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Laser Physics

Two-photon gain without inversion vs. linear absorption in a three-level scheme

Gennady A. Koganov, R. Shuker

Published May 25, 2011• 4 citations
Publisher

Abstract

A three level scheme interacting with three laser fields is considered. It is found that two competing processes contribute to the absorption/dispersion on a particular transition: (i) one-photon processes governed by the field applied to that transition, and (ii) two-photon processes governed by the two laser fields applied to two other transitions. Interference between the two processes can be both constructive and destructive. It is shown that the net absorption/gain and dispersion can be controlled by the relative phases of the laser fields.

DOI
10.1134/s1054660x11130160

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AIP conference proceedings•Jan 1, 1989•0 citations
A three level scheme interacting with three laser fields is considered. It is found that two competing processes contribute to the absorption/dispersion on a particular transition: (i) one-photon processes governed by the field applied to that transition, and (ii) two-photon processes governed by the two laser fields applied to two other transitions. Interference between the two processes can be both constructive and destructive. It is shown that the net absorption/gain and dispersion can be controlled by the relative phases of the laser fields.
We have calculated the interference spectra of a single electron in the excited state of a four-level atom interacting with three laser fields, where two of them are on two-photon resonance with the three levels forming a `Λ' scheme while the third laser operates between the second ground and the second excited states of the atom. It is shown that in the low-intensity limit of all three laser fields, the third laser field destroys the destructive interference that occurs between the spontaneous radiative decay and the process arising from the combined field of all the three laser fields. The result is that the overall intensity of the peak at the centre of the line depends on the strength of the third laser field and it takes negative values indicating that light amplification without population inversion (LAWI) is likely to occur at the one-photon frequency. The antisymmetric two-photon state of the two lower levels as well as the two-photon process, where one laser photon is absorbed and another laser photon is emitted simultaneously by the second excited state of the atom, is found to exhibit LAWI at the one-photon frequency. The derived results are graphically presented and discussed.
When a two-level system interacts resonantly with a saturating pump laser, the absorption profile of a probe wave in the Rabi sidebands resembles two separate dispersion profiles. At sufficiently high probe intensity this effect is accompanied by a dispersive profile in the pump-wave absorption, considered as a function of the probe-wave frequency, and also by a dip in the population of the upper state of the two-level system. The relative stabilization of the ground state is interpreted in terms of two-laser-photon processes where absorption of a pump (probe) photon is followed by stimulated emission of a probe (pump) photon. The dispersive absorption profile is attributed to the competition between three- and four-photon scattering processes.
We describe here an effect which occurs when a first laser is tuned near a dipole allowed three‐photon resonance and a second laser is used to complete a dipole allowed four‐photon resonance between the ground state ‖ 0≳ and an excited state ‖ 2≳. In this process three photons are absorbed from the first laser and one photon from the second; so that if the ‖ 0≳ to ‖ 2≳ transition is two‐photon allowed the transition is also pumped resonantly by the third harmonic field due to the first laser and the second laser field. When the second laser is strong enough to cause strong absorption of the third harmonic light, and the phase mismatch, Δk is large and dominated by the nearby resonance, a destructive interference occurs between the pumping of the ‖ 0≳ to ‖ 2≳ transition by two‐ and four‐photon processes.