Experiments

Coherent effects in alkali atomic vapors

Coherent Population Trapping (CPT) and Electromagnetically Induced Transparency (EIT) are coherent phenomena observed in atomic vapors. These are produced in atomic systems with three levels, coupled by suitable laser radiation or in presence of suitable external fields: the quantum interference between transition amplitudes causes a decrease of light absorption in resonant conditions. The transparency window has a small spectral width, is linked to a steep dispersion of the refractive index and exhibits characteristics that depend on external conditions, such as magnetic fields. Because of these reasons, such effects are of great interest for fundamental research and in view of their numerous applications, such as atomic magnetometers and atomic clocks, slowing and storage of light, ultra-high resolution spectroscopy and quantum computation.
Atomic coherences with long lifetime are a fundamental condition for the production of coherent resonances whose width is negligible compared to the natural linewidth of an optical transition. However, collisions of atoms in the vapor phase against the cell’s walls significantly decrease the coherences lifetime; this phenomenon is particularly severe in the case of micrometric cells, required because of the increasing demand for miniaturized atomic devices. Usually, a noble gas (buffer gas) is added in order to contain the influence of the atom/wall collisions. Nevertheless, this solution causes a further broadening of atomic transitions ought to collisions and imposes a limitation to the cells miniaturization, because of the high pressure of buffer gas (>10 Torr) required.
In order to overcome these limitations, our group developed a technique for the cell coating with organic anti-relaxation polymers, usually silanes or paraffines. In this way, the polarization of atoms and their coherences are maintained even after a collision against the cell wall. This produces a relevant increase of the coherence lifetime: indeed, thank to these coatings, ultra-narrow EIT resonances (down to 1/60 Hz) were observed by exploiting the so-called wall-induced Ramsey effect [1]. Moreover, organic coatings, which have a thickness negligible compared to the cell size, do not limit the miniaturization and exhibit stable characteristics, within a large temperature range (up to about 250°C).
In addition, the use of organic coatings allows the application of techniques for atomic density control and stabilization at room temperature based on the LIAD (Light-Induced Atomic Desorption) effect [2]. In details, atoms adsorbed in the coating surface cavities are quickly released into the vapor phase upon exposure to non-resonant radiation, even at low intensity. This effect is currently used for the loading of magneto-optical traps and atom chips, for the photo-induced production of metal nanoparticles and for the fast modulation of atomic vapor density [2].
In our laboratory, ECDL lasers are available for the first and the second resonance of K and Rb. A complete system for shielding from external magnetic fields is implemented by means of a μ-metal chamber and by three pairs of mutually orthogonal Helmholtz coils [3].
[1] Gozzini, S., Marmugi, L., Lucchesini, A., Gateva, S., Cartaleva, S., Nasyrov, K. Narrow Structure in the Coherent Population Trapping Resonance in Sodium, Phys. Rev. A 84, 013812, 2011
[2] Bogi, A., Marinelli, C., Burchianti, A., Mariotti, E., Moi, L., Gozzini, S., Marmugi, L., Lucchesini, A. Full control of sodium vapor density in siloxane-coated cells using blue LED light-induced atomic desorption, Opt. Lett. 34, 17, 2643 – 2645, 2009
[3] Marmugi L., Gozzini S., Lucchesini A., Bogi A., Burchianti A., Marinelli C. All-optical vapor density control for Electromagnetically Induced Transparency, J. Opt. Soc. Am. B 29, 2729-2733, 2012

Liad effect on sodium: left desorbing LED on; center desorbing LED on; right desorbing LED on with filter cutting the LED light.

EIT (green curve, upward peaks) conversion to EIA (blue curve, downward peaks); spectroscopy profile (red curve).