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Physical Review A

Frequency-comb-induced radiative force on cold rubidium atoms

Gordana Kregar, Neven Šantić, Damir Aumiler, Hrvoje Buljan, Ticijana Ban

Published May 23, 2014• 7 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.89.053421

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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.
Abstract We report on the direct observation of the frequency comb printed in the blue fluorescence excitation spectrum of rubidium vapor induced by the combined action of an ultrashort pulse train and a cw diode laser. Each laser drives one step of the 5S—5P—5D two-photon transition in a copropagating configuration and the excitation spectrum is obtained while the cw-laser frequency is scanned over the rubidium D 2 lines. Measurements of the fluorescence as a function of the diode intensity and atomic density allow us to investigate how the effect of power broadening and absorption of the diode laser blur the excitation process. The experimental results and the printing process of the comb on the Doppler profile are described well by numerical integration of the Bloch equations for a cascade three-level system.
In this thesis, we studied radiation force of a femtosecond pulse train on cold rubidium atoms. Cooling of rubidium atoms is established by magneto-optic trap. We also studied theory of radiation force on atoms made by one femtosecond pulse train and two counter-propagating pulse trains. In case of two counter-propagating pulse trains we studied two cases of phase differences between pulses, case when pulses are in phase and when they are out of phase. Radiation force is measured for one femtosecond pulse train and for two counter-propagating pulse trains. In case of one pulse train measured radiation force matches theoretical model. In case of two pulse trains we made measurement in two configurations: ”in phase” and ”out of phase”. For ”out of phase” configuration, it is expected that there is no radiation force on atoms. For ”in phase” configuration, displacement of the cold atoms cloud center of mass is effect of femtosecond beams imbalance. We also measured spectral width of lasers that are used in experiment.
In this dissertation a cloud of cold rubidium atoms confined in a magnetooptical trap (MOT) has been realized. The laser cooling and trapping technique was implemented in the Laboratory for femtosecond frequency comb spectroscopy at Institute of Physics for the first time, so this work opens a new area of research. The cloud of cold atoms, containing ~ 10^9 87^Rb, F_g = 2, in volume ~ 3 mm3, density of ~ 1011 atom/cm3 in typical MOT operating conditions was obtained. Detailed characterization of the cloud has been done and the Doppler model for the real atomic system of rubidium 87 atoms was applied to describe the radiative force. The main focus of this work is investigation of the coherent continuous (cw) or pulsed (femtosecond) radiation effects on rubidium atoms in MOT. Atomic motion in the trap can be described with a damped harmonic oscillator model. A new technique for monitoring cold cloud center of mass oscillations induced by cw laser, tuned to rubidium resonant transition has been introduced. With this method the basic trap parameters, such are spring constant, damping coefficient and atom temperature are determined. For various trap working conditions an effective saturation intensity of 87^Rb D1 (F_g = 2 → F_e = 2) transition is, for the first time, determined through experimentally and theoretically investigated rubidium excited F_e = 2 state population depending on the probe beam intensity. Also, the radiative force of the probe D1 laser on cold cloud is measured and theoretically modelled. Interaction of cold rubidium cloud with femtosecond (fs) pulses has been studied in three different experimental configurations; interaction of cold atoms with single fs train of pulses, with two in-phase counterpropagating fs pulses and with two out-of-phase counterpropagating fs pulses. In all three described geometries the LIF signal and radiative force are experimentally determined and compared with theoretical model, based on optical Bloch equations (OBE) for two energy level atom.
We study Doppler cooling of atoms using a frequency comb and develop a theory to quantitatively describe the process by investigating the interaction of two-level atoms with two counterpropagating pulse trains. We derive the radiative force on atoms and show that each comb mode effectively acts as a cooling laser, thereby indicating that simultaneous laser cooling in multiple cooling channels can be achieved using a single-frequency comb source. Possible experimental applications involving alkali-metal atoms are illustrated.
We investigate the modifications of the frequency-comb-induced radiation pressure force on cold 87^{87}87Rb atoms that are induced by the collective effects. Collective effects include both coherent and incoherent contributions and depend on the optical thickness of the atomic cloud. We observe reduction and broadening of the comb-induced force when the cloud's optical thickness increases, and compare the measured results with predictions of a coherent model based on the timed Dicke state approach and an incoherent scattering model based on the shadow effect explained by Beer-Lambert law. Both models describe the experimental results well, indicating that an incoherent scattering approach is sufficient to explain the observed modifications of the comb-induced force even for larger optical thicknesses. The results support the analogy between the frequency comb and continuous-wave laser-atom interaction and thus pave the way toward novel frequency comb applications in laser cooling, quantum communication, and light-atom interfaces based on structured and disordered atomic systems.
We investigate the frequency-comb-induced radiation pressure force acting on a cloud of cold 87 R b atoms. Reduction and spectral broadening of the frequency comb force are observed as the cloud’s optical thickness is increased. Since the radiation pressure force is uniquely determined by light scattered by an atomic cloud, we discuss different scattering mechanisms and point to the shadow effect as the dominant mechanism affecting the FC-induced force in resonantly excited dense atomic clouds. Our results improve the understanding of the interaction of frequency comb light with many-atom ensembles, which is essential for novel frequency comb applications in simultaneous multispecies cooling and self-ordering, multimode quantum memories, and quantum computing.