  In this paper we introduce a method for creating a transparent computational
boundary for the simulation of unidirectional propagation of optical beams and
pulses using leaky modes. The key element of the method is the introduction of
an artificial-index material outside a chosen computational domain and
utilization of the quasi-normal modes associated with such artificial
structure. The method is tested on the free space propagation of TE
electromagnetic waves. By choosing the material to have appropriate optical
properties one can greatly reduce the reflection at the computational boundary.
In contrast to the well-known approach based on a perfectly matched layer, our
method is especially well suited for spectral propagators.
