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., $\ensuremath{\approx}0.005$ a.u. for $J=60$ and $\ensuremath{\approx}0.01$ 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 $20\phantom{\rule{4pt}{0ex}}\text{MV}/\text{cm}$ ($\ensuremath{\approx}0.0038$ a.u.).