The superprism effect in lithium niobate photonic crystals for ultra-fast, ultra-compact electro-optical switching

We numerically analyze ultra-refraction and slow-light in lithium niobate photonic crystals in order to investigate and then optimize the efficiency of a tunable photonic crystal superprism. In contrast to a passive superprism 1-to-N demultiplexer, we describe a tunable bandpass filter with only three output ports. The electro-optic effect in lithium niobate is used to achieve tunability, with the filter bandwidth shifting in wavelength as the refractive index of the superprism is modified by an externally applied electric field. Such a device could be used to realize a compact and fast wavelength multiplexer/demultiplexer for telecommunications or optical interconnect applications. We calculate constant frequency dispersion contours (plane-wave expansion) to identify initial configurations that show significant ultra-refraction, and verify the expected behavior of light propagation inside the structure using 2D FDTD (finite difference time domain) simulations. We show that the voltage requirements of such an electro-optically tunable superprism could potentially be relaxed by exploiting the enhancement of the electro-optic effect recently discovered by our group [M. Roussey, M.-P. Bernal, N. Courjal, D. Van Labeke, F.I. Baida, Electro-optic effect exaltation on lithium niobate photonic crystals due to slow photons. Appl. Phys. Lett. 89 (24) (2006) 241110], which we believe to be due to the presence of slow-light in the nanostructure. We present a methodology that readily identifies superprism design points showing both strong ultra-refraction as well as low group velocity. However, we find that this improved voltage efficiency comes at the cost of reduced operating bandwidth and increased insertion losses due to proximity to the band-edge.