Tightly focused optical waves are ubiquitous in modern technology, Understanding and controlling the behavior of focused light is critical for many applications, including optical data storage, scanning microscopy and laser machining. The exact distribution of a converging field is essentially determined by the frequency and the material in which the wave travels. In isotropic materials where the momentum is parallel to the energy flow, the wavefronts converge when the intensity distribution converges. In metamaterials and photonic crystals (PCs), however, the anisotropy of the dispersion surface results in anomalous focusing properties.
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