Surface texturing allows infrared tuning

17 Jun 2002

Researchers at the US company Ion Optics have developed surface texturing processes that alter the emissivity of infrared sources, allowing them to be tuned for spectroscopic applications. This will allow night vision and surveillance infrared cameras to gather information about the chemical composition of a scene.

Ion Optics currently manufactures the PulsIR infrared radiator for non-dispersive infrared gas sensors to analyze car exhausts, coolants and indoor air quality. The emissivity of these metal filament sources is enhanced and controlled by ion beam etching that creates a random surface texture consisting of sub-micron rods and cones. This creates long and/or short wavelength cutoffs.

The researchers' goal was to develop an infrared thermal emitter with high emissivity over a narrow band of wavelengths and low emissivity in other bands. Such sources, emitting hundreds of milliwatts of power in-band, are attractive alternatives to infrared LEDs which have low quantum efficiencies (about 2 to 4%) and only tens of microwatts output power.

Periodic arrays, similar to metal mesh and photonic bandgap filters, were fabricated on the emitter's surface. These were readily fabricated using lithography on silicon, as feature sizes around the order of 0.5 micron were needed to give wavelengths in the range of 2 to 14 micron.

It was found that the emission wavelength is linearly related to feature size and that for narrow feature sizes, the enhancement in emissivity was small, but for larger features, the enhancement is significant.

Ion Optics recently won a small business innovation research grant from the US Air Force to develop a compact, high resolution, hyperspectral far infrared imager incorporating this technology.

The resultant Hypercam is expected to be lightweight, low power and rugged for use in military and industrial remote sensing applications.

Story courtesy of Opto and Laser Europe magazine.