Long-lasting Orientation induced by Two-Color Femtosecond Laser Pulses

Molecular alignment and orientation have attracted considerable interest due to their importance in various applications in physics, chemical reaction, and attosecond electron dynamics. Recently, we reported a new phenomenon of long-lasting orientation in chiral molecules excited by a laser pulse with twisted polarization and in non-linear molecules excited by THz pulses. ‘Long-lasting’ means that the time-averaged orientation remains non-zero on a time scale exceeding the duration of the external fields by several orders of magnitude. Here we show that non-resonant two-color laser pulses consisting of a fundamental wave and its second harmonic are effective for inducing the molecular orientation, and report long-lasting orientation of symmetric- and asymmetric-top molecules following excitation by two-color femtosecond laser pulses. The long-lasting orientation relies on the combined effect of field-polarizability and field-hyperpolarizability interactions. We also study the dependence of the degree of long-lasting orientation on temperature and pulse parameters. The long-lasting orientation can be measured utilizing second (or higher-order) harmonic generation and may be used to control the deflection of molecules traveling through inhomogeneous electrostatic fields.