretically optimal population distribution on the rotational states. Different jobs can be assigned to the relevant pulse parameters. The pulse number m in a cluster effectively control the population range of rotational states, and the increase of m expands the actual population range. The pulse separationΔt in a cluster is mainly used to achieve the phase matching, therefore, through varying ofΔt we always can find out a value to realize phase matching. It is understood easily the interaction parameterωis used to approach the optimal population distribution in a definite population range determined already by the m value.This scheme has a innovation point. It breaks out saturation limit with intensity in original orientational methods through increasing two controllable parameters assigned different jobs. Optimal population distribution calculated by variational method provides an aggressive goal and a theoretical direction for subsequent relevant reseaches.In this paper, these schemes are strongly dependent of the preparation of pulses and preparation of the cyclic initial state in experiments. These problems are also discussed in the thesis by referring to the relevant works of the other authors.