Controlling Condensate Collapse and Expansion with an Optical Feshbach Resonance

We demonstrate control of the collapse and expansion of an ${}^{88}\mathrm{Sr}$ Bose-Einstein condensate using an optical Feshbach resonance near the ${}^{1}S_0\mathrm{\text{−}}{}^{3}P_1$ intercombination transition at 689 nm. Significant changes in dynamics are caused by modifications of scattering length by up to $\pm 10a_{\mathrm{bg}}$, where the background scattering length of ${}^{88}\mathrm{Sr}$ is $a_{\mathrm{bg}}=-2a_0$ ($1a_0=0.053\mathrm{nm}$). Changes in scattering length are monitored through changes in the size of the condensate after a time-of-flight measurement. Because the background scattering length is close to zero, blue detuning of the optical Feshbach resonance laser with respect to a photoassociative resonance leads to increased interaction energy and a faster condensate expansion, whereas red detuning triggers a collapse of the condensate. The results are modeled with the time-dependent nonlinear Gross-Pitaevskii equation.

Figure 1

Line profiles through absorption images showing OFR-induced variation of BEC expansion. Data correspond to no OFR laser and an OFR laser blue and red detuned by 0.5 MHz with respect to the $-24\mathrm{MHz}$ PA line [42] applied for $\tau =1.2\mathrm{ms}$. Expansion times are 35 ms. Fits are a Bose distribution for the thermal atoms (dashed line) and a Gaussian density distribution for the BEC.

Figure 2

(a) BEC size after 35 ms of expansion versus the exposure time of the OFR laser with the intensity of $0.057\mathrm{W}/{\mathrm{cm}}^2$ and three different detunings from the $-24\mathrm{MHz}$ PA line. (b) Number of condensate atoms versus exposure time. Curves calculated by the Gross-Pitaevskii equation correspond to a combined fit of the data, yielding $\eta =19.5$, ${\ell }_{\mathrm{opt}}/I=2.2\times {10}^4{a}_0/(\mathrm{W}/{\mathrm{cm}}^2)$, and $K_0=5.8\times {10}^{-7}{\mathrm{cm}}^3/\mathrm{s}$. Error bars represent the standard deviation of the mean from multiple measurements.

Figure 3

The BEC size (a) and number (b) versus the detuning with respect to the $-24\mathrm{MHz}$ PA resonance for an intensity of $0.057\mathrm{W}/{\mathrm{cm}}^2$. The OFR beam is applied for 4.0 ms, and the data are recorded after 35 ms of expansion. The insets give the total scattering length $a$ and the loss rate constants.