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Quantum Electronics

Efficient excitation of media for laser isotope separation

S. K. Borisov, Marina A. Kuzmina, В. А. Мишин

Published Jul 31, 1995• 4 citations
Publisher

Abstract

A theoretical investigation is made of the photoionisation of a three-level medium intended for laser isotope separation. The results show that the highest ionisation efficiency can be achieved by dividing the process into two stages: coherent population inversion and photoionisation. A study is made of the possibility of inversion of three-level systems for various detunings of the laser frequencies from the radiative transition frequencies. It is shown that there are detunings for which the population of the third level is maximal for any a priori selected parameters of the medium and field. Such detunings lie near a two-photon resonance and depend on the energy density and duration of the laser pulses, and also on the dipole moments of the transitions. The suitability of this method in the case of inhomogeneously broadened optically dense media is considered. It is shown that the method is most efficient in the case of counterpropagating laser pulses. The efficiency of the method is demonstrated for ytterbium.

DOI
10.1070/qe1995v025n07abeh000446

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A theoretical investigation is made of the photoionisation of a three-level medium intended for laser isotope separation. The results show that the highest ionisation efficiency can be achieved by dividing the process into two stages: coherent population inversion and photoionisation. A study is made of the possibility of inversion of three-level systems for various detunings of the laser frequencies from the radiative transition frequencies. It is shown that there are detunings for which the population of the third level is maximal for any a priori selected parameters of the medium and field. Such detunings lie near a two-photon resonance and depend on the energy density and duration of the laser pulses, and also on the dipole moments of the transitions. The suitability of this method in the case of inhomogeneously broadened optically dense media is considered. It is shown that the method is most efficient in the case of counterpropagating laser pulses. The efficiency of the method is demonstrated for ytterbium.
A principle for the efficient excitation of a three-level medium in a selective photoionization method is examined. It is shown that with special detunings for a given set of the laser-atom interaction the third-level population can be maximized. The expression of the optimal detunings is derived near the two-photon resonance region. It is shown that if the optimal detuning method is applied to three-level systems of Yb and Ca, the first-level populations are inverted to their corresponding third levels by about 100% and 86%, respectively. Application of this method to a Doppler-broadened medium is discussed and it is found that the optimal detuning method with counterpropagating waves will be very effective for selective ionization of wanted atoms.
A theory is developed of multistage isotopically selective ionisation of ytterbium ions. This theory is based on the rate equations for the level populations and on the transfer equation for the laser line intensities. An analysis of the time characteristics of multistage photoionisation is used to find the optimal laser radiation intensities for different ionisation stages. A multilevel model of multistage photoionisation developed for a natural mixture of isotopes is used to identify the ranges of wavelengths where the photoionisation selectivity is the highest. A mechanism of degradation of a short but powerful laser pulse is considered and the length of an efficiently excited medium is found.
The population inversion in a three-level atomic system, interacting with two coherent laser pulses, occurs from the first level to the third level at the optimal detunings of the laser frequencies near the two-photon resonance. An examination is performed to see whether the energy density and time-delay of the photoionizing laser can significantly influence the ion yield for the case where the atom interacts with two coherent pulses and one photoionizing pulse incoherently coupled. The well-known three-colour, three-step, three-photon ionization scheme, which is available for the selective photoionization of 168Yb, is chosen for use in this research with the Bloch equation. It is revealed that the proper time-delay and energy density enable us to achieve higher efficiency for the selective photoionization of 168Yb, regardless of the Doppler-broadening, though the efficiency and selectivity of coherent excitation are prominently enhanced by applying the optimal detunings.
Population transfer in three-level atomic systems with high efficiency and selectivity is desirable for practical applications in laser spectroscopy, trace analysis, and isotope separation. The effective excitation method for achieving this population transfer in three-level atomic systems using coherent two-photon excitation with two optimally detuned counterpropagating pulsed lasers, initially investigated analytically assuming monochromatic lasers and Doppler-free systems, is studied numerically by solving the density matrix equations incorporating finite laser linewidth and effective Doppler width. A three-level Yb system is chosen for illustration of numerical results. The effects of laser linewidth and effective Doppler width on the values of optimal detunings and maximum third-level population are discussed.
Mathematical simulation methods are used to optimise of three-stage isotopically selective photoionisation of atoms of the ytterbium-168 isotope. The influence of nonlinear deformation of laser beams is taken into account. It is shown that exact tuning of the laser radiation frequencies to the frequencies of the lines of radiative transitions in the ytterbium-168 atom results in absorption of the laser radiation by atoms of other isotopes and in self-focusing of the laser beams. Introduction of a two-photon detuning reduces considerably the influence of the absorption and self-focusing. A determination is reported of the two-photon detuning, and of the dimensions and the spatial distribution of the power in the laser beam required to ensure that the absorption and nonlinear self-focusing of laser beams do not reduce significantly the ionisation efficiency.
The selectivity of three-step pulse photo-ionization of atoms versus the detuning delta of exciting radiation frequencies from intermediate resonance is investigated in powerful laser fields where the power broadening of the resonant transitions is at least of the order of isotope shifts. The Doppler broadening of the resonant transitions is taken into account and it is assumed that the sum frequency of both exciting laser radiations is tuned to the line centre of the two-photon transition of a selected isotope. It is shown that in this situation the photo-ionization selectivity increases so greatly with the absolute detuning mod delta mod that even in such strong fields the process of photo-ionization can be highly selective. The optimization of selective photo-ionization of atoms is discussed in connection with the decrease in the selective isotope yield with increase of the absolute detuning mod delta mod . Analytical consideration of selective three-step pulse photo-ionization in powerful laser fields, based on the density matrix formalism, is presented together with the results of numerical simulation of the 40 Ca- 48 Ca laser isotope separation process as an example.