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Physical Review A

Stimulated Raman processes in optically dense, inhomogeneously broadened media

I. E. Mazets

Published Oct 1, 1996• 9 citations
Publisher

Abstract

We present an analytical theory of propagation of electromagnetic pulses through a Doppler-broadened medium in the adiabatic population transfer regime. We show that thermal motion of atoms causes a strong asymmetric distortion of the pulse shape. Two different kinds of pulse distortions are found to be relevant for cases of (a) nondegenerate and (b) degenerate atomic states between which the population is transferred adiabatically. In case (a) the forward front of the nonlinear electromagnetic wave becomes steeper; otherwise, in case (b) a trailing front sharpening takes place. To increase the effect in the degenerate atomic level configuration to an experimentally significant value, the ratio of the Doppler profile width to the Rabi frequency must be much higher than in case (a). \textcopyright{} 1996 The American Physical Society.

DOI
10.1103/physreva.54.3539

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We present an analytical theory of propagation of electromagnetic pulses through a Doppler-broadened medium in the adiabatic population transfer regime. We show that thermal motion of atoms causes a strong asymmetric distortion of the pulse shape. Two different kinds of pulse distortions are found to be relevant for cases of (a) nondegenerate and (b) degenerate atomic states between which the population is transferred adiabatically. In case (a) the forward front of the nonlinear electromagnetic wave becomes steeper; otherwise, in case (b) a trailing front sharpening takes place. To increase the effect in the degenerate atomic level configuration to an experimentally significant value, the ratio of the Doppler profile width to the Rabi frequency must be much higher than in case (a). \textcopyright{} 1996 The American Physical Society.
In the present paper, we examine the propagation of frequency-chirped laser pulses through a medium of Lambda atoms, pulses that are capable of producing adiabatic population transfer between the stable states. We compare the propagation of these pulses to their propagation in a medium of two-level atoms, and show that the distortion of the pulses is greatly reduced in the three-level case. In effect, there exists a transparency phenomenon for frequency-chirped laser pulses propagating in the three-level medium that is somewhat similar to electromagnetically induced transparency. It is concluded that the population transfer induced by chirped pulses does populate the excited state during the process to a slight extent, even if the transfer is perfectly adiabatic. Thus the transparency of the medium is also imperfect, the pulse is eventually distorted and its capability to produce the population transfer is lost, even in the perfectly adiabatic limit.
In a recent paper [Gorshkov et al., Phys. Rev. Lett. 98, 123601 (2007)] and in the two preceding papers [Gorshkov et al., this issue, Phys. Rev. A 76, 033804 (2006); 76, 033805 (2006)], we used a universal physical picture to optimize and demonstrate equivalence between a wide range of techniques for storage and retrieval of photon wave packets in homogeneously broadened $\ensuremath{\Lambda}$-type atomic media, including the adiabatic reduction of the photon group velocity, pulse-propagation control via off-resonant Raman techniques, and photon-echo-based techniques. In the present paper, we generalize this treatment to include inhomogeneous broadening. In particular, we consider the case of Doppler-broadened atoms and assume that there is a negligible difference between the Doppler shifts of the two optical transitions. In this situation, we show that, at high enough optical depth, all atoms contribute coherently to the storage process as if the medium were homogeneously broadened. We also discuss the effects of inhomogeneous broadening in solid state samples. In this context, we discuss the advantages and limitations of reversing the inhomogeneous broadening during the storage time, as well as suggest a way for achieving high efficiencies with a nonreversible inhomogeneous profile.
We study the propagation of frequency-chirped laser pulses in optically thick media. We consider a medium of atoms with a $\ensuremath{\Lambda}$ level-scheme (Lambda atoms) and also, for comparison, a medium of two-level atoms. Frequency-chirped laser pulses that induce adiabatic population transfer between the atomic levels are considered. They induce transitions between the two lower (metastable) levels of the $\ensuremath{\Lambda}$-atoms and between the ground and excited states of the two-level atoms. We show that associated with this adiabatic population transfer in $\ensuremath{\Lambda}$-atoms, there is a regime of enhanced transparency of the medium---the pulses are distorted much less than in the medium of two-level atoms and retain their ability to transfer the atomic population much longer during propagation.
We develop a general method allowing one to construct the consistent theory of light pulse propagation through an atomic medium in arbitrary nonlinear regime with respect to the field strength, taking into account the light polarization, temporal (frequency), and spatial dispersions. The method is based on the reduced Maxwell equation, atomic density matrix formalism, and adiabatic approximation. In order to demonstrate the efficiency of our method we investigate in detail the case of propagation of the pulses under conditions of coherent population trapping in a medium of two-level atoms with degenerate energy levels. Equations describing the evolution of various field parameters are derived. It is shown that pulses of ellipticity and spatial orientation of polarization ellipse propagate with slowing. Analytical expressions for the slowing factor are obtained. Previously unknown effects of the stimulated phase modulation and the generation of a phase pilot pulse by the variation of spatial orientation of the polarization ellipse are predicted.
We present a systematic theoretical study to deal with linear and nonlinear light propagations in a Doppler-broadened three-level $\ensuremath{\Lambda}$ system via electromagnetically induced transparency (EIT), with incoherent population exchange between two lower energy levels taken into account. Through a careful analysis of base state and linear excitation, we show that the EIT condition of the system is given by $|{\ensuremath{\Omega}}_{c}|{}^{2}{\ensuremath{\gamma}}_{31}\ensuremath{\gg}2{\ensuremath{\gamma}}_{21}\ensuremath{\Delta}{\ensuremath{\omega}}_{D}^{2}$, where ${\ensuremath{\Omega}}_{c}$ is half the Rabi frequency of the control field, $\ensuremath{\Delta}{\ensuremath{\omega}}_{D}$ is the Doppler width, and ${\ensuremath{\gamma}}_{\mathit{jl}}$ is the decay rate of the coherence between states $|j\ensuremath{\rangle}$ and $|l\ensuremath{\rangle}$. Under this condition, the effect of incoherent population exchange is insignificant, while dephasing dominates the decoherence of the system. This condition also ensures the validity of the weak nonlinear perturbation theory used in this work for solving the Maxwell-Bloch equations with inhomogeneous broadening. We then investigate the nonlinear propagation of the probe field and show that it is possible to form temporal optical solitons in the Doppler-broadened medium. Such solitons have ultraslow propagating velocity and can be generated in very low light power. The possibility of realizing (1+11+11+1)-dimensional and (2+12+12+1)-dimensional spatial optical solitons in the adiabatic regime of the system is also discussed.