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Optics Letters

Spectroscopy on a modulated magneto-optical trap

Jens Lykke Sørensen, Jan Hald, Eugene Simon Polzik

Published Jan 1, 1998• 3 citations
Publisher

Abstract

We demonstrate a new type of high-resolution two-photon frequency modulation (FM) spectroscopy with cold atoms in a magneto-optical trap. Instead of modulating the probe as in ordinary FM spectroscopy, we modulate the trap itself by FM of the trapping beams. We present theoretical as well as experimental results for both absorption and polarization rotation spectroscopy. Finally, we demonstrate two-photon FM spectroscopy, using the intrinsic phase noise of the trapping diode lasers.

DOI
10.1364/ol.23.000025

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We demonstrate a new type of high-resolution two-photon frequency modulation (FM) spectroscopy with cold atoms in a magneto-optical trap. Instead of modulating the probe as in ordinary FM spectroscopy, we modulate the trap itself by FM of the trapping beams. We present theoretical as well as experimental results for both absorption and polarization rotation spectroscopy. Finally, we demonstrate two-photon FM spectroscopy, using the intrinsic phase noise of the trapping diode lasers.
A novel variation of the technique of two-photon frequency modulation (FM) spectroscopy has been developed, which allows signals to be obtained from small (∼105 atoms) and dilute (∼1010 cm−3) samples – in this case laser-cooled rubidium atoms. This was possible by detecting and demodulating the fluorescence signal rather than the transmission through the sample. Data are presented demonstrating the dependence of the signal on phase and frequency of the imposed modulation. Initially the atoms were held in a magneto-optic trap but, in order to eliminate line broadening due to the presence of the trapping fields, data were taken with the atoms in free fall. A similar signal, slightly shifted and broadened by the trapping fields, was obtained while the trap was on and used for long-term stabilisation of the probe laser.
The results of frequency-modulation (FM) spectroscopy of coherent dark resonances from the Zeeman sublevels of the transition F=2 ↔ F=1 of D1 line in absorption of 87Rb atoms are presented and discussed in detail. By contrast with the conventional spectroscopy of coherent dark resonances employing two laser beams, relative frequency of which can be varied, these data has been obtained with the help of a single frequency-modulated laser field. Variation of the modulation frequency plays then similar role with variation the relative frequency in conventional spectroscopy. Experimental data are fit to the theoretical calculations, which are based on the theory of FM spectroscopy of coherent dark resonances recently developed by us. Feasibility of using such experimental technique for accurate measurements of magnetic fields is also discussed.
We Study a modification of classical FM spectroscopy in the cases where several electromagnetic fields are FM modulated, each in a different manner. This complex spectrum scans a multi-photon resonant atomic medium with the output detected by a phase-sensitive scheme. The demodulated output signal reveals the spectroscopic features of the probed medium. The case in which two different carriers are FM modulated at the same frequency and index but with an opposite phase with respect to each other is analyzed theoretically. This configuration is essential for probing Coherent Population Trapping (CPT) resonances induced by a directly modulated diode laser. Employing a macroscopic model to describe the physical properties of CPT leads to a superb fit between predicted and measured CPT characteristics.
Frequency-modulation (FM) spectroscopy has been performed on the D(2) transitions of rubidium atoms with an AlGaAs diode laser at 780 nm. Doppler-broadened hyperfine-structure transitions of (85)Rb and (87)Rb were resolved with no residual amplitude-modulation-induced background signal by modulating the injection current of the laser diode at a low frequency (20-50 MHz) compared with the Doppler width. To obtain Doppler-free spectra, we combined FM spectroscopy with saturation spectroscopy. The results show that the FM spectroscopy technique is sensitive and should be useful for high-resolution spectroscopy, although the resolution was instrument limited and unusual double peaks were observed.
We have demonstrated phase-sensitive FM noise spectroscopy, using the frequency noise of a diode laser. We obtained the reference signal for this experiment by sending part of the diode-laser beam through a Mach-Zehnder interferometer, which acted as a frequency discriminator. Both the reference beam (prepared with an interferometer) and the probe beam (which probed a Fabry-Perot cavity) carried amplitude noise converted from the intrinsic frequency noise. Mixing the two noise signals allowed us to measure the absorptive and dispersive responses of the sample. This method may provide a simple and sensitive spectroscopic tool.
Abstract This paper presents a novel method for achieving parametric excitation in an optical polarization-modulated magneto-optical trap for a two-level atom. The proposed method has a unique mechanism that is different from that of the existing methods, which modulate the laser intensity, magnetic field gradient, or laser detuning, because the excitation realized through the polarization modulation originates from the competition between the repulsive and attractive forces on a two-level atom. The theoretical calculation is based on the simple Doppler cooling theory, and we derive atomic motion in a rotating frame and coupled equation for motional amplitude and phase, in detail. PACS numbers: 37.10.Vz, 37.10.De, 37.10.Gh