Abstract

We consider a standard four-beam optical lattice consisting of two x-polarized beams propagating in the yOz plane and making an angle 2θ and two y-polarized beams propagating in the xOz plane making the same angle. The variation of the optical potential along an x axis connecting the potential minima is shown in Fig. 1 for the case of a J=1/2 →J'=3/2 atomic transition. The distance between two adjacent wells is λ/2θ in the small angle limit. Even though most of the atoms are bound inside a well where they undergo an oscillatory motion characterized by the oscillation frequencies Ω_x,y,z, a few delocalized atoms still propagate along this periodic structure. A numerical simulation shows that the dominant propagation mode occurs through the steps presented in Fig.1. An atom travels from A to B in a time τ on the order of τ = π/Ω_x. The average atomic velocity $\overline{v}=\lambda {\Omega }_x/2\pi \theta$ thus appears as a characteristic velocity for the propagation of an excitation inside the lattice. Such a motion can be driven by a pump-probe excitation mechanism provided that the phase velocity of the interference pattern created by the superposition of the probe and pump beams is equal to $\pm \overline{v}$. If the probe is along Oz, the interference patterns drift in the lattice with the velocities v_± = ±δ/kθ (δ is the latticeprobe frequency difference).

© 1996 IEEE