Efficient field-free orientation of CO molecules using a three-step excitation scheme

Abstract

An efficient three-step excitation scheme is proposed to achieve the strong molecular orientation. By taking the CO molecule for example, via the Raman excitation by the femtosecond pulse L 1 and subsequently resonant excitations by a single-cycle THz pulse L 2, the high degree of the field-free orientation of |⟨ cosθ ⟩ |max = 0.8921 can be obtained at the rotational temperature T = 0 K. An additional single-cycle THz pulse L 3 is introduced to further regulate the population distribution on rotational states and decrease the reduction of the orientation originating from the offset phase. With the increase of the central frequency ω 3 of L 3, the optimal |⟨ cosθ ⟩ |max increases monotonically. When ω 3 = 1.0 THz, the degree of the field-free orientation reaches |⟨ cosθ ⟩ |max = 0.9401. Moreover, our scheme is robust against the rotational temperature. Although the molecular orientation weakens with the rise of rotational temperature, |⟨ ⟨ cosθ ⟩ ⟩ | > 0.5 is still obtained until T = 22.8 K.