  A finite element-based modal formulation of diffraction of a plane wave by an
absorbing photonic crystal slab of arbitrary geometry is developed for
photovoltaic applications. The semi-analytic approach allows efficient and
accurate calculation of the absorption of an array with a complex unit cell.
This approach gives direct physical insight into the absorption mechanism in
such structures, which can be used to enhance the absorption. The verification
and validation of this approach is applied to a silicon nanowire array and the
efficiency and accuracy of the method is demonstrated. The method is ideally
suited to studying the manner in which spectral properties (e.g., absorption)
vary with the thickness of the array, and we demonstrate this with efficient
calculations which can identify an optimal geometry.
