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Quantum and Semiclassical Optics Journal of the European Optical Society Part B

Observation of coherent transient effects in a magneto-optical trap

A. Pietiläinen, H. Ludvigsen, Hannu Talvitie, Erkki Ikonen

Published Aug 1, 1997• 4 citations
Publisher

Abstract

We present results obtained by sweeping the frequency of a probe laser beam across a hyperfine absorption line of laser-cooled and trapped atoms in a time of the order of the lifetime of the excited state. The method allows observation of coherent transient phenomena in laser spectroscopy. The results can be understood quantitatively on the basis of an analysis describing interference between the probe laser field and an induced polarization of the resonant sample.

DOI
10.1088/1355-5111/9/4/010

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We present results obtained by sweeping the frequency of a probe laser beam across a hyperfine absorption line of laser-cooled and trapped atoms in a time of the order of the lifetime of the excited state. The method allows observation of coherent transient phenomena in laser spectroscopy. The results can be understood quantitatively on the basis of an analysis describing interference between the probe laser field and an induced polarization of the resonant sample.
We investigated coherent transient oscillations in the transmission intensity of a frequency-modulated probe-laser beam through a sample of trapped atoms. The transients occur when the frequency of the probe-laser beam crosses a resonance of the atoms in a time of the order of the lifetime of the excited state. A theoretical description used previously to explain transient effects observed in Mössbauer spectroscopy has been developed further to adapt for the specific frequency- and amplitude-modulation characteristics of current-modulated laser diodes. The transients were recorded at various experimental parameters, and the results of the experiments were compared with results obtained from slow frequency sweeps and independent measurements of the modulation characteristics of the laser. The results show that the theory accurately explains the coherent transient phenomena.
The velocities of atoms can be manipulated by near-resonant laser light. After many absorption-spontaneous emission cycles momentum is transferred to the atoms in the direction of the laser beam. In this way atoms can be slowed down and cooled to milliKelvin temperatures and below. In three mutually orthogonal pairs of counter-propagating laser beams in combination with a magnetic field atoms can be cooled and trapped. In this so-called magneto-optical trap (MOT) two atoms can be excited to a molecular state by absorption of a photon. This technique is called photoassociation spectroscopy. In our case we study metastable helium atoms, which have a large internal energy. When two metastable helium atoms approach each other, one of the atoms ionizes and the ions are detected by a microchannelplates detector. When a probe laser beam is directed onto the MOT cloud and scanned in frequency, atom pairs can be excited to several ro-vibrational states. In the excited state the metastable helium atoms are accelerated towards each other and ionize at short internuclear distance. The ro-vibrational states appear as peaks in the ionization signal and from the position of these states information on the excited state interaction potential can be extracted.
The ultra-narrow resonances, obtained in the coherent spectroscopy of alkali atoms, originate from the destruction of the laser-induced coherence in the ground state. In the case of a single frequency, near-resonant excitation and a suitable geometry of irradiation and observation by scanning a weak magnetic field, one observes coherent population trapping in a Hanle - configuration. The laser field transfers the nonlinear interference resonances created in the degenerate Zeeman sublevels of the ground states to the upper level. As a result, the well-known ground state Hanle - effect is observed in the fluorescence. We investigated theoretically and experimentally the shape, amplitude and sign of these resonances and their dependence on the excitation and registration conditions. The experiment was performed on the D1 87Rb line. Results of the computations were compared with the measurements for corresponding experimental conditions.
Effects caused by fast laser frequency sweeping across absorption lines are investigated. Oscillations in the time dependence of intensity of radiation generated by the medium, which are caused by beats between oscillations at variable excitation frequency and constant eigenfrequencies, are discovered. The time intervals between local maxima of the oscillations are inversely proportional to the difference between the eigenfrequency of the atomic oscillator and the instant frequency of the external radiation. The method of using fast frequency sweeping for determining indices of absorption at high optical densities (k 0 z ~ 100) is proposed.