A bichromatic superlattice for neutral atoms
In our recent experiments, we extend the concept of a simple cubic monochromatic lattice by adding a second lattice with a different wavelength along one horizontal direction. We chose a ratio in wavelength of 1:2 by using light from a Ti:Sa laser at 765 nm and from a fiberlaser at 1530nm. Both lattices are created by retro-reflection giving rise to a ratio of 1:2 in the lattice spacings as well. By controlling both, the relative phase between the two standing waves and their respective depths independently, we can tune this combined lattice of double wells arbitrarily.
The relative phase is controlled by a frequency lock of the fiberlaser onto the Ti:Sa laser. In this locking scheme, a small portion of the fiberlaser light is frequency doubled in order to reach convenient beat frequencies in the GHz range when beatin with the Ti:Sa light.
Going away from the precise ratio of 1:2 in the wavelengths (in medium) leads to a shift in relative phase accumulated over the distance between the retro mirror and the atoms. Since the wavelength in the surrounding media (air, glass) is different for the two wavelengths 765nm and 1530nm, this phase shift will occur even when choosing the vacuum wavelengths to fulfill the ratio. The free choice of the offset frequency in the frequency lock in a range of 1-2 GHz yields a tuning range of 2π in terms of the relative phase. This allows to compensate for any shift in the relative phase due to the chromaticity of the media as well as for full dynamical control over the superlattice configuration.