Current issues of optics of atomic vapors

In the early 1900s, the optics of atomic vapors played a key role in the origin of quantum mechanics and its development into a full-scale quantitative theory. Optical transitions in individual atoms are now used not only for laboratory research but also in commercially manufactured products. Alkali-metal vapors play
a special role among all atomic media because vapor densities accessible for practical applications are easy to achieve, and the resonant transitions in the visible portion of the spectrum are very strong. These characteristics mean that atomic vapors are advantageous for use in a variety of engineering applications ranging from magnetometry to frequency standards. In recent years new approaches have been developed for the unique optical properties of atomic vapors.
This special-topic section of the journal includes several papers devoted exclusively to the optical properties of alkali-metal vapors [1–4]. The first two papers discuss the new capabilities associated with the development of ultrathin spectroscopic cuvettes. As long ago as the 1950s, Dicke [5,6] proposed two methods for suppression of Doppler broadening based on restricting the free movement of gas-phase atoms. In one well-cited paper [5], Dicke proposed what is now known as the “Dicke narrowing effect”—a technique for converting the mean free paths of atoms into slow atomic diffusion by adding a large amount of buffer gas to the active atoms. This technique works exceptionally well in the microwave region, where collisions between the active atoms and the buffer-gas atoms, which make it possible to manipulate the direction of motion of the atoms, do not cause any noticeable increase in the phase relaxation rate. At visible wavelengths, the Dicke narrowing effect generally does not cause any actual reduction in spectral line width, since the phase change in the atomic oscillator during collisions turns out to be of the order of or even larger than 1, so that when the buffer gas is added, the narrowing of the Doppler profile is masked by collisional broadening. However, even though the Dicke narrowing effect must be taken into account in high-precision spectroscopic calculations, this effect is very rarely observed in pure form at optical wavelengths.
A second technique for suppression of Doppler broadening [6] has been largely neglected until recently; this technique is based on enclosing the atomic vapors in a cell with longitudinal dimensions of the order of one wavelength. Romer and Dicke [6] presented calculations and microwave experimental results for ammonia vapor. The gas cuvette consisted of a right circular cylinder with a diameter of 5.75 cm and height of 0.62 cm—half the wavelength of the 24-GHz resonant transition. A short pulse at the resonant frequency excites the molecules into the upper level. Consistent with theoretical predictions, the ensemble of excited molecules turned out to have a narrower spectral width than the calculated Doppler width.
The phenomenon of Doppler narrowing in the optical region of the spectrum had been discovered a year earlier. The resonant reflection spectrum from the mercury vapor boundary revealed a structure free of Doppler broadening [7]. The interpretation proposed in this paper became the basis for all calculations involving the optics of dilute atomic vapors in containers with solid walls [8]. The primary difference between the proposed approach and the traditional method involves the methodology used to account for how the transient processes determine the polarization of an atom after it collides with the solid surface and returns to the gas phase. If the collisional broadening is smaller than the Doppler broadening, the spatial dispersion due to movement of the atoms turns out to be significant and no longer needs be taken into account. Development of this approach in [9] led to the discovery that the line profiles in transmission and absorption vary periodically as a function of the thickness of the gas layer. This is in contrast to resonances due to Fabry–Perot interference, where the period of variation in the reflection line profile is a full wavelength rather than a half-wavelength since the interference involves polarization waves originating on the rear wall of the cuvette that make a single, rather than a double, pass through the cuvette [10].
Experiments began to be conducted in the visible region of the spectrum [11] following the invention of the ultrathin cuvette. Since that time, a significant volume of very interesting research has been performed using ultrathin cuvettes. The nonlinear effects associated with transition processes that determine the polarization after collision with the surface used to contain the gas also have their own specific characteristic features [12,13].
The paper by Sargsyan et al. [1] discusses the use of an ultrathin cuvette to study the Zeeman effect. Doppler-free spectroscopy in ultrathin cells enables investigation of the splitting of atomic levels in the field of a permanent magnet. In this case, nonuniformity of the field created by the permanent magnets does not lead to nonuniform spectral broadening, since thecuvette thickness is much smaller than the typical scale on which the field is nonuniform. In addition, eliminating the Doppler broadening enables well-defined resonances to be identified, even in relatively weak magnetic fields.
The paper by Todorov et al. [2] presents a theory of linear and nonlinear optical processes in ultrathin cells to take into account the actual level structure of alkali-metal atoms. Unlike prior theoretical work, this paper incorporates all hyperfine structure levels as well as degeneracy due to magnetic quantum number. The use of irreducible tensor operators enabled the authors to produce a simplified description of the relaxation processes in a multilevel system and obtain results consistent with experimental results.
Interest in atomic vapors is clearly not limited to ultrathin cuvettes. Other linear and nonlinear processes in atomic vapors are also significant [14,15].
The paper by Kulyasov et al. [3] discusses the advantages and disadvantages of fluorescence filters based on cesium and rubidium vapor. This paper has great practical value at optical wavelengths, since fluorescence filters can be used to detect a narrow-band optical signal against the background of broadband illumination.
The paper by Sautenkov et al. [4] discusses determination of the self-focusing threshold in rubidium vapor. The results obtained in this paper are important in terms of modern filamentation theory. This paper used a unique high-temperature cuvette containing rubidium vapor.
We note in conclusion that all of the papers presented here are on warm atomic vapors. Despite the significant progress that has been made in experimental techniques for cooling atoms and holding them in traps, “cold” atoms still cannot compete with hot vapor in terms of the number of laser sources required, size, or stability against external effects.