Perfectly matched layer behavior in negative refractive index materials

The perfectly matched layer (PML) absorbing boundary condition is shown to be analytically unstable for a material with a bandwidth of negative refractive index. Physically, this is a consequence of the reversed phase and group velocities in such a medium. The instability is inherent in the PML and is not implementation-specific. We derive a modified PML (NIMPML) that is analytically stable in a negative index material (NIM), and we demonstrate its numerical stability in a specific NIMPML implementation.