The Doppler effect is related to the propagation of harmonic oscillations (optical, acoustic) and consists in oscillation frequency change with the propagation of its source. The Doppler effect is of fundamental importance in laser spectroscopy. The atoms and molecules in the ensemble under observation perform continuously movements with velocities grouped around one central frequency value which is determined by the ensemble’s temperature. This temperature movement of atoms leads to a spectral broadening of the transitions and so limits the application of many devices and methodologies. For atoms enclosed in cells with centimetre-scale size, the laser beam “sees” the atomic ensemble in the same way—with the same velocity distribution of atoms independently of the propagation direction trough the cell. In this case we say that the velocity distribution of atoms is spatially isotropic.

New kind of cells have been developed in the last couple of years for laser spectroscopy. They contain atomic vapours of different metals (mainly alkali metals). The dimensions of these cells differ significantly in different directions (Figure 1). The distance between their windows L is on the order of 10 μm. However, the windows diameter is about 1–2 cm. Therefore, a strong spatial anisotropy is present for the time of interaction between the atoms and the laser radiation. For example Cs atoms (Figure 1, v┴), which average thermal velocity at room temperature is on the order of 200 m s^–1 will cover the 10 μm distance in about 50 ns. Such a limit is not imposed on the atoms (Figure 1, vII), moving parallel to the windows of such kind of cell. They will interact with the laser radiation for a time determined by the diameter of the laser beam D. When the cell is irradiated with a laser beam propagating orthogonally to the windows surfaces the atoms with velocity direction close to parallel to the windows surface can be considered as “fixed” which leads to a strong reduction of the Doppler effect and to the related Doppler broadening of the spectral lines as well. From the point of view of the laser beam two kinds of atoms can be seen—“slow” (moving parallel to the windows and therefore having very small projection of the velocity vector on the laser beam propagation direction) and “fast” (moving parallel to the laser beam propagation direction in the narrow space between the cell windows).

The second-generation extremely thin cell (Figure 2), developed by Armenian scientists, allows the formation of an atomic nanolayer (L=100–1500 nm). The initial experiments performed by scientists from the Armenian Academy of Sciences and from the Paris-Nord University justify the expectations for a new direction in the ultra-high resolution laser spectroscopy to be created where the investigated medium has at least one dimension of the order of the laser wavelength λ.

With the new nano-cell, the presence of the two groups of atoms mentioned above leads to the observation of a large difference between the fluorescence and transmission spectra (Figure 3). Here the basic contribution to the creation of the fluorescence signal comes from the atoms moving parallel to the windows. The fast atoms moving along the laser beam direction do not have enough time to perform a complete absorption–emission cycle, i.e. to fluoresce. As can be seen from Figure 3, the fact that different groups of atom contribute to the fluorescence and absorption means that a big difference in the width of the corresponding optical transitions in each case occurs. The transitions are completely resolved in the fluorescence case. Only the “fast” atoms are able to contribute to the absorption spectrum in the nano-layer because the absorption of a quantum of light is a much faster process. In this way the absorption process “suffers” much more from the Doppler effect. It is to mention here that in the cm-size cells the different Cs transitions are completely overlapped and cannot be separated. In this manner the nano-cell gives the opportunity to obtain better resolution in laser spectroscopy as well as to study the dynamics of the absorption and fluorescence in more detail.

Further investigations show that the atom confinement between two surfaces spaced at less than (or in the order of) the light wavelength results not only in elimination of the Doppler broadening, but also in such an ensemble a series of coherent effects occurs which are subject of intensive study recently. Among them the authors highlight the Dicke effect observed in the optical part of the spectrum. The big difference between the absorption spectra shown in Figure 4 for two Cs nano-layers the first with thickness L = λ/2, and L = λ for the second can be seen easily. The first interesting conclusion is that doubling the thickness of the Cs layer does not increase its maximal absorption by about two times. Indeed, the opposite is true—in the centre of the strongest transition the absorption at L = λ/2 is bigger than at L = λ. The reason for this is the coherent response of the medium as a result of the Dicke effect, which is the strongest at L = λ/2 and disappears at L = λ. The second interesting detail is in the big shape difference in the two spectra for two different thicknesses of the Cs layer. The theoretical explanation leads to a new fundamental knowledge about the atom–light interaction.

The Institute of Electronics in the Bulgarian Academy of Sciences is coordinating a European contract in the INTAS programme called: “Study of atomic vapor layers of nanometric thickness and atom-surface interaction”. The partners in the project are two laboratories from France and one each from Italy, Armenia and Latvia. The project is studying the new properties of the thin metal–vapour atomic layers. The Coherent Population Trapping resonances obtained are very sensitive to weak interaction between atoms and the cell surfaces. They are promising for investigation of gas–surface interaction.

Teams from the Institute of Electronics, Sofia University, and the Paris-Nord University are studying laser spectroscopy of metallic-vapour nano-layers as part of the scientific programme, “Rila”.

These investigations and the knowledge obtained are very important for the development of compact optical magnetometers with high spatial resolution of the measurements without compromising their high sensitivity. The narrow resonances registered allow the application for laser frequency stabilisation, high-precision optical clocks and magnetometers. Another application is the investigation of the coherent effect in atoms confined in 2D and 3D micro- and nano-volumes using hollow optical fibres and porous media. This will increase their efficiency for the development of optical sensors with micrometre dimensions.

Sensors based on the coherent interaction between atoms and light show promise for a number of applications. One of the problems is related to their miniaturisation to the micrometre scale without compromising their properties. Moreover, recent investigations attempting to obtain narrow coherent resonances in solid media (which is much more suitable for applications) have not been successful. That is why the team considers the coherent resonances obtained in the intermediate case of atomic vapour confined in miniature volumes as a useful method for the development of new sensors and new methodology in photonics.

User Rating: / 2
PoorBest