A saturable absorber, coherent population oscillations, and slow light

Abstract

The paper presents a critical analysis of publications devoted to one of the methods for production of the so-called "slow light" (the light with an anomalously low group velocity) due to extremely high local steepness of the dispersion curve of the medium. The method in point employs for this purpose the effect of coherent population oscillations accompanied by burning of a narrow spectral hole in the homogeneously broadened absorption spectrum.Physical model of the effect proposed in the studies under consideration is based on analysis of response of a nonlinear medium to a low-frequency intensity modulation of the incident light beam. We show that all the observations described in these papers can be easily interpreted in the framework of the simplest model of saturable absorber and have nothing to do with the effects of hole burning and group velocity reduction.

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“…Full agreement of the experimental data obtained in those papers with the prediction of the simple model of saturable absorber was shown in [15,25], where one can find a more detailed and grounded criticism of these publications [19][20][21][22][23][24]. The same perverse interpretation of the effects of delayed photoresponse in a saturable absorbed has received further development in the experiments with bacteriorhodopsin molecules in a polymer film [26].…”

“…At the same time, the studies on 'slow light' frequently do not pay sufficient attention to the experimental facts that contradict canonical models of the effect. Among them may be mentioned, in particular, the dependence of the 'released' pulse shape on the pump beam intensity [34], the admissible nonadiabaticity of switching of the pump beam [44], and an ideal agreement of the results of the experiments on 'slow light' under conditions of 'coherent population oscillations' with the prediction of the trivial model of saturable absorber (see, [15,25]). …”

“…Fig. 5) at the entrance (1) and at the exit (2 -4) of the medium for different ratios of the pulse width δ and relaxation time of the absorber τ : τ = δ/2(2), δ(3), and 2δ(4) [15]. …”

“…The studies were performed in rather simple experimental conditions, and analysis of the results was made, in our opinion, not critically enough: Any apparent delay of the light pulse in the medium was attributed to its low group velocity. Some obviously erroneous works of this type were criticized in [13][14][15]. However, the essence of the matter, in our opinion, consists not in the errors committed in certain papers, but rather in the inconsistencies of the provided evidence.…”

“…In the same category of the effects it can also be attributed the effect (already mentioned above) of apparent increase of the light group velocity in a 'saturable amplifier' [2,28], when, like in the inverse saturable absorber, the pulse is distorted in favor of its front edge. Figure 4 shows the calculated curves that illustrate the pulse 'delay' for the usual (a) and inverse (b) saturable absorber for some ratios of the pulse width d and relaxation time t [15]. Note that the illusion of the pulse delay arises, in this case, due to the amplitude normalization of the output pulse.…”