Shaping of the time evolution of field-free molecular orientation by THz laser pulses

We present a theoretical study of the shaping of the time evolution of field-free orientation of linear molecules. We show the extent to which the degree of orientation can be steered along a desired periodic time-dependent signal. The objective of this study is not to optimize molecular orientation but to propose a general procedure to precisely control rotational dynamics through the first moment of the molecular axis distribution. Rectangular and triangular signals are taken as illustrative examples. At zero temperature we compute the quantum states leading to such field-free dynamics. A tera-Hertz laser pulse is designed to reach these states by using optimal control techniques. The investigation is extended to the case of nonzero temperature. Due to the complexity of the dynamics, the control protocol is derived with a Monte Carlo simulated annealing algorithm. A figure of merit based on the Fourier coefficients of the degree of orientation is used. We study the robustness of the control process against temperature effects and amplitude variations of the electric field.