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Optics Express

Dark Hanle resonances from selected segments of the Gaussian laser beam cross-section

Aleksandar J. Krmpot, Senka Ćuk, Stanko N. Nikolić, Milan Radonjić, Dimitar G. Slavov, B. M. Jelenkovic

Published Nov 24, 2009• 15 citations
Publisher

Abstract

We present the Hanle EIT resonances obtained from the various segments of the Gaussian laser beam cross-section, selected by moving the small aperture (placed in front of the detector) radially along the laser beam. Significant differences in the Hanle lineshapes are observed depending on whether the central or outer parts of the Gaussian laser beam are detected. The line narrowing and two counter-sign peaks occur at outer, less intense parts of the beam. The theoretical model suggests that the EIT lineshapes in the laser wings are result of the interference of the laser light and coherently prepared atoms coming from the central part of the beam. By blocking the central part of the laser beam in front of the detector, narrower, and for high laser intensities, even more contrasted Hanle resonances are obtained.

DOI
10.1364/oe.17.022491

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Optical Hanle effect

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Physical Review A•Mar 1, 1981•13 citations
We present the Hanle EIT resonances obtained from the various segments of the Gaussian laser beam cross-section, selected by moving the small aperture (placed in front of the detector) radially along the laser beam. Significant differences in the Hanle lineshapes are observed depending on whether the central or outer parts of the Gaussian laser beam are detected. The line narrowing and two counter-sign peaks occur at outer, less intense parts of the beam. The theoretical model suggests that the EIT lineshapes in the laser wings are result of the interference of the laser light and coherently prepared atoms coming from the central part of the beam. By blocking the central part of the laser beam in front of the detector, narrower, and for high laser intensities, even more contrasted Hanle resonances are obtained.
We present Hanle electromagnetically induced transparency (EIT) resonances obtained from the outer parts of the Gaussian laser beam. The signal from the outer parts only was obtained by placing circular opaque masks of different diameters in the center of the laser beam just in front of the detector. The Hanle EIT resonances obtained in that way are narrower and for high laser intensities even more contrasted. Suggested explanation for the line narrowing is based on lower power broadening in the wings of the Gaussian laser beam as well as on the traversing of the coherently prepared atoms through the beam. The resonance contrast to linewidth ratio, when the central part of the beam is blocked, is higher or equal to the ratio obtained when the whole laser beam is detected, for all laser intensities used in the experiment. Due to high ratio of contrast and linewidth, resonances obtained in proposed way could be useful in frequency metrology and magnetometry.
We studied experimentally and theoretically the evolution of open atomic systems in the constant intensity laser field. The study is performed by analyzing the line shapes of Hanle electromagnetically induced transparency (EIT) obtained in different segments of a laser beam cross section of constant intensity, i.e., a $\ensuremath{\Pi}$-shaped laser beam. Such Hanle EIT resonances were measured using a small movable aperture placed just in front of the photodetector, i.e., after the entire laser beam had passed through the vacuum Rb cell. The laser was locked to the open transition ${F}_{g}=2\ensuremath{\rightarrow}{F}_{e}=1$ at the D1{D}_{1}D1​ line of 87{}^{87}87Rb with laser intensities between 0.5 and 4 mW/cm2{}^{2}2. This study shows that the profile of the laser beam determines the processes governing the development of atomic states during the interaction. The resonances obtained near the beam center are narrower than those obtained near the beam edge, but the significant changes of the linewidths occur only near the beam edge, i.e., right after the atom enters the beam. The Hanle EIT resonances obtained near the beam center exhibit two pronounced minima next to the central maximum. The theoretical model reveals that the occurrence of these transmission minima is a joint effect of the preparation of atoms into the dark state and the optical pumping into the uncoupled ground level Fg=1{F}_{g}=1Fg​=1. The appearance of the transmission minima, although similar to that observed in the wings of a Gaussian beam [A. J. Krmpot et al., Opt. Express 17, 22491 (2009)], is of an entirely different nature for the $\ensuremath{\Pi}$-shaped laser beam.
We compared, experimentally and theoretically, Hanle electromagnetically induced absorption (EIA) obtained using Gaussian and $\ensuremath{\Pi}$-shaped laser beams 3 mm in diameter. The study was done by measuring the transmission of a laser locked to the ${F}_{g}=2\ensuremath{\rightarrow}{F}_{e}=3$ transition at the D2{D}_{2}D2​ line of 87Rb^{87}\mathrm{Rb}87Rb in a vacuum cell. EIA linewidths obtained for the two laser profiles were significantly different in the range of laser intensities 111--444 mW/cm2{}^{2}2. EIA with the $\ensuremath{\Pi}$-shaped laser beam has a broad intensity maximum and linewidths larger than those obtained with the Gaussian beam profile. We also studied Hanle EIA by measuring the transmission of selected segments of the entire laser beam by placing a small movable aperture in front of the detector. Waveforms so obtained in Hanle EIA resonances were strongly influenced both by the radial distance of the transmitted segment from the beam center and by the radial profile of the laser beam. We show that outer regions of Gaussian beam, and central regions of the $\ensuremath{\Pi}$-shaped beam generate the narrowest lines. The different behaviors of EIA owing to different beam profiles revealed by both theory and experiment indicate the importance of the radial profile of the laser beam for proper modeling of coherent effects in alkali metal vapors.
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.
Influence of two different laser beam profiles, the Gaussian and the Π (top hat) profile on the resonance line widths and amplitudes in the Hanle electromagnetically induced transparency was studied.The laser beam propagates through the vacuum Rb glass cell.Studies were done at D 1 line for the open 87 Rb: F g = 2 → F e = 1 transition.Hanle electromagnetically induced transparency was measured for the two beam profiles with the same total power and beam diameter and experimental results showed that Gaussian and the top hat profiles give different amplitudes and widths of the Hanle resonances.Resonances obtained from the top hat laser beam profile have lower amplitudes and higher line widths.
The optical Hanle effect (light-shift-induced zero-field level crossing) has been investigated theoretically and experimentally. In the case of a $J=0\ensuremath{\rightarrow}1\ensuremath{\rightarrow}0$ three-level system and of a $J=0\ensuremath{\rightarrow}1$ two-level system, the resonance line shape has been obtained using a nonlinear-response-function method. Approximate expressions, valid under the conditions for the observation of light shifts, are obtained and compared with exact calculations as well as with the corresponding expression for the magnetic Hanle effect. As concerns the weak laser beam which produces the coherent excitation of atomic substates, we have calculated the linear response in the limiting case of monochromatic and of broad-band excitation. The experimental investigation has been performed for the Ba resonance line using two cw dye lasers at $\ensuremath{\lambda}=5535$ \AA{} and an atomic beam. The absorption- and dispersion-shaped level crossing resonances have been observed, for narrow- and broad-band excitation, by varying the power of the strong nonresonant laser beam. We have checked that the line shapes are only dependent upon the reduced variable $\frac{{P}_{L}}{\ensuremath{\delta}}$ (PL{P}_{L}PL​; beam power; $\ensuremath{\delta}$: frequency detuning). Using the theoretical expressions, we have tried to fit the experimental curves. Provided that the Gaussian distribution of the beams is taken into account, we have obtained a good agreement between theory and experiment for the whole set of experimental data.