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Journal of Experimental and Theoretical Physics

Weak-pulse transparency enhancement in an optically dense three-level medium induced by a 2π pulse in a neighboring transition (V-scheme)

N. V. Denisova, V. S. Egorov, V. V. Kozlov, N. M. Reutova, Pavel Yu. Serdobintsev, Evald E. Fradkin

Published Jan 1, 1998• 3 citations
Publisher

Abstract

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.

DOI
10.1134/1.558517

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•Feb 3, 2009•1 citation
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.
The spatial and temporal dynamics of two short pulses propagating in an optically dense medium of resonant three-level Λ-atoms is investigated numerically and analytically. The maximum coherence for the Raman transition due to coherent population trapping. It is shown that, at the initial stage of propagation, the waveforms of such pulses only slightly change along the length of the medium, which may considerably exceed the length of linear absorption for a single weak pulse. As the length of the absorbing medium increases, the energy of the probe (first) pulse is completely transferred into the second (control) pulse.
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 results of theoretical research of the propagation effect of an ultrashort optical pulse under the conditions of coherent interaction with two resonant transitions are presented. One of the transitions is tuned to exact resonance with the pulse carrier frequency; the other is distanced by much more than the pulse spectral width, thus the adiabatic following approximation may be applied to it. The interaction between the pulse and the far-from-resonance transition is described by the wave equation with the nonlinear index of refraction in the form of the Lorentz function of the field amplitude. This wave equation is solved together with the Bloch equations for the exact resonant transition. The solution in the form of the stationary phase-modulated pulse is obtained, and (pi) -pulse of self-induced transparency is the special case of this solution. The obtained solution has the area under the pulse envelope less than (pi) , and with increasing the absorption coefficient of the far-from-resonance transition the pulse area decreases. The effect can be used for reducing the transparency threshold of the resonant medium below (pi) .
Theoretical research of propagation effect of ultrashort optical pulse under conditions of coherent interaction with two resonant transitions simultaneously is presented. One of the transitions is tuned in exact resonance with pulse carrier frequency; the other is distanced much more than spectral pulse width, thus the adiabatic following approximation may be applied. Interaction between the pulse and far-resonant transition is described by wave equation with nonlinear index of refraction in the form of Lorentz function for field amplitude. This wave equation is solved together with Bloch equations for the exact resonant transition. The solution in the form of stationary phase-modulated pulse is obtained, and 2π-pulse of self-induced - transparency is the special case of this solution. The obtained solution has a square under pulse envelope less than 2π, and with increasing of far-resonant transition absorption coefficient the pulse square decreases. The effect can be used for reducing of the transparency threshold of the resonant medium below pi.
This thesis deals with the study of propagation effects experienced by weak light pulses as they propagate in atomic media driven by strong pulses. We explore both the ultra-short and long pulse regime and investigate the phenomena that arise at these different time scales. In the short pulse regime, a strongly driven two level atomic system presents transient light shifts, and non-adiabatic transitions occur between these adiabatic levels. We have studied a method to probe these light shifts in real time by propagating a weak probe through the medium. The light shifts enrich the spectrum of the probe and the probe gets shaped as a result. In this way a strongly driven two-level system can act as an active pulse shaper, and can introduce oscillations in the temporal profile of an ultra-short pulse at a time scale shorter than the pulse duration. We also show that by driving the system with two time delayed non-resonant strong fields, the non-adiabatic effects can be rendered phase dependent. This gives very sensitive phase control of the excited state population and can be used to formulate new techniques in interferometry. In the long pulse regime we present a new method of slowing light that can be realized in a double two-level system interacting with two orthogonally polarized light pulses that propagate along different axis. Spatio-temporal dependence of the total polarization induces a grating in the ground Zeeman coherence. The stronger of the two fields (the control field) is diffracted from this grating into the direction of the weak probe field compensating for the absorption of this latter field. A transparency window is thus created in the absorption spectrum of the probe leading to the slowing down of light. The transparency window exhibits characteristics similar to the one obtained by EIT (electromagnetic induced transparency) method. However, the important difference between our method and the traditional EIT method, is that ours doesn't rely on realizing dark state in the system. This may open the possibility of slowing down light in more complex atomic media. Moreover, when the linear absorptive response of the medium is cancelled in this manner, the nonlinear response becomes more important. In the situation where fields propagate in the same direction and have same frequency, two regimes have been investigated. For small optical depths, the effective susceptibility behaves as chi_lin exp[2 i phi] (with phi the phase difference between two fields). This renders phase control of the medium response, and the medium can be changed from an absorber to an amplifier, with normal or anomalous dispersion, by adjusting the relative phase phi.  In the regime of large optical thickness, phase saturation takes place and the effective susceptibility turns into chi_lin*, changing an absorber into an amplifier without effecting the dispersive response (chi_lin is linear susceptibility).