Simulated trapping potential. The parameters are those of “config. 1” in Table II. (a) Cross section through the x–y plane. The white cross shows the position of the minimum. (b) Cross section through the y–z plane. (c) Trapping potential along the x, y, and z axis. Close to the center, in a range of ±50 μm, the potentials are well approximated by harmonic potentials.