Second-harmonic generation in atomic vapor with picosecond laser pulses

Doseok Kim, Christopher S. Mullin, Y. R. Shen
1997 Journal of the Optical Society of America. B, Optical physics  
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
more » ... HG 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. © 1997 Optical Society of America [S0740-3224 (97) 04110-6] PACS numbers: 42.65.Ky, 42.50.Md, 32.80.Wr REVIEW OF PAST EXPERIMENTS AND PROPOSED MECHANISMS A. Past Experiments Resonant SHG in atomic vapor was first observed by Mossberg et al. 8 in thallium vapor. Using a 7-ns, 0.2-kW dye laser pulse focused to 200 m in a vapor density of 7 ϫ 10 15 /cm 3 , they obtained ϳ5 ϫ 10 3 second-harmonic photons in the output. They reported that the output depended both on the square of the input power and on the square of the vapor density and proposed that the electric field produced by three-photon ionization of the Tl atoms could break the symmetry and induce the observed SHG. Subsequent experiments by many others with alkali, 9, 11, 13, 14, 17, 20 alkali-earth, and other metallic vapors 10,12,15 yielded similar results. In most of the ex-2530
doi:10.1364/josab.14.002530 fatcat:7sxule6nj5en7g5gfr5wdwpgsm