Laser Induced Molecular Alignment

 The physics of molecular alignment by ultrashort laser pulses can be described classically: A short non-resonant linearly polarized laser pulse induces a molecular dipole moment which in turn interacts with the electric field itself. Because of the generally anisotropic molecular polarizability, the laser field delivers a torque to the molecules, causing them to rotate towards the direction of the laser field polarization.

 In the cartoon below, a classical 2D model is shown: An ultrashort laser pulse is applied to the molecular ensemble at time 0.                                                                                                                         Left - Classical molecular rotation.                                                                                                        Right  -  Averaged alignment factor <cos²θ>  as a function of time.

 Short time after the pulse, the angular distribution of the molecules elongates along the direction of the field, a configuration we refer to as “cigar”, as shown in the following cartoon.