Proposed 3D PhC configuration for rainbow trapping. (a) Schematic illustration of the chirped woodpile PhC structure. For better visualization the number of periods of the illustrated PhC structure is reduced. The lower inset depicts the cross sectional view of the structure in the yz- plane. (b) The variation of the PBG along the propagation direction is representatively depicted. Due to the gradual variation of the layer-to-layer distances, the incident wave will be spatially separated into its frequency components along the propagation direction, thus forming a “trapped rainbow”, as it is representatively shown as a cross sectional view on the lower left of (a). The upper right inset in (b) representatively shows the change of the dispersion curves in the propagation direction, where the layer-to-layer distance of the solid black dispersion curve is smaller than that of the dashed blue dispersion curve. The black arrows reveal the propagation direction of the incoming wave.