Enhancement of intermediate-field two-photon absorption by rationally shaped femtosecond pulses

Abstract

We extend the powerful frequency-domain analysis of femtosecond two-photon absorption to the intermediate-field regime, which involves both two-and four-photon transitions. Consequently, we find a broad family of shaped pulses that enhance the absorption over the transform-limited pulse. It includes any spectral phase that is anti-symmetric around half the transition frequency.The spectrum is asymmetric around it. The theoretical framework and results for Na are verified experimentally. This work opens the door for rational femtosecond coherent control in a regime of considerable absorption yields.

Optimal control under spectral constraints: enforcing multi-photon absorption pathways

“…The resulting spectral widths by far exceed experimentally realistic values. As a result, only solutions using one-photon transitions are found while the experimental result of non-resonant two-photon control [24][25][26][27][28][29][30][31][32][33] could not be reproduced. Here we employ optimal control theory with spectral constraints to enforce a non-resonant two-photon solution.…”

“…Non-resonant two-photon absorption has been studied experimentally for ns to (n + 1)s transitions in alkali atoms in the weak [24][25][26], strong [27][28][29] and intermediate field regime [30][31][32][33]. To date, optimal control calculations of non-resonant two-photon absorption have been hampered by a spectral spread of the field.…”

Intermediate-field two-photon absorption enhancement by shaped femtosecond pulses: Tolerance to phase deviation from perfect antisymmetry

“…Recently [4,5] we have extended the rational control studies of femtosecond two-photon absorption from the weak-field regime to the intermediate-field regime, which corresponds to considerable absorption yields exceeding the weak-field yields by two orders of magnitude. The corresponding weak-field absorption is described by second-order perturbation theory and is induced by interfering nonresonant two-photon transitions of two absorbed photons [6], while the intermediate-field absorption is described by fourth-order perturbation theory and involves additional resonance-mediated four-photon transitions of three absorbed photons and one emitted photon [4,5].…”

“…The corresponding weak-field absorption is described by second-order perturbation theory and is induced by interfering nonresonant two-photon transitions of two absorbed photons [6], while the intermediate-field absorption is described by fourth-order perturbation theory and involves additional resonance-mediated four-photon transitions of three absorbed photons and one emitted photon [4,5].…”

“…One of our important intermediate-field findings [5] is the enhancement of the two-photon absorption by any shaped pulse having its spectral phase antisymmetric around one-half of the two-photon transition frequency, with its spectrum being red or blue detuned from this frequency according to the specific physical system. The enhancement is over the absorption induced by the unshaped transform-limited (TL) pulse.…”