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Frontiers in Optics

Control of Ultracold Collisions with Frequency-Chirped Light

Matthew J. Wright, J. A. Pechkis, J. L. Carini, C. E. Rogers, P. L. Gould

Published Jan 1, 2006• 0 citations
Publisher

Abstract

We report on ultracold collision experiments utilizing frequency-chirped laser light. The collision rate is measured through trap loss. We also present our recent progress towards producing the frequency-chirp through phase modulation of the laser light.
DOI
10.1364/fio.2006.jwd121

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We report on ultracold atomic collision experiments utilizing frequency-chirped laser light. A rapid chirp below the atomic resonance results in adiabatic excitation to an attractive molecular potential over a wide range of internuclear separation. This leads to a transient inelastic collision rate which is large compared to that obtained with fixed-frequency excitation. The combination of high efficiency and temporal control demonstrates the benefit of applying the techniques of coherent control to the ultracold domain.
We present results on coherent control of ultracold trap-loss collisions and molecule formation using frequency-chirped light. The chirps, either positive or negative, sweep 1 GHz in 100 ns and are red-detuned below the D2 line in either 85Rb or 87Rb. The pulses are Gaussian and have a full-width at half-maximum (FWHM) of 40 ns. In our collision experiments, we demonstrate coherent control of ultracold 85Rb collisions using nonlinear (either concave-down or concave-up) frequency chirps in the region of pulse detunings where coherent collision blocking occurs. We attribute this to the excitation radius of the negative chirp following the excited-wavepacket trajectory. We find that this process is dependent on the nonlinearity of the negative chirp. Specifically, the concave-down negative chirp yields a higher value of the collisional loss rate constant β than those of the concave-up and linear negative chirps. For nonlinear positive chirps, we find no significant dependence of β on the nonlinearity of the chirp. Our measurements are supported by quantum mechanical simulations of the collisional process. ^ In our molecule formation experiments, we use resonantly-enhanced multi- photon ionization to directly detect ground-state 87Rb 2 formed though photoassociation (PA) by linearly frequency-chirped pulses and subsequent spontaneous decay. In particular, we measure the rates of formation (R) and photodestruction (Γ PD) for positive and negative frequency-chirped pulses, as well as for unchirped pulses. We find that the unchirped pulse yields higher values of both R and ΓPD than those of the positively-chirped pulse, whose values in turn are greater than those of the negatively-chirped pulse. Our results are important steps towards coherent control of ultracold ground-state molecule formation. ^
We demonstrate the ability to coherently control ultracold atomic Rb collisions using frequency-chirped light on the nanosecond time scale. For certain center frequencies of the chirp, the rate of inelastic trap-loss collisions induced by negatively chirped light is dramatically suppressed compared to the case of a positive chirp. We attribute this to a fundamental asymmetry in the system: an excited wave packet moves inward on the attractive molecular potential. For a positive chirp, the resonance condition moves outward in time, while for a negative chirp, it moves inward, in the same direction as the excited wave packet; this allows multiple interactions between the wave packet and the light, enabling the wave packet to be returned coherently to the ground state. Classical and quantum calculations support this interpretation.
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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 85Rb^{85}\mathrm{Rb}85Rb, 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.