Observation of bidirectional and unidirectional ratcheting in a dissipative optical lattice

We confine cold atoms in a 3D tetrahedral lin-perp-lin dissipative optical lattice and illuminate these atoms, which are diffusing in all directions, with a weak probe beam propagating along a lattice axis. Probe-induced directed propagation is observed along a lattice axis perpendicular to the probe beam, in both positive and negative directions symmetrically, thus yielding a bidirectional ratchet. The experimental signature for bidirectional ratcheting manifests as ``Brillouin"-like resonances in the probe transmission spectrum. By angling the probe beam off-axis, we show that the atoms now preferentially propagate in either the positive or the negative direction, thus yielding a unidirectional ratchet. The experimental signature for unidirectional ratcheting is revealed as a splitting of the Brillouin resonance into a central feature corresponding to the vibrational frequency for a non-propagating atom oscillating inside a well, flanked by two side-features one of which corresponds to directed motion along the positive direction and the other along the negative direction. By analyzing pump-probe spectra with an angled probe beam, we show, with good theoretical agreement and greater resolution than before, how the split-Brillouin resonances shift with probe angle.