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Physical Review Letters

Multiphoton Resonance Meets Tunneling Ionization: High-Efficient Photoexcitation in Strong-Field-Dressed Ions

Yewei Chen, Hongbin Lei, Quanjun Wang, Hongqiang Xie, He Zhang, Xu Lu, Ning Zhang, Shunlin Huang, Yuzhu Wu, Jianpeng Liu, Qian Zhang, Yi Liu, Zengxiu Zhao, Jing Zhao, Jinping Yao

Published Sep 13, 2024• 16 citations

Abstract

Tunneling ionization, a fascinating quantum phenomenon, has played the key role in the development of attosecond physics. Upon absorption of a few tens of photons, tunneling ionization creates ions in different excited states and even enables the formation of population inversion between ionic states. However, the underlying physics is still being debated. Here, we demonstrate a significant enhancement in the efficiency of multiphoton excitation when ionization of neutral molecules and resonant excitation of ions coexist in strong laser fields. It facilitates the dramatic increase in population inversion and lasing radiation in N_{2}^{+} around 1000 nm pump wavelength. Utilizing the ionization-coupling theory, we discover that the synergistic interplay between tunneling ionization and multiphoton excitation enables the ionic coherence to be maximized by phase locking of the periodically created ionic dipoles and consistently maintain an optimal phase for the follow-up photoexcitation. This Letter provides new insights into the photoexcitation mechanism of ions in strong laser fields and opens up a route for optimizing ionic lasing radiations.

DOI: 10.1103/physrevlett.133.113201

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Tunneling ionization, a fascinating quantum phenomenon, has played the key role in the development of attosecond physics. Upon absorption of a few tens of photons, tunneling ionization creates ions in different excited states and even enables the formation of population inversion between ionic states. However, the underlying physics is still being debated. Here, we demonstrate a significant enhancement in the efficiency of multiphoton excitation when ionization of neutral molecules and resonant excitation of ions coexist in strong laser fields. It facilitates the dramatic increase in population inversion and lasing radiation in N_{2}^{+} around 1000 nm pump wavelength. Utilizing the ionization-coupling theory, we discover that the synergistic interplay between tunneling ionization and multiphoton excitation enables the ionic coherence to be maximized by phase locking of the periodically created ionic dipoles and consistently maintain an optimal phase for the follow-up photoexcitation. This Letter provides new insights into the photoexcitation mechanism of ions in strong laser fields and opens up a route for optimizing ionic lasing radiations.

Tunneling ionization followed by strong-field excitation leads to important ultrafast phenomena such as charge migration and lasing. Recent theoretical developments suggest that the population of the ionic excited state can be greatly enhanced due to the complex interplay between tunneling and excitation. In this Letter, using an adiabatic approach for both tunneling and excitation, semi-analytical solutions are derived for the population and coherence of a two-level ionic system. This approach removes the strong-field dressing, revealing novel sub-half-cycle processes for excited state population and coherence buildup. It predicts that the excited state population is enhanced by an order of magnitude, independent of the laser wavelength, while coherence amplitude can be boosted by over four orders of magnitude for a multi-cycle pulse. For a single-cycle pulse, it suggests that coherence amplitude decreases rapidly as the wavelength increases. This work introduces a novel framework for generating and controlling the electronic excited state and coherence using intense laser pulses, with applications in strong-field control of chemistry and lasing.

We develop a vibronic coherence model for multiorbital ionization and excitation of nitrogen molecules, based on an up-to-date concept of coherent ionization into excited states [C. H. Yuen and C. D. Lin, Phys. Rev. A 108, 023123 (2023); Phys. Rev. A 109, 033108 (2024)]. Compared to other models, where the coherence of excitation dynamics is described with fewer details, our model predicts larger populations at excited states, in particular, at levels corresponding to the transition wavelengths 391 or 428 nm relevant to air lasing in the atmospheric environment. The dependence of population inversion on the pump wavelength is investigated, showing that ionic coherence in ionization injection enhances the vibronic three-photon transition in the

{\mathrm{N}}_{2}

postionization process.

Multielectron tunneling ionization creates ionic coherence crucial for lasing and driving electron motion in molecules. While tunneling is well understood as a single active electron process, less emphasis has been placed on theoretical descriptions of bound electrons during tunneling. This work systematically investigates multielectron tunneling ionization based on the strong field approximation, establishing a theoretical foundation and demonstrating the equivalence of wave function and density matrix approaches for subcycle ionic dynamics. An accurate subcycle nonadiabatic ionization rate is also derived and incorporated into the theory to improve its quantitative accuracy. Applying the theory to N

_{2}

and CO

_{2}

, this work showcases how an intense laser field can induce ionic coherence in molecules as observed in previous experiments. These findings encourage future investigations into multielectron tunneling ionization and its applications in lasing and in controlling chemical reactions.

Supercontinuum (SC) light sources hold versatile applications in many fields ranging from imaging microscopic structural dynamics to achieving frequency comb metrology. Although such broadband light sources are readily accessible in the visible and near infrared regime, the ultraviolet (UV) extension of SC spectrum is still challenging. Here, we demonstrate that the joint contribution of strong field ionization and quantum resonance leads to the unexpected UV continuum radiation spanning the 100 nm bandwidth in molecular nitrogen ions. Quantum coherences in a bunch of ionic levels are found to be created by dynamic Stark-assisted multiphoton resonances following tunneling ionization. We show that the dynamical evolution of the coherence-enhanced polarization wave gives rise to laser-assisted continuum emission inside the laser field and free-induction decay after the laser field, which jointly contribute to the SC generation together with fifth harmonics. As proof of principle, we also show the application of the SC radiation in the absorption spectroscopy. This work offers an alternative scheme for constructing exotic SC sources, and opens up the territory of ionic quantum optics in the strong-field regime.

Multiphoton ionization of Ca and Sr was studied at a wavelength of 532 nm with optical pulses of 12-ns and 35-ps duration. Excess photon absorption followed by autoionizing decay leads to populations of excited states of singly charged ions that can be observed via their spontaneous emission. Relative measurements were performed on fluorescence from

{\mathrm{Ca}}^{+}

(4p),

{\mathrm{Ca}}^{+}

(5s), and

{\mathrm{Ca}}^{+}

(4d), and from

{\mathrm{Sr}}^{+}

(5p) and

{\mathrm{Sr}}^{+}

(6s). The contribution of the highly excited states was found to be favored under short-pulse (35-ps) irradiation.

Neutral Rydberg state excitation in strong UV laser fields of Xe atoms has been studied and compared with direct ionization. The yields of both strong-field excitation and ionization have been measured as a function of UV laser intensity and ellipticity. The underlying physical mechanism has been discussed based on the experimental results, indicating that the Rydberg states are populated via multi-photon resonance excitation in strong 400 nm laser fields. Finally, a comparative study of Xe and

O_2

with similar ionization potentials have been performed, showing the suppressed excitation of molecules in strong UV laser fields.