A photonic crystal sets a cunning trap

In 1929 John von Neumann and Eugene Wigner proposed a most unusual electron trap—one that can confine an electron whose energy exceeds that of the potential barrier. The trick was to shape the trapping potential so that the electron's available paths of escape interfere destructively. Known as a bound state in the continuum, such a trap has never been achieved in an experiment. But now John Joannopoulos, Marin Soljačić, and coworkers at MIT have created its optical analogue inside a photonic crystal—a periodically nanostructured material designed to control the propagation of light.