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Soviet Journal of Quantum Electronics

Coherent low-frequency effects in three-level media with asymmetric optical transitions

Olga A. Kocharovskaya

Published Jan 31, 1990• 1 citation
Publisher

Abstract

An asymmetry of the spectral characteristics of optical transitions influences strongly the nature of a coherent nonlinear interaction between a broad-band three-level absorber and a series of ultrashort pulses. It is predicted that the absorption of the field will be reduced substantially in a three-level medium with asymmetric 1–3, 2–3 transitions. It is shown that the strong asymmetry results in coherent bleaching of the medium even in the absence of a resonance between the low-frequency 1–2 transition and the pulse repetition frequency (also under the action of high-power single ultrashort pulses). There is a range of parameters where a resonant dependence of the absorbed power on the pulse repetition frequency is observed and this dependence is enhanced appreciably by increasing the asymmetry.

DOI
10.1070/qe1990v020n01abeh004781

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An asymmetry of the spectral characteristics of optical transitions influences strongly the nature of a coherent nonlinear interaction between a broad-band three-level absorber and a series of ultrashort pulses. It is predicted that the absorption of the field will be reduced substantially in a three-level medium with asymmetric 1–3, 2–3 transitions. It is shown that the strong asymmetry results in coherent bleaching of the medium even in the absence of a resonance between the low-frequency 1–2 transition and the pulse repetition frequency (also under the action of high-power single ultrashort pulses). There is a range of parameters where a resonant dependence of the absorbed power on the pulse repetition frequency is observed and this dependence is enhanced appreciably by increasing the asymmetry.
The coherent V -configuration interaction between an optically dense resonantly-absorbing three-level medium (neon) and two ultrashort superradiance pulses with converging wave fronts is investigated experimentally and theoretically. Both separate and combined propagation of pulses with wavelengths λ 1 =614.3 nm (strong field, θ 1 ⩾ π ) and λ 3 =594.5 nm (weak field, θ 3 ≈/20) are studied. For propagation of a separate strong-field pulse, supertransparency of the absorbing medium was observed, which is associated with the generation of a soliton-like pulse at the difference frequency (Δ ν ≈1700 MHz) and the dispersion-diffraction stabilization effect. Under these conditions a weak-field pulse is completely absorbed. Combined propagation of the pulses leads to novel effects. A below-threshold pulse (weak field) was observed to pass through the absorber while interacting coherently with a strong-field pulse at a neighboring transition. It is shown theoretically that absorption of the weak pulse is reduced for two reasons: first, as a result of incoherent transparency of the resonance transition caused by emptying of the lower level by the field of the strong pulse, and second, as a result of coherent transfer of polarization between the upper levels via the two-photon processes. When the conditions for combined propagation are met, the latter mechanism ensures inversionless amplification of a weak pulse over a wide band of frequencies. In this case, the gain can even exceed the linear absorption coefficient in absolute value. A difference in propagation velocities of the weak and strong pulses was recorded experimentally, along with a shift in the carrier frequency of the weak field towards the red (≈600 MHz). A mechanism for transfer of phase modulation from a strong pulse to a weak pulse via the common lower level is discussed theoretically.
A numerical investigation is made of third harmonic generation (THG) under conditions of two-photon resonance between the fundamental frequency and a selected transition in the atoms of a medium. It is shown that under conditions of coherent interaction between the pump pulse and the medium, a THG efficiency of ∼20% or higher may be achieved. In this case, THG in the form of high-power ultrashort pulses may be obtained. It is shown that experimental observation of similar effects in specific media should not present particular difficulties.
A review is given of recent theoretical investigations on coherent propagation and nonlinear interaction of multifrequency ultrashort radiation pulses in resonantly absorbing media. An analysis is made of frequency conversion by parametric interaction and stimulated Raman scattering of laser pulses in resonant three-level media. New ways of controlling the parameters of coherent pulses and the use of multifrequency pulses in the spectroscopy of multilevel media are discussed. The experimental capabilities of the investigated effects are considered.
Propagation of broad-band ultimately short light pulses in a partially absorbing medium is analyzed in the framework of the three-level model. Nonlinear wave equations are obtained describing propagation of light pulses in media with quadratic (all three transitions are allowed) or cubic (one of the transitions is forbidden) nonlinearity in the range of optical transparency or with the sine-Gordon-type nonlinearity in the region of absorption. Using the averaged variational principle, the approximate solutions of equations in the form of unipolar soliton-like signals are found and conditions of their transverse stability are determined. A stable propagation of a broad-band pulse is shown to be possible under conditions when monochromatic signals exhibit self-focusing.
A numerical experiment was carried out on resonant interaction of ultrashort light pulses with a radially symmetric power distribution. It is shown that in the case of an exact resonance the result of diffraction by a light-induced bleached stop in a beam gives rise to ring-shaped power distribution zones. An increase in the area under the profile of an ultrashort pulse results in a redistribution of the beam power out of its core to the peripheral ring zones. Detuning of the field frequency from the exact resonance blurs the ring structure of the beam.