  This paper describes the implementation and performance of adiabatic
absorbing layers in an FFT-accelerated volume integral equation (VIE) method
for simulating truncated nanophotonics structures. At the truncation sites, we
place absorbing regions in which the conductivity is increased gradually in
order to minimize reflections. In the continuous setting, such adiabatic
absorbers have been shown via coupled-mode theory to produce reflections that
diminish at a rate related to the smoothness of the absorption profile
function. The VIE formulation we employ relies on uniform discretizations of
the geometry over which the continuously varying fields and material properties
are represented by piecewise constant functions. Such a discretization enables
the acceleration of the method via the FFT and, furthermore, the introduction
of varying absorption can be performed in a straightforward manner without
compromising this speedup. We demonstrate that, in spite of the crude discrete
approximation to the smooth absorption profiles, our approach recovers the
theoretically predicted reflection behavior of adiabatic absorbers. We thereby
show that the FFT-accelerated VIE method is an effective and fast simulation
tool for nanophotonics simulations.
