Fig. 1. Experimental setup schematic.

Fig. 2. Sequences of the cooling laser and clock laser for in-field detection and free-of-field detection.

Fig. 3. (a) Normalized atom number in the MOT under the in-field (blue) and the free-of-field (red) spectroscopy fitted with a Gaussian function at 1 mW. (b), (c), (d) Free-of-field spectroscopy of the clock transition at incident powers of 0.7, 0.2, and 0.03 mW and their fittings.

Fig. 4. (a) Calculated temperature of the atoms in MOT from the FWHM of the spectroscopy at different clock laser powers. (b) Contrast of the spectroscopy (i.e., depletion rate) at different clock laser powers. (c) The center frequency of the free-of-field spectroscopy at different clock laser power.

Fig. 5. Doppler-free signal in the Doppler-broadened spectroscopy.