Ultrashort Laser Pulse Phenomena, 2nd Edition

268 Coherent Phenomena

for the Stark shift. As the pulse propagates through the mercury vapor, the third

harmonic generation is seen to saturate after a short distance, while the ionization

drops to zero. The third harmonic being generated with a phase opposite to that

of the fundamental, the interaction vanishes, because the second order (ω

3

+ω



)

and fourth order (4ω



) mechanisms of four photon resonant excitation [Eq. (D.1)]

cancel each other. This problem could be solved by adjusting the relative phase

of the fundamental and third harmonic and by using phased pulse sequences to

control the relative phase of the coherent excitation (term of amplitude

˜

Q

4

) and

that of the radiation.

Multiphoton resonances reduce the energy requirements on higher-order ...