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. The team’s crystal, shown above, is fashioned with an array of 80-nm-diameter holes that give rise to a discrete set of internal optical modes: Some modes, usually the lower-energy ones, undergo total internal reflection at the crystal’s surface and remain permanently trapped; others, typically of higher energy, are resonances that couple to free space and allow photons to escape. When the researchers excited resonances with an incident laser beam, they found that at certain beam orientations the resonance lifetimes tended toward infinity. Simulation and theory confirmed that destructive interference was preventing photons from escaping—just as it did for von Neumann and Wigner’s hypothetical electron. The MIT group anticipates practical payoffs for their newfound trapped state: As a means to concentrate a large, tunable amount of energy near a surface, it could be ideal for biomolecule sensing. (C. W. Hsu et al., Nature 499, 188, 2013 .)—Ashley G. Smart