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 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 -dependent field-free orientation (equal to for and for at 100 K) in the near–single-cycle THz pulse region is close to 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 (e.g., a.u. for and a.u. for ) 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 ( a.u.).
- Received 3 July 2012
DOI:https://doi.org/10.1103/PhysRevA.87.013429
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