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Optics Letters

Effect of a dressing radiation field on the laser-induced collisional energy-transfer line shape

Roberto Buffa, M. Matera, Stefano Cavalieri

Published Dec 1, 1992• 3 citations
Publisher

Abstract

We describe a new scheme suitable for the study of strong-field effects on the laser-induced collisional energytransfer process. The use of two independently tuned radiation fields, to induce the interatomic transition and to modify the energy levels, is found to give rise to a splitting of the excitation spectrum peak, similar to the Autler-Townes doublet. Owing to the resonant coupling, an observable splitting of a few inverse centimeters can be induced by a dressing laser intensity of the order of 10 MW/cm(2).

DOI
10.1364/ol.17.001685

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We describe a new scheme suitable for the study of strong-field effects on the laser-induced collisional energytransfer process. The use of two independently tuned radiation fields, to induce the interatomic transition and to modify the energy levels, is found to give rise to a splitting of the excitation spectrum peak, similar to the Autler-Townes doublet. Owing to the resonant coupling, an observable splitting of a few inverse centimeters can be induced by a dressing laser intensity of the order of 10 MW/cm(2).
We discuss a scheme suitable for the study of strong-field effects on the excitation spectrum of the laser-induced collisional energy-transfer process. The use of a weak radiation field, to induce the interatomic transition, and a strong radiation field, to modify the energy-level position, is found to give rise to a splitting of the resonance peak. The position and the relative intensity of the two spectral components are strongly dependent on the intensity and detuning of the strong field. Line-shape calculations performed for the Eu-Sr system show that the effect is expected to be detectable even for moderate intensities of the strong field.
We consider the response of a laser-induced quadrupole-dipole collisional system driven by a strong dressing laser field with the aim of calculating the collisional cross section of a weak inducing laser probe. The addition of a second driving field to the traditional arrangement will cause magnitude changes of the spectra and modify the profile. The calculation results show that the bichromatic laser-induced collisional energy-transfer process can be an efficient way to probe Stark splitting of both the final state and intermediate state. The magnitude and position of the splitting spectral lines are strongly dependent on the intensity of the dressing laser field. The peak cross section almost reduces by a factor of 2 with the presence of the dressing laser. Also, in the antistatic wing, bright and dark lines are periodic, appearing with the increasing of the dressing laser intensity.
Summary form only given. It is well known that strong resonant laser radiation can significantly modify absorption and emission spectra of matter, producing such features, as the Autler-Towns doublet, the dark resonance, etc. Laser fluctuations, which are often inevitable, usually tend to smear out the field-induced spectral features. Here, however, we show that in an experimental scheme, usually used to obtain electromagnetically induced transparency (EIT), intensity fluctuations of a strong coupling field can induce a narrow peak in the probe absorption spectrum.
Based on the existing four-level model of laser-induced collisional energy transfer, the collision process in Ba-Sr system in both weak and strong fields is calculated by immediate integration of the equation of motion, and profiles of laser-induced transition probability and collision cross section under both conditions are obtained. Numerical results for the weak field are in fair to good agreement with former analytical results, while the ones for the strong field show that the effect of laser-induced collision is enhanced when the transfer laser intensity increases. Moreover, the peak of the laser-induced collisional energy transfer spectrum shows an evident shift from resonant frequency to the violet side in strong field, and full width at half maximum (i.e. tuning range) of the spectrum becomes obviously narrower compared with that in weak field.
We report an experimental observation of Autler-Townes doublet splitting in electromagnetically induced transparency (EIT) resonance. The splitting is introduced by a coherent microwave field, which perturbs a three-level A-type EIT system through an auxiliary level. The doublet splitting of EIT resonance is demonstrated in two cases, where the microwave field shares a common lower level with coupled or probe transition, respectively. The dependence of doublet splitting on microwave field intensity and detuning is measured. This may provide a feasible way of manipulating the atomic states with both laser and microwave field.
We report on the experimental observation of a strong-field dressing of an autoionizing two-electron state in helium with intense extreme-ultraviolet laser pulses from a free-electron laser. The asymmetric Fano line shape of this transition is spectrally resolved, and we observe modifications of the resonance asymmetry structure for increasing free-electron-laser pulse energy on the order of few tens of Microjoules. A quantum-mechanical calculation of the time-dependent dipole response of this autoionizing state, driven by classical extreme-ultraviolet (XUV) electric fields, evidences strong-field-induced energy and phase shifts of the doubly excited state, which are extracted from the Fano line-shape asymmetry. The experimental results obtained at the Free-Electron Laser in Hamburg (FLASH) thus correspond to transient energy shifts on the order of a few meV, induced by strong XUV fields. These results open up a new way of performing nonperturbative XUV nonlinear optics for the light-matter interaction of resonant electronic transitions in atoms at short wavelengths.