TIME-DELAYED 2ND-HARMONIC GENERATION

Second harmonic generation in Ca vapor is produced by the combined act ion of a polarized laser pulse and a magnetic field. It is delayed fro m the 10 ps excitation pulse by several hundred ps. The laser is two-p hoton resonant with the 6S(1/2)-6D(3/2) transition and initially induc es a macroscopic quadrupole moment whose radiation pattern has a null in the forward direction. The magnetic held reshapes this pattern to p roduce second harmonic light, which is simply a free polarization deca y signal evolving in a manner akin to the free induction decay followi ng an adiabatic demagnetization.