Picosecond study of transient optical second harmonic generation (SHG) in K vapor

SHG is forbidden in materials with centrosymmetry such as an atomic vapor, yet it has been observed with appreciable efficiency on atomic resonances in metal vapors with focused nanosecond lasers. We have used a 3 psec tunable laser pulse in a pump-probe geometry to determine how the symmetry is broken in atomic media. We observe a significant instantaneous response with a strong intensity dependence. Enhancement of the SHG of a delayed probe pulse by the pump pulse is weak up to 1 nsec. Other factors such as ionization of K vapor, linewidth and phase matching are also studied