Photonic Crystal Improves Mid-IR Generation

Kevin Robinson

A photonic crystal designed into a silicon wafer promises users the ability to controllably generate radiation at mid-infrared wavelengths. The emitter, developed by Ion Optics Inc. in Waltham, Mass., in conjunction with researchers at the Jet Propulsion Laboratory in Pasadena, Calif., and at Iowa State University in Ames, is being readied for use in environmental sensing applications.

Martin U. Pralle of Ion Optics explained that mid-IR radiation typically is generated using a hot filament, an LED or a quantum cascade laser, but the new device has at its core a photonic crystal lattice. The researchers coat a silicon wafer with glass and then gold or platinum. Using photolithography, they etch a pattern of micron-size holes into the wafers to a depth of 5 µm. This structure turns the wafer into an infrared emitter, with the lattice size and placement determining the wavelength of the output.

The addition of a photonic crystal structure to a silicon blackbody emitter produces narrowband mid-IR output by surface-enhanced emission. The wavelength of the output depends on the periodicity of the crystal. Courtesy of Martin U. Pralle.

When heated, the silicon acts like a hot filament and produces a wide spectrum of blackbody radiation. The photonic crystal reshapes this spectrum, centering it on the resonant wavelength defined by the lattice, but the device does not simply filter the blackbody emission. The photons excite plasmons at the buried metal interface that couple with plasmons on the surface, which decay to release narrowband mid-IR radiation.

To drive the emitters, the researchers initially placed them on a hot plate at 325 °C. Pralle said that they have incorporated the device into a micro hot plate, in which it is driven electrically.

The new emitter already has a commercial application, he said. The researchers have produced proto-type gas sensors for methane, carbon dioxide and water that display resolutions as low as 1000 ppm. They plan to work on improving the bandwidth and the emission intensity of the emitters to increase the sensitivity of the sensors.