Fast Acting Optical Forces From Far Detuned, High Intensity Light

We are exploring fast acting, strong optical forces from standing wave light fields with high intensity and large detuning $\delta\gg\gamma$, where $\gamma$ is the transition linewidth. We observe these fast acting forces on a time scale of a few times the excited state lifetime $\tau\equiv 1/\gamma$; thus an atom may experience at most one or two spontaneous emission events. The dipole force is typically considered when the Rabi frequency $\Omega\ll\delta$, but we use $\Omega\sim\delta$ so the usual approximations break down because a significant excited state population can occur, even for our short interaction times that limit spontaneous emission. Our experiment measures the transverse velocity distribution of a beam of 2$^3$S He after a chosen interaction time with a perpendicular standing wave detuned from the 2$^3$S$\rightarrow$3$^3$P transition near 389 nm. The distribution shows velocity resonance effects that persist over a large range of $\Omega$. We also simulate the experiment numerically using the Optical Bloch Equations and the results are consistent with our measurements.