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.
The pump-probe spectra in a cell of micrometric thickness containing cesium vapor are reported. The line shape and nonlinear features observed in the case of fluorescence in the direction parallel to the cell windows and the transmission spectra observed along the propagation direction of the probe beam show considerable differences in the spectral profiles. We observed Electromagnetically Induced Transparency (EIT) and enhanced Velocity Selective Optical Pumping (VSOP) signals. Atoms moving nearly parallel to the windows and perpendicular to the collinear pump and probe beams will see much lower Doppler shift of incident frequencies and hence will lead to considerable narrowing of the Doppler background in the fluorescence spectra. The coherence decay rate is also low for such atoms as they do not meet with the cell walls. A theoretical model based on five level optical Bloch equations is used to simulate the spectra. The Doppler convolution includes all possible orientation of atomic velocities with respect to the laser beam direction. The simulated curves reproduce the observed sharp EIT peaks and enhanced broad VSOP signals for the closed probe transition in the fluorescence and absorption spectra. The observed effect of the light intensity and temperature change on the non-linear features is reproduced by the simulation.
Laser spectroscopy experiments are reported on rubidium atoms by using two or three external cavity diode lasers to
study multiple resonance transitions in Λ- and V-type systems as well as on (Λ+V)-type system. Electromagnetically
induced transparency (EIT) peaks having sub-natural line-widths are found on the Doppler broadened transmission
backgrounds with velocity selective enhanced absorption dips. In the presence of a pump laser, probe spectrum shows
enhancement of EIT signal by tuning the control laser frequency when the Λ- and V-type EIT signals are made to
overlap. Theoretical analysis is carried out by solving the optical Bloch equations for five-level atomic model under Λ
configuration. With the numerically simulated spectra, variation of EIT transmission peak with ground state decay rate
and excited state spontaneous decay rate are investigated. Effect of pump Rabi frequency on the transmission peak is