Second-harmonic generation in potassium vapor with picosecond laser pulses

Stoyan Dinev

Naohiro Tanno, Yoshio Adachi, Kenichi Yokoto +1 more

Doseok Kim, Christopher S. Mullin, Y. Ron Shen

Picosecond laser pulses were used to study the highly forbidden resonant second-harmonic generation (SHG) in potassium vapor. The input intensity dependence, vapor density dependence, buffer-gas pressure dependence, and spatial profile of the SHG were measured. A pump–probe experiment was conducted to probe the time dependence of the SHG signal. The experimental results can be understood from an ionization-initiated dc-field-induced SHG model. A theory of a dc-field-induced SHG model is developed that takes into account the time development of the dc electric field in detail. This temporal buildup of the dc field along with transient coherent excitation between two-photon-allowed transitions can explain the experimental results quantitatively, including the previous vapor SHG results with nanosecond laser pulses.

V. M. Arutyunyan, N. N. Badalyan, V. A. Iradyan +1 more

J.Y. Zhang, Kam Sing Wong, George K. Wong +1 more

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.>

Christopher S. Mullin, Doseok Kim, Marla B. Feller +1 more

Picosecond laser pulses are used to study the forbidden resonant second harmonic generation (SHG) in atomic vapor. The results can be understood quantitatively by an ionization-initiated dc-field-induced SHG model taking into account the temporal buildup of the dc field and transient behavior of the two-photon coherent excitation of the vapor.