Department of Physics
Condensed Matter Seminar

The interaction of waves with inhomogeneous media leads to the natural focusing of light, the channelling of waves into stable caustics. We have extended natural focusing to x-rays, observing caustics in topographs of ferroelectric lithium niobate. Voltage across domains of reversed polarity induces perturbations to the local crystal planes, producing dramatic variations in the images. Ray-tracing shows a “catastrophic” discontinuity, causing bright focal lines. (Similar effects have been seen with visible light in so-called "Oriental magic mirrors, recently analyzed by M. V. Berry.) Analysis reveals details of boundary strains and local ferroelectric properties. Controlled focusing could be extended to designed domain patterns to probe microstructural properties, and also as a type of voltage-controlled ferroelectric optics for x-rays. These results also suggest entirely new x-ray monochromators based on MEMS micromirror arrays