Giant dispersive properties of planar graded photonic crystals

摘要

The dispersive properties of planar photonic crystals (PhCs) have been envisaged for years. In particular, the superprism effect has been considered to obtain a strong influence of input beam conditions (e.g. wavelength or input angle) on the light group velocity direction, enabling the design and fabrication of on-chip infra-red spectrometers and integrated optical demultiplexers. We extend here the properties of PhCs to the study of graded photonic crystals (GPhCs) made of a two-dimensional chirp of lattice parameters and show that GPhCs enable solving several drawbacks of dispersive PhCs like the beam divergence issues or the need of long preconditioning regions to precompensate beam diffraction effects. The proposed approach is applied to a square lattice air-hole PhC with a gradual filling factor that was fabricated using e-beam lithography and ICP etching techniques. A nearly-constant 0.25μmm spatial dispersion is demonstrated for a 60μm square GPhC structure in the 1470-1600nm spectral range without noticeable spatial or spectral spreading. Moreover, contrary to PhC superprism structures, a linear dispersion is obtained in the considered wavelength range.