Losses in slab photonic crystals induced by fabrication tolerances

Abstract

We investigate the effect of fabrication tolerances on the propagation losses of light guided in photonic crystal waveguides. We focus on slab photonic crystals defined as a regular array of holes in high index material such as silicon. The deviation of photonic crystal lattice parameters, such as the hole diameter, from the ideal results in the shift of the guided mode in the photonic band gap. This shift occurs randomly along a photonic waveguide as the electromagnetic wave propagates in it. Because the range of frequencies supported in a photonic crystal waveguide is typically very narrow, even a small deviation of the lattice parameters may result in a sufficiently large shift for the wave to fall out of the guided range and become lossy. Indeed, we demonstrate that a deviation in hole size as small as a few percent can result in a guided mode becoming radiative. Consequently, a wave propagating in a real photonic crystal waveguide undergoes multiple transitions between guided and radiative modes, depending on the local variations of the photonic crystal parameters. This results in lossy propagation of the wave. We present simulations of the light propagation in a photonic crystal waveguide incorporating real-life fabrication tolerances.