FM SPECTROSCOPY IN FLUORESCENCE IN LASER-COOLED RUBIDIUM

A novel variation of the technique of two-photon frequency modulation (FM) spectroscopy has been developed, which allows signals to be obtai ned from small(similar to 10(5) atoms) and dilute (similar to 10(10) c m(-3)) samples - in this case laser-cooled rubidium atoms. This was po ssible by detecting and demodulating the fluorescence signal rather th an the transmission through the sample. Data are presented demonstrati ng 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 trap ping fields, data were taken with the atoms in free fall. A similar si gnal, slightly shifted and broadened by the trapping fields, was obtai ned while the trap was on and used for long-term stabilisation of the probe laser. (C) 1998 Elsevier Science B.V. All rights reserved.