Coherent Transients in the Femtosecond Photoassociation of Ultracold Molecules

We demonstrate the photoassociation of ultracold rubidium dimers using coherent femtosecond pulses. Starting from a cloud of ultracold rubidium atoms, electronically excited rubidium molecules are formed with shaped photoassociation pump pulses. The excited state molecules are projected with a time-delayed probe pulse onto molecular ion states which are detected in a mass spectrometer. Coherent transient oscillations of the excited state population are observed in the wings of the pump pulse, in agreement with the time-dependent solution of the Schrödinger equation of the excitation process.

Figure 1

Photoassociation and detection of ultracold molecules in a pump-probe scheme using ultrashort laser pulses. Pulse shaping restricts the femtosecond pump pulse spectrum to below the molecular potential asymptote (inset).

Figure 2

(a) Experimental pump-probe signal of molecular ions for different cut positions below the $5s+5p_{1/2}$ asymptote. Curves correspond to cutoff detunings from $-6$ (yellow) to $-12{\mathrm{cm}}^{-1}$ (red) in steps of $-2{\mathrm{cm}}^{-1}$. A constant background of molecular ions at a rate of 310 Hz, which originates from the ionization of excited state molecules formed by the trap light, is subtracted from the data. The inset shows the variation of modulation frequencies in wave numbers versus the cutoff detuning. (b) $5s+5p_{1/2}$ population from quantum dynamical simulation of the pump excitation with shaped femtosecond pulses. The dashed blue line shows the temporal envelope of the pump pulse electric field below $3\times {10}^6\mathrm{V}/\mathrm{m}$. (c) Fluorescence signal of the trapped Rb atoms as a function of the pump-probe delay. Signals are normalized to the trap fluorescence without femtosecond lasers.

Figure 3

Measured femtosecond pump-probe photoassociation traces (black curve) for three different chirp settings of the pump pulse. Chirp is found to shift the phase of the transient oscillation leading to a smaller population in the excited molecular state at large delay times. Good agreement is found with the solution of the time-dependent Schrödinger equation depicted as the gray (red) curve. As in Fig. 2, a background count rate of 350 Hz is subtracted from the data. The calculated curves are scaled to fit the experimental data.