Polarization Detection of Molecular Alignment Using Femtosecond Laser Pulse

Femtosecond laser is becoming a powerful tool to manipulate the behaviors of molecules. When molecules are irradiated by strong laser field with intensity below the ionization threshold of molecules, the interaction between molecules and the laser electric field tends to align the molecules with the most polarizable axis along the laser polarization vector. If the laser pulse duration is larger than the rotational period of the molecule, the free rotor transforms into a pendular state that liberates about the polarization vector. Upon turning off the laser, the librator adiabatically returns to the isotropic free rotator from which it originates. If the laser pulse duration is less than the molecular rotational period, the lasermolecule interaction gives the molecules a rapid “kick” to make the molecular axis toward the laser field vector. After extinction of the laser, the transient alignment can periodically revive as long as the coherence of the rotational wave packet is preserved. Therefore, the former is also called adiabatic alignment and the latter field-free alignment. Even though both adiabatic alignment and field-free alignment can produce macroscopic ensembles of highly aligned molecules, field-free alignment has the obvious advantages that will not interfere with subsequent measurements. A variety of new and exciting applications of field-free aligned molecules are currently emerging. For example, Litvinyuk et al. measured strong field laser ionization of aligned molecules and obtained directly the angle dependent ionization rate of molecules by intense femtosecond laser field. Another novel application is to accomplish a tomographic reconstruction of the highest occupied molecular orbital of nitrogen by using high harmonic generation from intense femtosecond laser pulses and aligned molecules. Recently, Kanai et al. observed the quantum interference during highorder harmonic generation from aligned molecules and demonstrated that aligned molecules could be served as an ideal quantum system to investigate the quantum phenomena associated with molecular symmetries. The weak field polarization technique has homodyne and heterodyne detection modes. The alignment signal is proportional to ( -1/3)2 for homodyne detection and ( -1/3+C)2 for heterodyne detection, where C describes the constant external birefringence contribution. Because the magnitude and the polarity of the external birefringence are hard to precisely control, homodyne detection is commonly used up to now. However, the homodyne signal cannot indicate whether the is larger or smaller than 1/3. In other words, the homodyne signal cannot demonstrate whether the aligned molecule is parallel or perpendicular to the laser