A passively self-aligning crossed optical dipole trap for cold atoms

Crossed optical dipole traps (ODTs) provide three-dimensional confinement of cold atoms and other optically trappable particles. However, keeping the two beams precisely crossed at the waists requires high alignment stability. Here we demonstrate a novel self-aligning crossed ODT. A single high-power laser beam is folded back on itself, forming the cross, and the two waists at the cross are conjugated by an imaging system. By adding an intermediate image plane, the lateral magnification perpendicular to the plane of the crossed beams is made positive. The waist of one beam follows the transverse movement of the other, and the intersection is preserved when the magnification is tuned to 1. We load a sample of lithium-6 atoms into the ODT, after cooling in a magneto-optical trap and gray molasses, and show that the trap remains aligned after millimeter-scale displacement of the first beam perpendicular to the crossing plane. Beyond alignment stability, by allowing an atomic sample to be repositioned transversely to the plane of a high-power crossed ODT, our scheme has potential applications to interfacing atomic clouds with, e.g., material samples, integrated photonic devices, or superconducting circuits.