Probing ultracold collisional dynamics with frequency-chirped pulses

Abstract

We report on the dynamics of ultracold collisions induced by near-resonant frequency-chirped light. A series of identical chirped pulses, separated by a variable delay, is applied to an ultracold sample of 85 Rb, and the rate of inelastic trap-loss collisions is measured. For small detunings of the chirped light below the atomic resonance, we observe that the rate of collisions induced by a given pulse can be increased by the presence of an earlier pulse. We attribute this to the enhancement of short-range collisional flux by the long-range excitation of atom pairs to an attractive molecular potential. For larger detunings and short delays, we find that a leading pulse can suppress the rate of collisions caused by a following pulse. This is due to a depletion of short-range atom pairs by the earlier pulse. Comparison of our data to classical Monte-Carlo simulations of the collisions yields reasonable agreement. Recent years have witnessed enhanced capabilities in controlling both the external and internal degrees of freedom of atoms and molecules. Laser cooling and evaporative cooling of atoms [1] and the coherent control of excitation processes in molecules [2,3] represent prime examples. The possibility of combining these two areas, i.e., applying coherent control techniques to ultracold systems, has generated a great deal of interest, especially in the context of using short laser pulses to produce ultracold molecules by photoassociating ultracold atoms [4,5,6,7,8,9,10,11,12,13,14,15]. Part of the appeal is the prospect of controlling the internal state distribution of the resulting molecules. Understanding and controlling the dynamics of the formation process will be key to these efforts. In the present work, we explore the nanosecond time-scale dynamics of a closelyrelated process: ultracold atomic collisions induced by frequency-chirped light [16], shown in Fig. 1. By varying the delay between successive pulses of chirped light, we observe that the collisions induced by a given pulse can be either enhanced or suppressed by the presence of a preceding pulse, depending on the range of frequencies spanned by the chirp. If the chirp encompasses frequencies close to the atomic resonance, long-range excitation to the R −3 potential (R is the internuclear separation) leads to collisional flux enhancement. For chirps centered well below the atomic resonance, efficient adiabatic excitation by the first chirp depletes the short-range atom pairs available to be excited by the second chirp.A number of previous experiments [6,17,18,19,20] have employed time-dependent excitation with fixedfrequency light to investigate dynamical effects in ultracold atomic interactions. The frequency-chirped light utilized in the present work is unique in two important ways [16]: 1) the chirp provides adiabatic, and therefore * Present address: Institut für Experimentalphysik, Universität Innsbruck, Technikerstraße 25, 6020 Innsbruck, Austria very efficient, excitation of atom pairs; and 2) the wide range of frequencies spanned by the c...

Coherent control of ultracoldRb85trap-loss collisions with nonlinearly frequency-chirped light

“…Multiple-pulse effects were also investigated. We found that the rate of collisions induced by a given pulse can be enhanced or suppressed by a preceding pulse, depending on the delay [30]. In work most relevant to that described here, we demonstrated coherent control of these ultracold trap-loss collisions [31].…”

“…The MOT light is turned off for 150 µs, during which time a number (typically 60) of frequency-chirped pulses with peak intensity I = 67 W/cm 2 illuminates the atoms at a repetition rate of 1 MHz. This 1-µs delay between pulses ensures that each chirped pulse acts independently [30]. The overall sequence is repeated every 722 µs.…”

Interatomic potentials, electric properties and spectroscopy of the ground and excited states of the Rb2 molecule: ab initio calculations and effect of a non-resonant field*

“…Trapping rubidium in an optical lattice has facilitated studies of atom-molecule dark states [10] and transferring the molecules into their vibrational ground state [11]. The Rb 2 molecule continues to draw attention in the context of the coherent control of ultracold collisions [12][13][14][15][16] and femtosecond photoassociation [17][18][19][20]. These experiments as well as those employing photoassociation with continuous wave lasers [21][22][23][24][25] require precise spectroscopic knowledge not only of the ground but also the excited states for both interpretation and detection.…”

Perspectives for coherent optical formation of strontium molecules in their electronic ground state

“…the efficiency of the pump step. While Feshbach-optimized photoassociation [39] is limited to the odd isotopes of strontium, flux enhancement via coherent control of ultracold collisions [40,41,42] could represent a general means of pushing the atom pairs closer together before photoassociating them.…”