We present the experimental demonstration of a novel, efficient, and selective technique to prepare population inversion. The technique is an extension of Stark-chirped rapid adiabatic passage (SCRAP), i.e., SCRAP among three states. In this process a Lambda-type quantum system is driven by two laser pulses, the pump and Stokes pulses, which are appropriately detuned from transition frequencies. A third laser pulse induces a dynamic Stark shift in the upper energy level, and the timing of all three pulses is controlled in order to prepare population inversion between the two lower states in the Lambda-type level scheme. Our data on population transfer in nitric oxide (NO) molecules clearly show that SCRAP among three states provides an advantageous alternative to such techniques as stimulated Raman adiabatic passage.

We present extended experimental investigations and numerical studies on coherently driven population inversion in a three-state molecular quantum system by sequential double-Stark-chirped rapid adiabatic passage ͑SCRAP͒ ͑D-SCRAP͒ and three-state-SCRAP ͑T-SCRAP͒. D-SCRAP and T-SCRAP are alternative extensions of SCRAP. In D-SCRAP and T-SCRAP, a ⌳-type quantum system is coherently driven by two laser pulses, the pump and Stokes pulses, which are slightly detuned from transition frequencies. A third strong laser pulse induces dynamic Stark shifts of the relevant transitions. If the timing of the three pulses is appropriately chosen, the quantum system is prepared to almost complete population inversion between the two lower states in the ⌳-type level scheme. The transfer process is robust with regard to fluctuations of experimental parameters, provided some limitations are met. The paper presents convincing experimental data on D-SCRAP and T-SCRAP, driving efficient and selective population transfer to a highly excited vibrational level in nitrous oxide ͑NO͒ molecules. T-SCRAP yields transfer efficiencies of up to 95%. The efficiency of T-SCRAP is almost uneffected by the limited lifetime of the intermediate state in the ⌳ system. The paper also presents data on the accurate experimental determination of Rabi frequencies and Stark shifts. This involves measurements of power broadening, Autler-Townes splittings, and Stark-shifted spectral lines in NO molecules. Moreover, we discuss the calibration of absolute transfer efficiences by comparison with stimulated emission pumping. The experimental data are confirmed by extended numerical simulations. The simulations also serve to intensively study the properties and dynamics of D-SCRAP and T-SCRAP.

We present systematic investigations of harmonic generation in Argon, driven in the vicinity of a five-photon resonance by intense, tunable picosecond radiation pulses. When properly matching the laser frequency with the Stark-shifted multi-photon resonance, we observe a pronounced enhancement not only of the 5th, but also the 7th and 9th harmonic of the driving laser (i.e. at orders higher than the involved multi-photon resonance). We study the harmonic yield at different intensities and wavelengths of the driving laser to determine optimal conditions for resonantly-enhanced harmonic generation.