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"Dark state polariton" is a manifestation of inequality of forward and reversed processes in optics

V. A. Kuz’menko

Published Aug 3, 2006• 0 citations
OpenAlex

Abstract

The physical origin of the so-called "dark states" is connected not with the low absorption efficiency of the ground state of atoms, but with the ultra high efficiency of stimulated emission transition into the initial state. Simple pump-probe experiments for study the efficiency of the reversed transitions in a two-level system are discussed.

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The physical origin of the so-called "dark states" is connected not with the low absorption efficiency of the ground state of atoms, but with the ultra high efficiency of stimulated emission transition into the initial state. Simple pump-probe experiments for study the efficiency of the reversed transitions in a two-level system are discussed.
The origin of recently discovered reverse (opposite-sign) dark resonances has been explained theoretically and verified experimentally. It is shown that the reason for these resonances is a specific optical pumping of the ground state magnetic sublevel in a transition when the ground state angular momentum is smaller than the excited state momentum. An experiment was conducted on 85 Rb atoms in a cell, when a diode laser using the ground state hyperfine level F g = 3 simultaneously excites spectrally unresolved hyperfine levels with total angular momentum quantum numbers F e = 2,3 and 4. It is shown that due to differences in the transition probabilities the dominant role in total absorption and fluorescence signals is played by absorption on a transition F g = 3→ F e = 4.
Dark states were first observed in optical pumping experiments and interpreted as a quenching of fluorescence due to a destructive interference between two absorption amplitudes connecting two ground state sublevels g1 and g2 to an excited sublevel e. The atom can absorb light from g1 and jump to e. Similarly, it can absorb light from g2 and jump to e. But, if it is in a certain linear superposition of g1 and g2, , the two absorption amplitudes from g1 to e and from g2 to e interfere destructively and cancel out. The fluorescence stops. The state from which light cannot be absorbed is called a dark state. This paper will start with a description of the first experiments demonstrating the existence of dark states and of the first theoretical interpretations that have been proposed. Dark states appeared to play an essential role in several new physical effects, such as electromagnetically induced transparency, slow light, stimulated Raman adiabatic passage (STIRAP), which will be briefly described. A special emphasis will be given to the applications of dark states in the field of ultracold atoms and molecules. It will be first shown how the use of velocity dependent dark states allows atoms to be cooled below the so-called recoil limit, corresponding to a velocity dispersion of the cooled atoms smaller than the recoil momentum communicated to the atom by the absorption of a single photon. It turned out that a quantitative interpretation of this subrecoil cooling may be given in terms of anomalous random walks and Lévy statistics. Finally, recent applications of dark states to the production of ultracold molecules will be described: production of ultracold polar molecules by the combination of Feshbach resonances and STIRAP; dark states in the photo-association of ultracold atoms allowing a precise determination of the scattering length describing their collisions.
We have considered the induced processes that occur in a driven two-level atom, where a laser photon is absorbed and emitted by the ground and by the excited states of the atom, respectively. In the low-intensity limit of the laser field, the induced spectra arising when a laser photon is absorbed by the ground state of the atom consist of two peaks describing induced-absorption and stimulated-emission processes, respectively, where the former prevails over the latter. Asymmetry of the spectral lines occurs at off-resonance and its extent depends on the detuning of the laser field. The physical, process where a laser photon is emitted by the excited state is the reverse of that arising from the absorption of a laser photon by the ground state of the atom. The former differs from the latter in that the emission of a laser photon by the excited state occurs in the low-frequency regime and that the stimulated-emission process prevails over that of the induced absorption. In this case, amplification of ultrashort pulses is likely to occur without the need of population inversion between the optical transitions. The computed spectra are graphically presented and discussed.
We present a physical picture explaining the asymmetries between absorption and stimulated emission occurring in the driven three-level systems. Based on a dressed-state picture, we obtain transition rates for absorption and stimulated emission separately, and show that the cancellation of absorption in the $\ensuremath{\Lambda}$- and V-type systems are originated from different physical mechanisms. Furthermore, a simple analogy can be applied to cascade-type systems to show the absorption enhancement at two-photon resonance.