Frequency-comb-induced radiative force on cold rubidium atoms

We experimentally investigate the radiative force and laser-induced fluorescence (LIF) in cold rubidium atoms induced by pulse-train (frequency-comb) excitation. Three configurations are studied: (i) single-pulse-train excitation, (ii) two in-phase counterpropagating pulse trains, and (iii) two out-of-phase counterpropagating pulse trains. In all configurations, measured LIF is in agreement with calculations based on the optical Bloch equations. The observed forces in the first two configurations are in qualitative agreement with the model(s) used for calculating mechanical action of a pulse train on atoms; however, this is not the case for the third configuration. Possible resolution of the discrepancy is discussed.

Figure 1

Time and frequency representation of (a) a single fs pulse train [configuration (i)], (b) two identical fs pulse trains delayed by $\tau$ [configuration (ii)], and (c) two identical fs pulse trains delayed by $\tau$ with $\pi$ phase differences [configuration (iii)].

Figure 2

Time evolution of the excited-state population ${\rho }_{22}$ of ${}^{87}\mathrm{Rb}$-like atoms excited by a single pulse train [black curve; configuration (i)] and two in-phase [red (light blue) curve; configuration (ii)] and two out-of-phase [blue (dark gray) curve; configuration (iii)] counterpropagating pulse trains delayed by $\tau =0.9$ ns. The insets shows close-ups of one-period ($T_R$) dynamics in the stationary state for two in-phase [red (light gray) curve] and two out-of-phase [blue (dark gray) curve] counterpropagating pulse trains.

Figure 3

Measured trap loss as a function of the fs laser peak wavelength for a laser power of 700 mW and typical ${}^{87}\mathrm{Rb}$ MOT parameters.

Figure 4

Fluorescence from the cold rubidium cloud induced by the $5S_{1/2}(F_g=2)\to 5P_{1/2}(F_e=1,2)$ excitation using a single frequency comb (points). Data are fitted to a saturation curve (red line).

Figure 5

(left) Fluorescence intensity distribution and (right) LIF spectra of the cold rubidium cloud for (a) no fs excitation, (b) excitation by a single pulse train, and (c) excitation by two in-phase counterpropagating pulse trains. The green dashed line indicates the position of the c.m. of the cloud. Note the different intensity scales of the 780.2- and 795-nm LIF signals.

Figure 6

(left) Fluorescence intensity distribution and (right) LIF spectra of the cold rubidium cloud for (a) no fs excitation, (b) excitation by a single pulse train, and (c) excitation by two out-of-phase counterpropagating pulse trains created by retroreflecting the fs laser beam. The green dashed line indicates the position of the c.m. of the cloud. Note the different intensity scales of the 780.2- and 795-nm LIF signals.

Figure 7

(left) Fluorescence intensity distribution and (right) time evolution of the c.m. coordinate of the cold rubidium cloud for the cases of no fs excitation (blue triangles), excitation by a single pulse train (black dots), and excitation by two out-of-phase counterpropagating pulse trains [red (gray) dots] created by retroreflecting the fs laser beam. The green dashed line in the left panel indicates the position of the c.m. of the cloud. The black dashed line in the right panel indicates the expected position of the cloud calculated using Eq. (2).