Alignment echoes by phase-shaped femtosecond laser pulse

Abstract In recent years, molecular alignment echo induced by a pair of time-delayed femtosecond laser pulses has been proposed and aroused wide research interest. We demonstrate that, however, the alignment echo can be alternatively produced by a shaped femtosecond laser pulse with the V-style spectral phase modulation. The full, fractional and imaginary alignment echoes are formed by the excitation of the tailored two time-delayed sub-pulses. Both the delay time and the ratio of the intensity between two sub-pulses can be easily manipulated by designing the modulation parameters to induce various types of the echoes. We further show that the optimal ratio of the intensity between two sub-pulses that results in the maximal alignment degrees of the full echo closely correlates with the energy of the sub-pulse. When the pulse energy is relatively low, the maximal alignment degree of the full echo is obtained as the two sub-pulses with equal intensity. The optimal ratio of the intensity increases with the excitation energy of the first pump pulse.