A Bright Metastable Helium Beam for Neutral Atom Lithography

We have used non-monochromatic light to produce large optical forces over a wide range of atomic speeds, e.g., slowing a beam of metastable helium (He*)\footnote{M. Cashen and H. Metcalf, {\it J. Opt. Soc. Am. B} \textbf{20}, 915 (2003). }. Our He* beam has now been brightened by active collimation\footnote{M. Partlow et al., {\it Phys. Rev. Lett.} \textbf{93}, 213004 (2004). } using large transverse bichromatic forces on the $2^{3}S_{1} \rightarrow 2^{3}P_{2}$ transition at $\lambda = 1083$ nm. An LN$_{2}$ cooled discharge source yielding $10^{14}$ atoms/sr-s with $\overline{v} \approx$ 1000 m/s forms the beam. We have captured atoms from a transverse velocity range of $\pm$ 87 m/s (175 mrad cone) in an interaction length of only 5 cm comprised of four interaction regions. The collimated beam has an integrated flux of $1.4\times10^{11}$ atoms/s and thus contains $\sim1/4$ of the total source output. Further collimation with a subsequent optical molasses yields an overall increase in brightness by a factor of 4100. Small improvements to the collimation will produce a flux density high enough to expose a resist for atomic nanolithography in less than one minute. The bichromatic detuning was $\delta = \pm 2\pi \times 60 \,{\rm MHz}\, (\pm 37\gamma)$. For this $\delta$, the bichromatic force is optimum for $I \sim 0.7$ W/cm$^{2}$ (4100 $\times I_{sat}$) for each of four frequencies. The light originates from a single, extended-cavity DBR diode laser and is injected into two fiber amplifiers.