Experimental and theoretical investigation of loss issues in photonic crystal slab waveguide devices

In this paper we demonstrate ultra-low loss transmission across a photonic crystal super-prism device consisting of 600 lattice periods etched into a slab waveguide at wavelengths both above and below the primary band-gap. By modifying the refractive index of the holes we have reduced overall insertion loss to 4.5 dB across the entire visible region of the spectrum, greatly enhancing transmission and extinction in higher order stop-bands. In addition we show that the remaining loss is predominantly due to impedance mismatch at the boundary between patterned / unpatterned slab waveguide regions and so is no longer proportional to the length of the photonic crystal or the number of lattice periods. This is an important step forward for the realization of functional photonic crystal time delay elements, dispersion compensators and super-prism spectrometer devices. Experimental loss measurements compare extremely well with Finite difference time domain simulations which were used to investigate the effect of etch depth on scattering loss. We find that partial penetration into the underlying buffer region causes massive scattering loss to substrate modes due to loss of waveguiding in the holes