Optimal Control Simulation of Field-Free Molecular Orientation: Alignment-Enhanced Molecular Orientation

Abstract

Nonresonant optimal control simulation is applied to a CO molecule to design two-color phase-locked laser pulses (800 nm + 400 nm) with the aim of orienting the molecule under the field-free condition. The optimal pulse consists of two subpulses: the first subpulse aligns the molecule and the second one orients it. The molecular alignment induced by the first subpulse considerably enhances the degree of orientation, the value of which is close to an ideal value at temperature T = 0 K. To confirm the effectiveness of this alignment-enhanced orientation mechanism, we adopt a set of model Gaussian pulses and calculate the maximum degrees of orientation as a function of the delay time and the intensity. In finite-temperature (T = 3.0 K and T = 5.0 K) cases, although the alignment subpulse can improve the degree of orientation, the control achievement decreases with temperature rapidly; this decrease can be attributed to the initial-state-dependent (phase-shifted) rotational wave packet motion.

Alignment-assisted field-free orientation of rotationally cold CO molecules

“…The former provides control of the rotational wave packet and enhances the orientation by a factor of two. The latter has been shown computationally to be very efficient for orienting molecules at both low [33] and high [34] rotational temperatures. In simulations, a three-fold enhancement in orientation can be obtained when the single-color field is positioned correctly with respect to the two-color field [33,34].…”

“…The latter has been shown computationally to be very efficient for orienting molecules at both low [33] and high [34] rotational temperatures. In simulations, a three-fold enhancement in orientation can be obtained when the single-color field is positioned correctly with respect to the two-color field [33,34]. In this article, we report the experimental demonstration of this technique using only the hyperpolarizability mechanism in CO. By applying an aligning pulse before the orienting two-color pulse, the degree of orientation [characterized by cos θ 2D , where θ 2D is the angle between the laser polarization axis and the projection of the C 2+ fragment's momentum vector measured by using a velocity map imaging (VMI) spectrometer [35] ] is increased from 0.07 to 0.2 in rotationally cold CO molecules.…”

Control of molecular orientation with combined near-single-cycle THz and optimally designed non-resonant laser pulses: Carrier-envelope phase effects

“…The difficulty lies in the requirement of parity-violating interactions that give sufficiently strong asymmetric torques to the molecules. For example, a phaselocked two-color laser pulse that typically consists of 800 nm and 400 nm components is used to 1D orient linear molecules through the hyperpolarizability interaction [14][15][16][17][18][19][20]. Because of the weak coupling between molecules and laser fields, a strong laser pulse is needed [19].…”

Orienting CO molecules with an optimal combination of THz and laser pulses: Optimal control simulation with specified pulse amplitude and fluence

“…The difficulty stems from the requirement of asymmetric interactions. When a phase-locked two-color laser pulse is used to introduce asymmetric interactions through higher-order-induced dipole interactions [39][40][41][42][43][44][45][46][47], its intensity is usually so high that it may induce undesirable transitions and/or cause damage to molecules. Consequently, we need to utilize permanent dipole moment coupling [48][49][50][51][52][53][54][55][56][57][58][59][60][61][62] with low-frequency electric fields, such as half-cycle-pulse trains [48] and few-cycle pulses [49][50][51][52][53], to realize molecular orientation in the field-free condition.…”