Welcome to Optical Lattices
Press coverage: Anyons in 1D Optical LatticesIntroduction
An optical lattice is simply a set of standing wave lasers. The electric field of these lasers can interact with atoms - the atoms see a potential and therefore congregate in the potential minima. In the case of a typical one-dimensional setup, the wavelength of the opposing lasers is chosen so that the light shift is negative. This means that the potential minima occur at the intensity maxima of the standing wave. Furthermore, the natural beam width can constrain the system to being one-dimensional.
To keep the atoms from distributing over too large a distance, the lattice is superimposed with an additional trap. This trap is generated by a dipole laser beam focused at the position of the atom cloud. Perpendicular to the beam axis, this creates a Gaussian intensity profile. For small excursions from the trap centre this is a near harmonic trap. Along the beam axis, the trapping frequency is too low, though: atoms could spread out many 100 µm. To close the trap in this direction, a second (and later a third) perpendicular laser beam is focussed onto the atom cloud.
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If one of these laser beams is now collimated after passing through the atom cloud and retro-reflected on a mirror, the intensity and thus the trap-depth at the trap centre is doubled. But now a standing wave forms, with its first node at the surface of the retro-reflecting mirror. The interference pattern extends back to the atom cloud, producing an intensity modulation with a distance of half the laser wavelength between intensity maxima. A 2D or 3D lattice is formed by also retro-reflecting the other laser beams. The standing waves intersect and lattice sites are where all standing waves have an intensity maximum. Consider the oblate traps of one standing wave as parallel planes. Then two perpendicular groups of planes intersecting with each other form an array of cigar-shaped traps in a regular 2D lattice. A third group of parallel planes divide sthese 2D lattice sites into spherically symmetric traps arranged in a 3D optical lattice.