Multiphoton pi pulses

George N. Gibson, University of Connecticut

Single photon interactions are well understood, including pi pulses - pulses of light that produce a 100% population transfer to an excited state. Multiphoton pi pulses would be very useful for producing population inversions on highly excited states. However, it is also well known that small multiphoton excitation rates, large AC Stark shifts and ionization all inhibit efficient population transfer. In this talk, I will present a novel 3-level system in which the AC Stark shift is greatly reduced and the multiphoton coupling strength is greatly enhanced over a 2-level system, to such an extent that higher order pi pulses are achievable. To demonstrate the feasibility of this process, I will present a fully correlated 2-electron calculation in a model 1-D molecular potential, including ionization that shows a 12-photon pi pulse driven with 800-nm photons. In other words, a single excited state 18.6 eV above the ground state can be populated with over 90% efficiency with little ionization while the ground state is almost completely depopulated. This creates a prompt population inversion in the vacuum-ultraviolet spectral region that may be suitable for amplifying femtosecond laser pulses created by high harmonic generation. Moreover, this 3-level configuration may open the door to other effects previously restricted to single photon interactions, such as adiabatic passage and inner shell ionization.