Filtering light by angle

Light comes to us with an infinitude of frequencies and polarizations, and from every direction. Photonic crystals isolate desired frequencies and familiar filters block specific polarizations. Now the first material that selects a specific incidence angle of light for a wide range of optical frequencies has been fabricated by a group of physicists led by MIT’s Marin Soljačić. The key element in the new device is a stack of alternating layers of two oxide films with different dielectric constants ε₁ and ε₂. Each of the 14 layers in the stack has the same thickness, and the stack as a whole blocks certain frequencies and incidence angles of light from getting through. But light incident at the Brewster angle, defined by tan θ_B = (ε₂/ε₁)^1/2, and polarized in the plane of incidence (a restriction that in principle can be relaxed) passes through the stack, regardless of frequency. In the MIT device, six stacks are deposited on top of each other. Each stack has a different layer thickness—70 nm for the thinnest stack, 150 nm for the thickest. The six-stack gauntlet is no problem for light incident at the Brewster angle, but for incidence angles more than about 4° off from θ_B, one of the stacks reflects the light back. The figure shows the researchers’ experimental test. In the top panel, a camera faces the six-stack material, measuring 2 cm × 4 cm, and photographs its own reflection. In the bottom panel, light incident on the stacks at θ_B from the rainbow poster passes through the now-transparent material and is captured by the camera. (Y. Shen et al., Science 343, 1499, 2014, doi:10.1126/science.1249799 .)