Maximum attainable field-free molecular orientation of a thermal ensemble with near–single-cycle THz pulses

Abstract

Recently, single-cycle THz pulses have been demonstrated in the laboratory to successfully induce field-free orientation in gas-phase polar molecules at room temperature [Phys. Rev. Lett. 107, 163603 (2011)]. In this paper, we examine the maximum attainable field-free molecular orientation with optimally shaped linearly polarized near-single-cycle THz laser pulses of a thermal ensemble. Large-scale benchmark optimal control simulations are performed, including rotational energy levels with the rotational quantum numbers up to J = 100 for OCS linear molecules. The simulations are made possible by an extension of the recently formulated fast search algorithm, the two-point boundary-value quantum control paradigm, to the mixed-states optimal control problems in the present work. It is shown that a very high degree of field-free orientation can be achieved by strong, optimally shaped near-single-cycle THz pulses. The extensive numerical simulations showed that the maximum attainable J -dependent field-free orientation (equal to 0.714 for J = 60 and 0.837 for J = 100 at 100 K) in the near-single-cycle THz pulse region is close to 92% of the corresponding optimal bound that can be attained by arbitrarily long pulses. It is also found that a smaller amplitude for the optimal control field corresponds to a smaller J (e.g., ≈ 0.005 a.u. for J = 60 and ≈ 0.01 a.u. for J = 100) in the model simulations. The latter finding may underline the actual experimental performance of the field-free molecular orientation, since presently the available amplitude of single-cycle THz pulses can only reach slightly beyond 20 MV/cm (≈0.0038 a.u.).

Unified parameter for localization in isotope-selective rotational excitation of diatomic molecules using a train of optical pulses

“…The rotation of small molecules has been studied from the viewpoint of non-adiabatic molecular alignment [1][2][3] or molecular orientation [4][5][6][7][8][9][10]. Non-adiabatic molecular alignment has been experimentally realized using non-resonant broadband optical pulses [11,12].…”

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

“…The numerical simulation by Liao et al [24] showed that quite high degrees of orientation of OCS molecules at 100 K are achieved by applying strong near-single-cycle THz pulses with a maximum amplitude of several GV/m. Although such strong THz pulses are available at accelerator facilities, the techniques for generating them mainly use optical rectification [26,27].…”

“…An alternate approach is to use (resonant) permanent dipole interaction [21][22][23][24][25]. The numerical simulation by Liao et al [24] showed that quite high degrees of orientation of OCS molecules at 100 K are achieved by applying strong near-single-cycle THz pulses with a maximum amplitude of several GV/m.…”

External constraints on optimal control strategies in molecular orientation and photofragmentation: role of zero-area fields

“…In view of experimental applications in quantum control, it is also desirable to design pulse sequences with a zero global time-integrated area. Several works have pointed out the inherent experimental difficulties associated with the use of non-zero area fields [21][22][23][24][25], in particular for laser fields in the THz regime. Since the dc component of the field is not a solution of Maxwell's equation, such pulses are distorted when they propagate in free space as well as through focusing optics.…”