Shifty surfaces bend the rules of ray optics

The traditional laws of reflection and refraction rest on the assumption that the phase of light is continuous everywhere, even at the interface between two materials. Now, Federico Capasso and colleagues at Harvard University have figured out what happens when that assumption is relaxed. With elegantly simple math, they’ve shown how a surface that imposes abrupt, position-dependent phase shifts can bend light in ways that defy the centuries-old rules of ray optics. Negative angles of reflection and refraction, multiple critical angles for total internal reflection, and other anomalies become possible. The researchers created phase-shifting surfaces in the lab with the help of micron-sized plasmonic antennas, which scatter light with a phase delay that depends on their shape. Guided by Maxwell’s equations, they designed an assortment of antennas that, ordered appropriately on a silicon wafer, could shift the refraction and reflection angles of a mid-IR beam by as much as 40°. Arranged as shown in the scanning electron microscope image at left, the antennas converted an incident plane wave into the vortex beam shown at right. The Harvard team anticipates a host of applications for the phase-shifting surfaces, including birefringent polarizers and low-aberration lenses. (N. Yu et al., Science, in press, doi:10.1126/science.1210713.)—Ashley G. Smart