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We demonstrate and study the enhancement of intermediate-field two-photonabsorption by shaped femtosecond pulses having spectral phases of antisymmetricnature. The intermediate-field regime corresponds to pulse intensities, wherethe two-photon absorption is coherently induced by the weak-field nonresonanttwo-photon transitions as well as by additional resonance-mediated four-photontransitions. It is a regime of significant excitation yields, exceeding theweak-field yields by two orders of magnitudes, reaching about 10-20% populationtransfer. The considered antisymmetric nature is with respect to one-half ofthe (initial-to-final) two-photon transition frequency. The corresponding pulsespectrum is detuned from this frequency (the detuning direction is according tothe system). We study in detail the coherent interference mechanism leading tothe observed enhancement using forth-order frequency-domain perturbativeanalysis. We also show that, even though the maximal enhancement is achievedwith phase patterns of perfect antisymmetry, at high enough intermediate-fieldintensities absorption enhancement beyond the transform-limited level is stillachievable even with patterns having some degree of deviation from perfectantisymmetry. The degree of tolerance to deviations from perfect antisymmetryincreases as the pulse intensity increases. The theoretical and experimentalmodel system of the study is atomic sodium. These findings are of particularimportance for coherent control scenarios that simultaneously involve multipleexcitation channels.

Intermediate-field two-photon absorption enhancement by shaped femtosecond pulses with spectral phases of antisymmetric nature