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arXiv:physics/0509109 · Physical Review Letters

Coherent Control of Ultracold Molecule Dynamics in a Magneto-Optical Trap by Use of Chirped Femtosecond Laser Pulses

Benjamin L. Brown, Alexander J. Dicks, Ian A. Walmsley

Published May 5, 2006• 79 citations
arXivHTMLPDF

Abstract

We have studied the effects of chirped femtosecond laser pulses on the formation of ultracold molecules in a Rb magneto-optical trap. We have found that application of chirped femtosecond pulses suppressed the formation of (85)Rb and (87)Rb(2) a(3)sigma(+)(u) molecules in contrast to comparable nonchirped pulses, cw illumination, and background formation rates. Variation of the amount of chirp indicated that this suppression is coherent in nature, suggesting that coherent control is likely to be useful for manipulating the dynamics of ultracold quantum molecular gases.

DOI
10.1103/physrevlett.96.173002

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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. ^

•Jan 1, 2010•0 citations
We have studied the effects of chirped femtosecond laser pulses on the formation of ultracold molecules in a Rb magneto-optical trap. We have found that application of chirped femtosecond pulses suppressed the formation of (85)Rb and (87)Rb(2) a(3)sigma(+)(u) molecules in contrast to comparable nonchirped pulses, cw illumination, and background formation rates. Variation of the amount of chirp indicated that this suppression is coherent in nature, suggesting that coherent control is likely to be useful for manipulating the dynamics of ultracold quantum molecular gases.
We report on coherent control of excitation processes of translationally ultracold rubidium dimers in a magneto-optical trap by using shaped femtosecond laser pulses. Evolution strategies are applied in a feedback loop in order to optimize the photoexcitation of the Rb2{\mathrm{Rb}}_{2}Rb2​ molecules, which subsequently undergo ionization or fragmentation. A superior performance of the resulting pulses compared to unshaped pulses of the same pulse energy is obtained by distributing the energy among specific spectral components. The demonstration of coherent control to ultracold ensembles opens a path to actively influence fundamental photoinduced processes in molecular quantum gases.
Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text I. A. Walmsley, A. J. Dicks, B. L. Brown, D. McCabe, and A. Monmayrant, "The Effect of Chirped Femtosecond Laser Pulses on the Formation of Ultracold Molecules in a Magneto-Optical Trap," in Frontiers in Optics, OSA Technical Digest (CD) (Optica Publishing Group, 2006), paper LTuH3. Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article
Calculations relating to two experiments that demonstrate coherent control of preformed rubidium-85 molecules in a magneto-optical trap using ultrafast laser pulses are presented. The two experiments are a step toward the stabilization of ultracold rubidium dimers using ultrafast lasers. In the first experiment, it is shown that preassociated molecules in an incoherent mixture of states can be made to oscillate coherently using a single ultrafast pulse. A mechanism that can transfer molecular population to more deeply bound vibrational levels is used in the second. Optimal parameters of the control pulse are presented for the application of the mechanism to molecules in a magneto-optical trap. The calculations make use of an experimental determination of the initial state of molecules photoassociated by the trapping lasers in the magneto-optical trap and use shaped pulses consistent with a standard ultrafast laser system. The experiment's purpose is to demonstrate and evaluate the use of ultrafast shaped pulses to manipulate ultracold rubidium dimers with a view to the eventual stabilization of the molecules.
We use frequency-chirped light on the nanosecond time scale to produce ultracold 87{}^{87}87Rb2{}_{2}2​ molecules in the lowest triplet state via the process of photoassociation. Comparing to quantum simulations of the molecular formation, we conclude that coherent stimulated emission plays an important role and is primarily responsible for the significant difference observed between positive and negative chirps.
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. ^