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

Continuous Wave (cw) Doppler Free Two-Photon Frequency Modulated (FM) Spectroscopy

Werner Zapka, M. D. Levenson, Gary C. Bjorklund, A. C. Tam, Frank M. Schellenberg

Published Jun 16, 1983• 0 citations
Publisher

Abstract

oppler-free two-photon absorption and two-photon dispersion spectra of Rb87 have been taken by FM spectroscopy. Sensitivity achieved is comparable to fluorescence detection. The technique can be readily applied where fluorescence is difficult to detect.
DOI
10.1117/12.934392

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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.
Frequency-modulation spectroscopy was employed successfully to measure the Doppler-free absorption and dispersion signals originating from several two-photon transitions in (85)Rb and (87)Rb. The two-photon cross section of the hyperfine transition 5S(1/2) (F = 2) ? 5D(5/2) (F = 4) in (87)Rb was determined to be sigma((2)) asymptotically equal to 4 x 10(-20) cm(4)/W. The transition isotope shift (TIS) of the transition 5S(1/2) ? 5D(5/2) between (87)Rb and (85)Rb was found to be Deltanu(TIS) = +166 +/- 2 MHz.
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.
Heterodyne Doppler-free spectroscopy, previously demonstrated on single-photon resonances (heterodyne saturation spectroscopy), has been extended to two-photon transitions in rubidium (5S-5D and 5S-7S). It provides a simple method of observing the two-photon dispersion lineshape directly. The noticeable sensitivity of this absorption technique, which has been used for an optical measurement of the hyperfine structure of the 5D level, is discussed in comparison with that currently obtained in two-photon-induced fluorescence.
The spectral features of FM two-photon resonant interaction processes have been calculated for five different frequency modulation versions of counter-propagating incident fields. It is found that the proposed new modulation version (case b in the text) provides novel spectral features for a completely canceled absorption and a sharp dispersion shape at the fundamental beat note. Moreover, its absorption feature appears at the second harmonic of the RF modulation frequency generated by the joint modes via six interaction pathways without mutual phase shift. Such features persist even when the effects of the second-order sidebands of the incident fields are taken into account. Application potentials are emphasized.