Acceleration of neutral atoms in strong short-pulse laser fields

A charged particle exposed to an oscillating electric field experiences a force proportional to the cycle-averaged intensity gradient. This so-called ponderomotive force(1) plays a major part in a variety of physical situations such as Paul traps(2,3) for charged particles, electron diffraction in strong (standing) laser fields(4-6) (the Kapitza-Dirac effect) and laser-based particle acceleration(7-9). Comparably weak forces on neutral atoms in inhomogeneous light fields may arise from the dynamical polarization of an atom(10-12); these are physically similar to the cycle-averaged forces. Here we observe previously unconsidered extremely strong kinematic forces on neutral atoms in short-pulse laser fields. We identify the ponderomotive force on electrons as the driving mechanism, leading to ultrastrong acceleration of neutral atoms with a magnitude as high as similar to 10(14) times the Earth's gravitational acceleration, g. To our knowledge, this is by far the highest observed acceleration on neutral atoms in external fields and may lead to new applications in both fundamental and applied physics.