(a) Experimental setup: ECDL—external cavity diode laser; DDAVLL—Doppler-free dichroic atomic vapour laser lock; SMF—single-mode fiber; P—polarizer; BE—beam expander; PD—large area photo diode
Figure 1. (a) Experimental setup: ECDL—external cavity diode laser; DDAVLL—Doppler-free dichroic atomic vapour laser lock; SMF—single-mode fiber; P—polarizer; BE—beam expander; PD—large area photo diode. For certain measurements the small aperture on the translation stage is placed in the laser beam allowing only a selected part of the laser beam cross-section to reach the detector, while the rest of the laser beam is blocked. Π-shaped beam profiles were recorded by a beam profiler placed at 3 cm (b) and 30 cm (c) from the 3 mm circular aperture. (b) The dashed (red) curve is the profile of the Gaussian laser beam of the same power and diameter as the Π-shaped beam. Note that, in order to have the same overall power of the two laser beams, the peak of the Gaussian beam in the present graph has to have double the value of the flat region of the Π-shaped beam if the diameter of the Gaussian beam is measured at 1/e2 of the peak intensity.
Experimental and theoretical analyses show the effect of laser beam radial intensity distribution on line-shapes and line-widths of the electromagnetically induced transparency (EIT). We used Gaussian and Π (flat top) laser beam profiles, coupling the D1 transition of 87Rb atoms in the vacuum cell in the Hanle experimental configuration. We obtained non-Lorentzian EIT line-shapes for a Gaussian laser beam, while line-shapes for a Π laser beam profile are very well approximated with Lorentzian. EIT line-widths, lower for Gaussian than for Π, show nonlinear dependence on laser intensity for both laser beam profiles. EIT amplitudes have similar values and dependence on laser intensity for both laser beams, showing the maximum at around 0.8 mW cm−2. Differences between the EIT line-shapes for the two profiles are mainly due to distinct physical processes governing atomic evolution in the rim of the laser beam, as suggested from the EIT obtained from the various segments of the laser beam cross-section.