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Journal of Applied Physics

The possibility of x-ray pulse shortening through laser induced ac Stark shift in atoms

Li Chen

Published Mar 15, 1993• 2 citations
Publisher

Abstract

It is suggested that an originally long x-ray pulse may be shortened substantially, or, alternatively, a short sharp dip may be introduced into an originally long x-ray pulse, by passing the x rays through a resonant absorbing medium that is illuminated by an intense ultrashort laser pulse at certain chosen times. The various conditions for this scheme are stated. Neon and xenon gas media are discussed as examples.

DOI
10.1063/1.353080

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An ultrashort weak pulse that propagates resonantly on the $\ensuremath{\mid}a⟩\ensuremath{\rightarrow}\ensuremath{\mid}b⟩$ transition of a three-level system $(\ensuremath{\mid}a⟩,\ensuremath{\mid}b⟩,\ensuremath{\mid}c⟩)$ experiences reshaping effects on a time scale longer than its duration. When a strong field is applied (nonresonantly) on the $\ensuremath{\mid}b⟩\ensuremath{\rightarrow}\ensuremath{\mid}c⟩$ transition, important light shifts are induced. We have theoretically studied the novel behavior of both the medium and weak propagating laser pulses in such conditions. For the transmitted field profile, drastic changes occur: at long times, efficient reduction of the distortion is obtained, while strong oscillations appear for short times, mapping out the dynamical light shift. These shaping effects, which are accompanied by new features in the spectrum for the propagating pulse, are interpreted as the result of the interference between incident field and induced dipole radiated field, whose frequency sweeps in time because of the induced light shift.

Physical Review A•Jun 22, 2004•8 citations
It is suggested that an originally long x-ray pulse may be shortened substantially, or, alternatively, a short sharp dip may be introduced into an originally long x-ray pulse, by passing the x rays through a resonant absorbing medium that is illuminated by an intense ultrashort laser pulse at certain chosen times. The various conditions for this scheme are stated. Neon and xenon gas media are discussed as examples.
We propose a universal method to produce extremely short pulses of electromagnetic radiation in various spectral ranges. The essence of the method is a resonant interaction of radiation with atoms under the conditions of adiabatic periodic modulation of atomic transition frequencies by a far-off-resonant control laser field via dynamic Stark shift of the atomic levels and proper adjustment of the control field intensity and frequency, as well as the optical depth of the medium. The potential of the method is illustrated by an example in a hydrogenlike atomic system.
The propagation of ultrashort x-ray pulses through a resonant attenuating two-level atom medium is investigated on the basis of the temporal point-source (the Green-function) formalism. A general case of the small-area pulse (SAP) approximation of a traveling coherent wave is considered. The patterns of the SAP envelope E(t,z) and energy U(z) evolution within the medium are calculated in the cases of incident Lorentzian and exponential pulses and their dependence on the temporal bandwidth ${\mathrm{\ensuremath{\tau}}}_{\mathrm{band}}$ in comparison with the total dissipative relaxation time (lifetime) T2{\mathrm{T}}_{2}T2​ . It is shown that if ${\mathrm{\ensuremath{\tau}}}_{\mathrm{band}}$ is the same order or smaller than T2{\mathrm{T}}_{2}T2​ , the reshaping (oscillations) of the pulse envelope and/or low energy-loss effects occur in accord with the general conclusions pointed out by Crisp [Phys. Rev. A 1, 1604 (1970)] in the case of the SAP for coherent light traveling through a resonant medium. The experimental conditions for the observation of penetration effects of the SAP of x rays are discussed. Based on the theoretical study, it is found that an ultrashort x-ray pulse emitted by an ultrashort laser-produced plasma propagates through thin resonant medium foils with low energy loss.
The coherent control of atomic energy level population is important for several areas physics and engineering including fluorescent spectroscopy, resonantly enhanced multiphoton ionization, harmonic generation, remote sensing and lasing. At low intensities where the Stark shift can be neglected, coherent control mechanisms usually exploit resonances created from stationary energy states dressed by integer multiples of the photon energy. However, in the strong field limit where large laser pulse intensities lead to significant Stark shifts, the resonant conditions are dynamic both in time and space and will likely not be satisfied for the entire laser pulse or beam.\n\nIn this research, we experimentally and theoretically demonstrate coherent control of atomic energy level population in sodium vapor as well as the selective excitation of sodium Rydberg levels using high intensity laser pulses. We also make a direct measurement of the ponderomotive shift of the continuum. While techniques based on resonance interferences and rapid adiabatic passage have been introduced for selective excitation, the conditions that must be satisfied for these phenomena can be hampered by short femtosecond pulse durations and large Stark shifts. In contrast, a technique referred to here as Resonance Sampling, uses the AC Stark shifts to selectively induce Freeman resonances and is resilient to changes in the spectral phase. Freeman resonances established in this way can result in population inversion of the excited state, and may lead to new lasing mediums and remote sensing applications.\n\n\nIn this research, we experimentally and theoretically demonstrate coherent control of atomic energy level population in sodium vapor as well as the selective excitation of sodium Rydberg levels using high intensity laser pulses. While techniques based on resonance interferences and rapid adiabatic passage have been introduced for selective excitation, the conditions that must be satisfied for these phenomena can be hampered by short femtosecond pulse durations and large Stark shifts. In contrast, a technique referred to here as Resonance Sampling, uses the AC Stark shifts to selectively induce Freeman resonances and is resilient to changes in the spectral phase. Freeman resonances established in this way can result in population inversion of the excited state, and may lead to new lasing mediums and remote sensing applications.
Resonant absorption in a laser pulse can be reduced to zero if the trailing part induces stimulated emission and returns the absorbed energy back to the pulse. This kind of effect is called Self-Induced Transparency (SIT). Analytic descriptions of short pulse propagation in a resonant medium, based on Slowly Varying Envelope Approximation (SVEA) and in linear media, have been extended in studies of SIT along two principal directions: First, to improve the SVEA by introducing Short Pulse Approximation (SPA); and second, to consider the effect of nonlinear refractive index.
An ultrashort weak pulse that propagates resonantly on the $\ensuremath{\mid}a⟩\ensuremath{\rightarrow}\ensuremath{\mid}b⟩$ transition of a three-level system $(\ensuremath{\mid}a⟩,\ensuremath{\mid}b⟩,\ensuremath{\mid}c⟩)$ experiences reshaping effects on a time scale longer than its duration. When a strong field is applied (nonresonantly) on the $\ensuremath{\mid}b⟩\ensuremath{\rightarrow}\ensuremath{\mid}c⟩$ transition, important light shifts are induced. We have theoretically studied the novel behavior of both the medium and weak propagating laser pulses in such conditions. For the transmitted field profile, drastic changes occur: at long times, efficient reduction of the distortion is obtained, while strong oscillations appear for short times, mapping out the dynamical light shift. These shaping effects, which are accompanied by new features in the spectrum for the propagating pulse, are interpreted as the result of the interference between incident field and induced dipole radiated field, whose frequency sweeps in time because of the induced light shift.