Field-Free Orientation Of Linear Molecules By Using Half-Cycle Pulses

Molecular alignment/orientation plays an important role in a variety offields ranging from molecular reaction dynamics, gas-surface interaction,nanoscale design, quantum information, to strong field problems such ashigh-order harmonic generation and ultrashort laser pulse production, etc.Generally speaking, molecular alignment/orientation can be divided intotwo categories. When the external field is turned on/off slowly compared withthe rotational period of the molecule, the molecule aligns/orients within theexternal field. In this case, once the external field becomes extinct, thealignment/orientation disappears. When the external field is turned on/offrapidly compared with the rotational period of the molecule, the moleculebecomes aligned/oriented after the external field has died off. This is so calledfield-free alignment/orientation.In many applications of molecular alignment and orientation, it isrequired 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 became a focus of intensive studiesin recent years. It is also the main topic of the present thesis.Molecular orientation is more difficult than alignment as an additionalspatial symmetry breaking scheme has to be involved. However, the rapiddevelopment of laser technology provides us with preponderant conditions toachieve the subject. In particular, the half-cycle pulse (HCP), which is highlyasymmetric in its positive and negative field strengths, serves as a powerfultool to realize molecular orientation under field-free condition.However, confined by the peak amplitude and the pulse duration ofHCPs experimentally available, the HCP orientation can only be applied tomolecules with large permanent dipole moments. For molecules with smallerdipole moments, the orientation degree is extremely low due to their weakerresponse to the external electric fields. In this thesis, we proposed a strategy inwhich by using a train of HCPs with repetition period π / B( B is therotational constant of the molecule) or its integer times, the orientation degreeof the molecules with smaller dipole moments can be promoted effectively.We further analyzed the dynamical mechanism of the scheme by referring to asimplified two-rotational-state model. It is found that the mechanism can beinterpreted in terms of the intermittent Rabi oscillations. Thus one can controlthe orientation degree by adjusting the number of the HCPs. As the incidenttime of the HCPs can not be very exact in real experiments, we also examinedthe susceptibility of the orientation to the succeeding instants of the HCPs.In field-free molecular orientation researches, the duration of theobtained orientation is of equal importance as the orientation degree. Thus, inaddition to the promotion of orientation degree, the enhancement oforientation duration becomes another primary goal.Under the action of a train of identical HCPs with a definite repetitionperiod, molecular orientation can be maintained as long as desired when theinitial state of the molecular evolution is prepaired as one of the optimal cyclicstates of the system. The cyclic state, which is a particular superposition of thefield-free rotational eigenstates, is found by the generalized Floquet theorem.While the cyclic states can lead to sustainable molecular orientation, theunderlying causation is remained unanswered. In the present thesis, we foundthat when the cyclic state is expanded as the linear superposition of thefield-free rotational eigenstates, the phases of all the expansion coefficientswill differ only by 0 or π at the midpoints of every two adjacent pulses. Asfor the optimal cyclic state which is chosen to produce sustainable orientation,the expansion coefficients of the major rotational states (the rotational statesthat have non-trivial populations) possess a common phase factor at theseinstants. While in usual orientation schemes, the phase matching between thecomponents of the orientation wave packet is often spontaneous, thisoutstanding characteristic of the cyclic state orientation makes it an optimalscheme to maximize the orientation degree or its duration: At a givenpopulation distribution, the orientation degree will reach the highest possiblevalue. Or conversely, for a given orientation degree requirement, one can letthe fewest rotational states be involved in the orientation process, and hencewe are provided with an opportunity to maximize the orientation duration.Also, it is just this characteristic of the optimal cyclic state that results in thesustainability of the orientation.In addition, we investigated concretely the regularities concerning thecyclic state orientation. The main ones are as follows: At a smaller repetitionperiod of the pulses, the orientation degree that can be reached is higher andthe oscillation of orientation is smaller. However, when the repetition periodincreases, the orientation degree is lowered, and at the same time the obtainedorientation oscillates more violently. These regularities can be explained bytaking into account the above described characteristic of the optimal cyclicstate and the related dynamics.Under the action of HCPs with a single repetition period and for agiven value, the highest orientation degree that can be achieved isrestricted by the above described regularities. In order to breach the restriction,we introduced the dual repetition period scheme. With this scheme, theorientation degree can be locally promoted at the same pulse intervals as thesingle repetition period case. Besides, we can also produce the orientation thatappears alternatively in time, which may be found useful for the real timeexperimental comparison for a certain process with and without theorientation.TTIn previous orientation schemes the evolution usually starts from anatural molecular initial state such as the ground field-free rotationaleigenstate or the initial state at a certain temperature. An outstanding featureof the cyclic state orientation scheme lies in that it uniformly examines anddesigns both the molecular initial state and the applied external fields, i.e.,what external fields will be applied, the optimal cyclic state is then used as theinitial state of the evolution, while the total set of the cyclic states isdetermined by the actually applied fields through solving the related Floquetequations. We believe that this is an important advance in concept and that thishas opened a novel route towards better and better orientation.Just because of this feature, the experimental preparation of the cyclicinitial state is of crucial importance for the scheme. This problem is alsodiscussed in the thesis by referring to the relevant works of the other authors.