  We present two spatial-shaping approaches -- phase and amplitude -- for
creating two-dimensional optical dipole potentials for ultracold neutral atoms.
When combined with an attractive or repulsive Gaussian sheet formed by an
astigmatically focused beam, atoms are trapped in three dimensions resulting in
planar confinement with an arbitrary network of potentials -- a free-space atom
chip. The first approach utilizes an adaptation of the generalized
phase-contrast technique to convert a phase structure embedded in a beam after
traversing a phase mask, to an identical intensity profile in the image plane.
Phase masks, and a requisite phase-contrast filter, can be chemically etched
into optical material (e.g., fused silica) or implemented with spatial light
modulators; etching provides the highest quality while spatial light modulators
enable prototyping and realtime structure modification. This approach was
demonstrated on an ensemble of thermal atoms. Amplitude shaping is possible
when the potential structure is made as an opaque mask in the path of a dipole
trap beam, followed by imaging the shadow onto the plane of the atoms. While
much more lossy, this very simple and inexpensive approach can produce dipole
potentials suitable for containing degenerate gases. High-quality amplitude
masks can be produced with standard photolithography techniques. Amplitude
shaping was demonstrated on a Bose-Einsten condensate.
