Femtosecond resonant second harmonic generation (SHG) in potassium vapor

It is well-known that the second-order nonlinear effects, such as second harmonic generation (SHG), difference frequency generation (DFG), and sum frequency generation (SFG), in atomic vapors are theoretically forbidden by symmetry on both a microscopic level because the individual atoms in the vapor do not possess a dipole, and on a macroscopic level because the atoms are isotropically distributed. However, SHG is vapor was first observed by Mossberg et al. (1978) in Tl and then in many other atomic vapors and some noble gases by other groups. Several explanations for this phenomenon have been presented. A model proposed by Mossberg et al. and later modified by Bethune (1981), attributes it to a DC field, which is created by laser-induced multi-photon ionization and spatial separation of free photoelectrons from the heavier ions either by the pondermotive potential of the laser or by the kinetic energy of photoelectrons and makes SHG allowed. All experiments reported so far have been carried out by using either nanosecond or picosecond laser pulses. The authors report the first femtosecond SHG in vapor. The experimental study is done in potassium vapor using femtosecond pulses from a mode-locked Ti:sapphire laser