A new class of man-made materials could hold the key to creating X-ray-like cameras that can see through walls and clothing.
Called metamaterials, these substances could harness terahertz radiation, light with energies between infrared waves and microwaves. Terahertz waves are essentially low-level heat created by the movement of molecules. They’re largely unexplored as a regime of energy because they are very difficult to detect, but if harnessed could lead to devices with some remarkable abilities.
“Terahertz can do things like see through cardboard, styrofoam, or clothes, which is unique compared to infrared, visible, or microwave,” researcher Richard Averitt told Livescience.com.
When stimulated by terahertz radiation, many molecules absorb and re-emit the energy in specific ways, creating a spectral fingerprint that researchers can use to identify them. If researchers could find a way to get terahertz waves in and out of a package, they would be able to “see” chemicals inside a box, said Averitt.
“There’s no way to do that right now. That’s probably the grand challenge of terahertz and what’s so unique about it," he said.
Current full-body airport scanners use frequencies just below the terahertz range, but terahertz waves would yield greater spatial resolution and uncover chemical signatures not present at lower levels, said Averitt.
A library of distinct spectral signatures for water, explosives, and compounds such as cocaine and saccharine is already in development and will someday be used to identify substances, he added.
Terahertz waves, or "t-rays," could also hold great promise for medical imaging because they can detect tumors and vibrating proteins without the destructive, ionizing effects of x-rays.
Yet, despite their great potential, terahertz waves have been nearly impossible to detect. Low-frequency terahertz waves are obscured by background heat in thermal sensors, and the high-frequency ones can’t be picked up by conventional antennas.
But recently, Averitt and his team have made some headway in manipulating t-waves with metamaterials, substances that through their fine structure bend light in ways not possible in nature.
The internal grid of their metamaterial pane is comprised of layers of sensor arrays that act like pixels. Each sensor is smaller than a wavelength of light and patterned with a copper design uniquely capable of soaking up terahertz radiation. As each sensor absorbs energy, it heats up and causes tiny "legs" that connect them to the substrate layer to bend.
“If you can detect that bending from another way, then you have a detector,” Averitt told Livescience.com.
Depending on their rotation, the tiny sensors absorb more or less of incoming radiation. This allows the filter to also act as a lens. Different focuses, intensities, and phases of transmitted light are possible through different angles with respect to the beam.
Scientists have already achieved rigid filters that act as detectors and lenses for terahertz energies in laboratory settings. But Averitt and co-researchers want to create reconfigurable metamaterial lenses that can be manipulated on the fly.
“The idea is that you can manipulate your terahertz beam by reorienting the metamaterial elements as opposed to reorienting your beam,” Averitt said.
The scientists presented their research at the Conference on Lasers and Electro-Optics/Quantum Electronics and Laser happening this week in San Francisco, Calif.