Experimental demonstration of non-near-field image formed by negative refraction

We have experimentally investigated the negative refraction and focusing by a two-dimensional photonic crystal (PC) made from a triangular lattice of metal-core dielectric-shell coated cylinders. At a certain frequency, the PC structure has an isotropic band at which the phase velocity is antiparallel to the group velocity. As a result, the PC structure can exhibit negative refraction and the corresponding effective refractive index can be close to the ideal value of -1. A flat lens formed from such a PC has been designed and its imaging properties have been investigated systematically. The measurement results show that the flat lens can form non-near-field images, and the source and image distance follow the conventional wave-beam negative refraction law for the refractive index of -1 irrespective of the slab thickness and the source distance.