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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Dark Hanle resonance narrowing by blocking the central part of the Gaussian laser beam

Aleksandar J. Krmpot, S. N. Nikoilić, Senka Ćuk, Milan Radonjić, B. M. Jelenkovic

Published Oct 8, 2010• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.882831

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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 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.
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.
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.
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.
Electromagnetically induced transparency (EIT) due to Zeeman coherences in the Rb buffer gas cell is studied for different laser beam profiles, laser beam radii and intensities from 0.1 to 10 mW cm −2 . EIT line shapes can be approximated by the Lorentzian for wide Gaussian laser beam (6.5 mm in diameter) if laser intensity is weak and for a Π laser beam profile of the same diameter. Line shapes of EIT become non-Lorentzian for the Gaussian laser beam if it is narrow (1.3 mm in diameter) or if it has a higher intensity. EIT amplitudes and linewidths, for both laser beam profiles of the same diameter, have very similar behaviour regarding laser intensity and Rb cell temperature. EIT amplitudes are maximal at a certain laser beam intensity and this intensity is higher for narrower laser beams. The EIT linewidth estimated at zero laser intensity is about 50 nT or 0.7 kHz, which refers to 1.5 ms relaxation times of Zeeman coherences in 87 Rb atoms in our buffer gas cell. Blocking of the centre of the wide Gaussian laser beam in front of the photo detector yields Lorentzian profiles with a much better contrast to the linewidth ratio for EIT at higher intensities, above ∼2 mW cm −2 .
We observe resonances with width 20 times below the natural linewidth for a probe laser on an optical transition, when the same transition is driven by a control laser with a Laguerre-Gaussian (LG) profile. The basic mechanism is the phenomenon of electromagnetically induced transparency and absorption (EITA). Aligning the Gaussian probe beam to the central hole in the LG control beam simultaneously allows a strong control intensity required for high signal-to-noise ratio and a low intensity in the probe region required to get narrow resonances. We observe these resonances on the D2D_2D2​ line of Rb in a room-temperature vapor cell and show that the linewidth is reduced by a factor of 4 compared to the use of a Gaussian control beam, which should prove advantageous in all applications of EITA.