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arXiv:2412.04223

Generation of Subfemtosecond Deep and Vacuum UV pulses via Two-Photon Rabi Oscillations in Alkali Atoms or Alkaline Earth Ions

I. R. Khairulin, Alexander A. Romanov, Alexander A. Silaev, Mikhail Yu. Ryabikin, V. A. Antonov

Published Dec 5, 2024• 0 citations
arXivHTMLPDF

Abstract

A method is proposed for the formation of femto- and subfemtosecond pulses of the deep ultraviolet and vacuum ultraviolet radiation via generating the third harmonic of femtosecond laser pulses during their resonant interaction with alkali atoms or alkaline earth ions. The pulse formation occurs due to two-photon Rabi oscillations between quasi-equidistant energy levels of atoms or ions. The duration of the generated third harmonic pulse is several times shorter than far from resonance, while the generation efficiency is up to 3-4 orders of magnitude higher.

DOI
10.48550/arxiv.2412.04223

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A method is proposed for the formation of femto- and subfemtosecond pulses of the deep ultraviolet and vacuum ultraviolet radiation via generating the third harmonic of femtosecond laser pulses during their resonant interaction with alkali atoms or alkaline earth ions. The pulse formation occurs due to two-photon Rabi oscillations between quasi-equidistant energy levels of atoms or ions. The duration of the generated third harmonic pulse is several times shorter than far from resonance, while the generation efficiency is up to 3-4 orders of magnitude higher.
Two-photon resonant and near-resonant four-wave difference-frequency mixing in gases in the interaction regime when laser pulse durations are comparable to or shorter than the medium polarization relaxation time ${T}_{2}^{\ensuremath{'}}$ is investigated. The results of experimental studies of the process in Ar and Kr using pump pulses from the ArF-excimer laser are presented demonstrating a generation of tunable short pulse radiation in the range 102--124 nm. The results are discussed in terms of a theoretical model based on a self-consistent solution of the Bloch equations for the atomic transitions and the Maxwell equations for the fields. This enables one to interpret specific nonstationary resonant and quasiresonant phenomena involved in the frequency conversion process. It is shown that the femtosecond-pulse four-wave frequency-mixing technique with probe pulses significantly shorter than the pump pulses makes it possible to study the coherent dynamics of an atomic transition exposed to an intense field. Using atomic Kr as the nonlinear medium, coherent Rabi oscillations and the subsequent phase relaxation of excitation were observed under the condition of two-photon interaction of Kr with femtosecond 193-nm laser pulses. The obtained information is important for controlling and optimizing processes of two-photon resonant frequency conversion and for time-resolved studies of Rydberg states in atoms and molecules.
We demonstrate Rabi oscillations in nonlinear ionization of argon by an intense femtosecond extreme ultraviolet (XUV) laser field produced by high-harmonic generation. We monitor the formation of Ar2+\mathrm{A}{\mathrm{r}}^{2+}Ar2+ as a function of the time delay between the XUV pulse and an additional near-infrared (NIR) femtosecond laser pulse, and show that the population of an Ar+∗\mathrm{A}{\mathrm{r}}^{+*}Ar+∗ intermediate resonance exhibits strong modulations both due to an NIR laser-induced Stark shift and XUV-induced Rabi cycling between the ground state of Ar+\mathrm{A}{\mathrm{r}}^{+}Ar+ and the Ar+∗\mathrm{A}{\mathrm{r}}^{+*}Ar+∗ excited state. Our experiment represents a direct experimental observation of a Rabi-cycling process in the XUV regime.
We investigate the third order harmonic generation for model atoms interacting with intense short laser pulses. We focus on the cases where the ponderomotive energy is not big enough to generate high-order harmonics, and where resonant transitions between bound states are also as important as multi-photon ionization and low-order harmonic generation. We find that the third order harmonic intensity from a model hydrogen atom can decrease as the laser frequency draws near a resonance between bound states. We explain this result by looking into the ionization and Rabi oscillation from the numerical calculation of the time-dependent 1-D Schrödinger equation. In order to support our explanation we also investigate the populations of bound states that play an important role in this atomic process.
We introduce a new scheme for generating short-wavelength coherent broadband radiation with well-controlled spectral characteristics. It is based on shaping long-wavelength femtosecond pulse to coherently control atomic resonance-mediated (2+1) three-photon excitation to a broad far-from-resonance continuum. Here, the spectrum (central frequency and bandwidth) of deep-ultraviolet coherent broadband radiation generated in Na vapor is experimentally controlled by tuning the linear chirp we apply to the driving phase-shaped near-infrared femtosecond pulse. This is a first step in implementing the full scheme for producing shaped femtosecond pulses at wavelengths down-to the vacuum-ultraviolet range.
In coherent light-matter interactions the coherent state produced by the light pulse remains coherent during the entire period of observation. One of the well-known examples of such interactions is the Rabi oscillation—a periodic transfer of population in a two-level system induced by an external electromagnetic field. While these oscillations have been studied in light-matter interactions since the discovery of laser itself, most of the studies are limited to long wavelengths, typically ranging from microwave to visible radiation. Here, we discuss how such oscillations can be generated at shorter wavelengths, for example in the extreme ultraviolet domain, using intense femtosecond pulses from seeded free-electron lasers. When a two-level system is undergoing Rabi oscillations, it can be described via an ‘atom + photon’ dressed-state, the properties of which can be further manipulated to study quantum entanglement between massive particles. Our results offer opportunities to employ these intense laser pulses for generating and controlling coherent light-matter interactions across ultrashort timescales.
Shaped near-infrared (NIR) femtosecond pulses are used for the first time to control the generation of coherent deep-ultraviolet (UV) radiation in an atomic resonance-mediated (2+1) three-photon excitation. The broadband excitation coherently involves pathways that are on resonance with the intermediate resonance state as well as pathways that are near resonance with it. Experimental and theoretical results are presented for phase controlling the total emitted UV yield in atomic sodium. Depending on the NIR spectrum of the excitation pulse, the coherent UV emission is either predominantly due to a single excited real state that is accessed resonantly or due to a manifold of virtual states. The former leads to a narrowband UV emission, while the latter leads to a broadband UV radiation. Basic phase control is exercised in both cases, with excellent agreement between experiments and calculations. The tunability is over an order-of-magnitude UV-yield range.