  We have achieved stimulated laser cooling of thermal rubidium atomic beams on
a silicon chip. Following pre-collimation via a silicon microchannel array, we
perform beam brightening via a blue-detuned optical molasses. Owing to the
small size of the chip elements, we require only 8 mW, or nine times lower
power than earlier free-space experiments on cesium [Aspect et al., Phys. Rev.
Lett. 57, 1688 (1986)]. Silicon micromirrors are fabricated and hand-assembled
to precisely overlap a strong elliptical standing wave with a sheet-shaped
atomic density distribution, with dimensions chosen precisely to match these.
We reduce the transverse velocity spread to below 1 m/s within a total travel
distance of 4.5 mm on a silicon substrate. We use Doppler-sensitive two-photon
Raman spectroscopy to characterize the cooling. In contrast to time-of-flight
methods utilized previously, this approach requires a much shorter apparatus to
achieve similar resolution. This hybrid of passive and active collimation paves
the way toward the construction of full-fledged atomic instruments, such as
atomic beam clocks and gyroscopes, entirely on-chip through batch-fabricated
processes.
