Transparent boundary condition for beam propagation

A new boundary condition algorithm is presented that passes outgoing radiation freely with a minimum reflection coefficient (typically 10(-5)) while inhibiting the flux of incoming radiation. In contrast to the commonly used absorber method, this algorithm contains no adjustable parameters and is thus problem independent. It adapts naturally to a standard Crank-Nicholson difference scheme and is shown to be accurate and robust for both two-and three-dimensional problems.