COSMA: Coherent Optics Sensors for Medical Application is an European Marie Curie Project running from 2012 to March 2016, with the participation of 10 teams from Armenia, Bulgaria, India, Israel, Italy, Poland, Russia, UK, USA. The main objective was to focus theoretical and experimental research on biomagnetism phenomena, with the specific aim to develop all-optical sensors dedicated to their detection and suitable for applications in clinical diagnostics. The paper presents some of the most recent results obtained during the exchange visits of the involved scientists, after an introduction about the phenomenon which is the pillar of this kind of research and of many other new fields in laser spectroscopy, atomic physics, and quantum optics: the dark resonance.
We report simultaneous observation of the <i>N</i>- and EIT- resonances formation using multi-region (MR) cell, which contains micrometric –thin region with the thickness <i>L</i> varying in the range of 1μm - 90μm. MR cell is filled with natural Rb and 150 Torr neon gas. To form <i>N</i>- and EIT- resonances in Λ-system, two lasers are used: the probe (with tunable frequency), and the coupling (with the fixed frequency) with λ ≈ 795 nm wavelength and 1MHz- line-width. Although, the best parameters of <i>N</i>-resonance could be obtained for a cell thickness about 1cm, the use of MR cell with the thickness 30-40 μm still allows to obtain a good contrast and a narrow line-width. The <i>N</i>-resonance demonstrates subnatural increase of the probe absorption and is formed when probe frequency <i>ν</i><sub>P1</sub> is in resonance with the <sup>85</sup>Rb transition F<sub>g</sub>=2 →5P<sub>1/2</sub>, while for the coupling frequency the condition υ<sub>C</sub> = υ<sub>P1</sub> + Δ<sub>1</sub> (Δ<sub>1</sub>=3036 MHz) is fulfilled. The EITresonance is formed when ν<sub>P2</sub> scans the transition <sup>87</sup>Rb, F<sub>g</sub>=2 →5P<sub>1/2</sub> and for the same coupling frequency the condition υC = υ<sub>P2</sub> +Δ<sub>2</sub> (Δ<sub>2</sub>=6835 MHz) should fulfill. The frequency separation between <i>N</i>- and EIT-resonances is equal to Δ<sub>2</sub>-Δ<sub>1</sub>≈ 3.8 GHz. We also detected the low-frequency beating signal when the condition υ<sub>C</sub> ≈ ν<sub>P</sub>' is fulfilled, which allows a directly determine the coupling frequency υ<sub>C</sub>. Splitting of <i>N</i>-resonance in the external longitudinal magnetic field into five components is detected. Possible applications are addressed.