The Bichromatic Optical Force on the Atomic Life- time Scale

Our experimental and theoretical studies of the bichromatic force (BF) have shown that its strength and velocity range are very much larger than those of the usual radiative force.\footnote{Phys. Rev. Lett. {\bf 93} 213004 (2004), Phys. Rev. A {\bf 70}, 063402 (2001)} Since the BF relies on stimulated effects, the role of spontaneous emission in laser cooling has come into question.\footnote{Phys. Rev. A {\bf 77}, 061401 (2008)} We drive the $2^3S\rightarrow 3^3P$ transition of He at $\lambda = 389$ nm with laser frequencies $\omega_{\ell} = \omega_a \pm \delta$, where $\omega_a$ is the atomic transition frequency and $\delta \sim 30$ MHz. Thus the velocity range of the force is $\Delta v \sim \delta /2k = 6$ m/s. Because of the large and nearly constant strength of the BF, $F \sim \hbar k \delta / \pi$, all atoms can reach the velocity limit in a time $ \leq M\Delta v / F = \pi/4\omega_r = 380$ ns, where $\omega_r$ is the atomic recoil frequency. In our experiment a beam of He atoms crosses perpendicular through the BF laser beams in 380 ns so the relatively long lifetime of the excited state ($\tau = 106$ ns) allows one or at most two spontaneous emission events, despite $\Delta v$ of many tens of recoils. We will present our initial measurements of the BF in this new domain.