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Bulletin of the American Physical Society

Efficient population transfer in a ladder system by chirped pulses

Bart Noordam

Published May 1, 1993• 0 citations
OpenAlex

Abstract

Enhanced population transfer from the lowest to the highest level in the three-state ladder 5s->5p->5d of the rubidium atom by frequency swept ultra short laser pulses is observed if the frequency sweep first makes the pulse resonant with the 5s->5p transition, and later with the 5p->5d transition. With this intuitive scheme it is possible to achieve complete population inversion via the intermediate level by two successive adiabatic passages. Complete inversion is also obtained by a {open_quote}counterintuitive{close_quote} direction of the frequency-sweep, in which case population of the intermediate level during the pulse is strongly reduced. The atomic system can be seen as a model system for multistep vibrational excitation in an anharmonic molecular potential.

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Journal of Shaanxi Normal University•Jan 1, 2005•0 citations
Enhanced population transfer from the lowest to the highest level in the three-state ladder 5s->5p->5d of the rubidium atom by frequency swept ultra short laser pulses is observed if the frequency sweep first makes the pulse resonant with the 5s->5p transition, and later with the 5p->5d transition. With this intuitive scheme it is possible to achieve complete population inversion via the intermediate level by two successive adiabatic passages. Complete inversion is also obtained by a {open_quote}counterintuitive{close_quote} direction of the frequency-sweep, in which case population of the intermediate level during the pulse is strongly reduced. The atomic system can be seen as a model system for multistep vibrational excitation in an anharmonic molecular potential.
Enhanced population transfer from the lowest to the highest level in the three-state ladder 5s\ensuremath{\rightarrow}5p\ensuremath{\rightarrow}5d of the rubidium atom by frequency-swept ultrashort laser pulses is observed if the frequency sweep first makes the pulse resonant with the 5s\ensuremath{\rightarrow}5p transition, and later with the 5p\ensuremath{\rightarrow}5d transition. With this ``intuitive'' scheme it is possible to achive complete population inversion via the intermediate level by two successive adiabatic passages. Complete inversion is also obtained by a ``counterintuitive'' direction of the frequency sweep, in which case population of the intermediate level during the pulse is strongly reduced.
Generally, high optical intensity is required for effective multiphoton excitation of quantum systems to highly excited states. In certain situations, however, lower-intensity, chirped pulses can provide more efficient transfer of population to the upper states by the process of adiabatic passage. We have studied the relative importance of these two mechanisms in the anharmonic $5s\ensuremath{-}5p\ensuremath{-}5d$ quantum ladder system of rubidium using frequency chirped laser pulses from an amplified Ti:sapphire laser (\ensuremath{\lambda}=780 and \ensuremath{\Delta}\ensuremath{\lambda}=10 nm). We measure simultaneously the three-photon ionization signal due to the Ti:sapphire and the population that remains in the 5d5d5d state with a postionizing 532-nm Nd:YAG pulse (where YAG denotes yttrium aluminum garnet). At low infrared fluences (80 \ensuremath{\mu}J/cm2{\mathrm{J}/\mathrm{c}\mathrm{m}}^{2}J/cm2), the transfer to the 5d5d5d state is significantly enhanced when the pulse frequency is swept from the red to the blue, such that it follows the frequency spacing of the rubidium ladder. Counterintuitively, population is also transferred efficiently for the blue-to-red chirp at high fluences (>5 mJ/cm2{\mathrm{m}\mathrm{J}/\mathrm{c}\mathrm{m}}^{2}mJ/cm2). We attribute both of these effects to adiabatic passage from the 5s5s5s state to the 5d5d5d state. Even at the highest fluences, more efficient transfer occurs for either direction of chirp than occurs at zero chirp, where the intensity is maximal. A comparison to theoretical predictions reveals striking agreement in both absolute magnitude and functional form. These results have important implications for the understanding of population transfer in complex ladder systems, such as molecular anharmonic vibrational ladders.
Excitation of a quantum ladder system by frequency-swept (-chirped) laser pulses has been investigated by studying an atomic model system, the 5s-5p-5d ladder of rubidium. The population transfer is significantly enhanced when the frequency is swept such that it follows the spacing of the ladder. At intermediate intensities we observe oscillations in the population transfer to the upper level (5d), which can be attributed to interference between two routes of excitation: sequential transfer and direct two-photon excitation via a virtual intermediate state. We determine simultaneously the population transfer to the upper level (5d) and the direct three-photon ionization, the latter corresponding to a three-step ladder with the final state being in the continuum: 5s-5p-5d-\ensuremath{\varepsilon}p/\ensuremath{\varepsilon}f. Experiments were performed, using 100 fs pulses at \ensuremath{\sim}780 nm from a self-mode-locked Ti:Al2{\mathrm{Al}}_{2}Al2​O3{\mathrm{O}}_{3}O3​ oscillator amplified at 10 Hz.
Using the time-dependent multilevel approach, we have calculated the coherent population transfer between the quantum states of potassium atom by a single frequency-chirped laser pulse. The result shows that a pair of sequential 'broadband' frequency-chirped laser pulses can efficiently transfer population from the initial state of the ladder system to the target state. It is also found that the population can be efficiently transferred to a target state and trapped there by using an 'intuitive' or a 'counterintuitive' frequency sweep laser pulse in the case of 'narrowband' frequency-chirped laser pulse. Our research shows that the complete population transfer is related to the pulse duration, chirp rate, and amplitude of the laser pulse.
The authors have demonstrated that appropriately time-ordered pulses of diode-laser light can provide efficient population transfer in a three-level cascade system. Laser-trapped Rb atoms are excited from the 5S ground state to the highly-excited 5D level (via the 5P intermediate level) with nearly 100% efficiency. Diode-lasers at 780 nm (5S{yields}5P) and 776 nm (5P{yields}5D) are pulsed with acousto-optic modulators to provide the excitation. The variation of transfer efficiency with relative delay between the two pulses is in good agreement with theoretical expectations. Optimum excitation occurs for the counterintuitive pulse ordering, i.e., when the upper transition is driven first.
The stark-chirped rapid adiabatic passage (SCRAP) is analyzed, which can give a complete transfer of atomic or molecular population between two bound states in the two level states and Λ(-three)-level state, by means of level-crossing technique and dressed states theory. SCRAP uses two laser pulses: a nearly-resonant pump laser pulse drives the population between a ground state and an excited state, and an intense far-off-resonance Stark laser pulse modifies the transition frequency and resonates with the Bohr transition frequency for two level crossings. A population-transfer maximal efficiency approaching unity is provided from the ground state to the excited state, when the delay of the Stark-shifting pulse with respect to the pulse of the coupling-laser frequency is appropriate and guarantees the pump pulse is strong at one of these crossings and is weak at second one.