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Applied Physics Letters

X-ray stimulated Raman scattering in Li and He

E. Hudis, P. L. Shkolnikov, A. E. Kaplan

Published Feb 14, 1994• 10 citations
Publisher

Abstract

We propose to use initially neutral Li and He gases to observe x-ray stimulating Raman scattering. We solve the Maxwell–Bloch equations for the Raman scattering in the presence of a strong photoionization atomic depletion and show that very high conversion efficiencies can be obtained, although the required intensities are significantly high to cause very fast photoionization.

DOI
10.1063/1.111024

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We propose to use initially neutral Li and He gases to observe x-ray stimulating Raman scattering. We solve the Maxwell–Bloch equations for the Raman scattering in the presence of a strong photoionization atomic depletion and show that very high conversion efficiencies can be obtained, although the required intensities are significantly high to cause very fast photoionization.
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The effects of ground state depletion of atoms on the characteristics of stimulated Raman scattering in metal vapors are studied by simultaniously solving the rate-equation of the atom energy levels related in the Raman scattering, and the equations describing the energy-conversion from the focused pump laser pulses to the Raman down-converted pulses. The ground state of metal atoms are depleted most strongly in the regions near the focal spots of the pump beams because of the higher intensities of light and smaller available medium volume in thosa regions. This will result in the unsymmetrical shapes of the Raman-shifted pulses for the front and rear edges, and will reduce the energy-conversion efficiency.
A semiclassical theory of hyper-Raman scattering and three-photon resonant ionization via the coupled density-matrix and Maxwell equations is presented. A simplified three-level atom model is obtained, which includes two-photon resonant pumping and time-dependent photoionization rates. We consider conditions typically encountered in atomic vapors to simulate numerically the pulse propagation. A strong depletion of the photoionization probability in the hyper-Raman field-saturation regime is predicted.
Stimulated two-photon induced Raman scattering (hyper-Raman scattering) in an amplifier consisting of four-level atoms is analyzed semiclassically, via the coupled density matrix and Maxwell equations. We consider conditions typically encountered in the optical regime and in vapors. We show that even in the linear regime, it is characterized by a significant depletion of the atomic ground-state population, thus invalidating perturbative analyses based, for instance, on the nonlinear susceptibility tensor. Two major regimes of operation are found, one in which the population is completely consumed by the interaction ("linear regime"), and one in which the pump laser is depleted ("nonlinear regime"). Both regimes are, however, characterized by small photon conversion efficiencies. The tuning range is also briefly discussed.
The nonlinear regime of stimulated Raman scattering (SRS) may be reached in two ways. The best known is that of depletion of the pump laser as a large number of photons are scattered into the Stokes field. There is, however, another way in which gain saturation can occur, that is, by depletion of the atomic population while the laser remains undepleted. The equations which describe SRS, including damping and population inversion,1 are remarkably similar to those of well-known theories of two-level superfluorescence, and it is possible by analogy to define a delay time τ D for cooperative Raman scattering (CRS). This delay time is inversely proportional to the spontaneous scattering rate, which depends on the pump laser intensity. A sufficient condition for CRS is that the spontaneous Stokes linewidth Γ is very much greater than τ D − 1 . The only other requirement is that there be more laser photons than scatterers, so that τ D is independent of the number of scatterers. The S(1) transition in molecular hydrogen, when excited by a short (≅ 100-ps) laser pulse, provides a possible system for observing pure CRS. Similar studies have been carried out previously2 for Γ ≅ τ D − 1 .
We have observed collisional redistribution of laser light scattered by a three-level atom, thallium, in the presence of argon. The two observed spectral components correspond to Raman scattering and collision-induced fluorescence. We have seen the growth of both components from spontaneous to stimulated scattering as the laser intensity is increased. While the gain for the Raman component agrees with theory, the observed gain of the collision-induced fluorescence is over an order of magnitude lower than predicted.