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Thursday, February 7, 2008

Hidden Art Could Be Revealed By New Terahertz Device

Like X-rays let doctors see the bones beneath our skin, "T-rays" could let art historians see murals hidden beneath coats of plaster or paint in centuries-old buildings, University of Michigan engineering researchers say.
T-rays, pulses of terahertz radiation, could also illuminate penciled sketches under paintings on canvas without harming the artwork, the researchers say. Current methods of imaging underdrawings can't detect certain art materials such as graphite or sanguine, a red chalk that some of the masters are believed to have used.

The team of researchers, which includes scientists at the Louvre Museum, Picometrix, LLC and U-M, used terahertz imaging to detect colored paints and a graphite drawing of a butterfly through 4 mm of plaster. They believe their technique is capable of seeing even deeper.

In March, the scientists will take their equipment to France to help archaeologists examine a mural they discovered recently behind five layers of plaster in a 12th century church.

"It's ideal that the method of evaluation for historical artifacts such as frescoes and mural paintings, which are typically an inherent part of a building's infrastructure, be non-destructive, non-invasive, precise and applicable on site. Current technologies may satisfy one or more of these requirements, but we believe our new technique can satisfy all of them," said John Whitaker, an author of the paper who is a research scientist and adjunct professor in the Department of Electrical Engineering and Computer Science at U-M.

Terahertz imaging can reveal depth and detail that other techniques cannot, Whitaker said. And it's not potentially harmful like X-ray imaging because terahertz radiation is non-ionizing. Its rays don't have enough energy to knock electrons off atoms, forming charged particles and causing damage, like X-rays do.

While terahertz radiation is all around us in nature, it has been difficult to produce in a lab because it falls between the capabilities of electronic devices and lasers.

"Terahertz is a strange range in the electromagnetic spectrum because it's quasi-optical. It is light, but it isn't," said Bianca Jackson, first author of the paper who is a doctoral student in applied physics.

The device used for this research is a hybrid between electronics and lasers. It was developed by the Ann-Arbor based company Picometrix. It's called the T-Ray™ system, and it uses pulses from an ultra-fast laser to excite a semiconductor antenna, which in turn emits pulses of terahertz radiation.

The rays permeate the plaster, and some reflect back when there is a change in the material. When they bounce back and how much energy they retain depends on the material they hit. Different colors of paint, or the presence of graphite, for example, cause tell-tale differences in the amount of energy in the returning waves. A receiver measures this energy, and the scientists can use the data to produce an image of what lies beneath, Jackson explained.

A similar device made by Picometrix is used routinely to examine the foam on the space shuttle's fuel tanks for underlying damage, said Irl Duling, director of terahertz business development at Picometrix and an author of the paper. This paper discusses a new application, rather than a new device.

Gèrard Mourou, a U-M electrical engineering professor emeritus, said he believes this technique will be especially useful in Europe, where historic regime changes often resulted in artworks being plastered or painted over. This was common in places of worship, some of which switched from churches to mosques and vice versa over the centuries.

"In France alone, you have 100,000 churches," Mourou said. "In many of these places, we know there is something hidden. It has already been written about. This is a quick way to find it."

And Leonardo DaVinci's "The Battle of Anghiari," for example, is believed to lurk beneath other frescos at the Palazzo Vecchio in Florence, Italy, Mourou said.

The paper "Terahertz imaging for non-destructive evaluation of mural paintings," is published in the February edition of Optics Communications.

Mourou is the A. D. Moore Distinguished University Professor Emeritus of Electrical Engineering and Computer Science. He currently holds a position at the Laboratoire d'Optique Appliquèe. Other authors are: Steven Williamson of Picometrix; Marie Mourou, a U-M undergraduate student; and Michel Menu, of the Center for Research and Restoration at The Louvre Museum.

Scientists develop new terahertz material
Researchers at Los Alamos National Laboratory have created a device for manipulating terahertz (THz) radiation. The device could be the basis for novel electronics and photonics applications ranging from new imaging methods to advanced communication technologies. The THz range of the electromagnetic spectrum lies between the infrared and microwave wavelengths.
In research published in the journal Nature, Los Alamos scientist Hou-Tong Chen and his colleagues explain how metamaterials (artificial materials with properties derived from their sub-wavelength structures instead of their compositions) can be designed to efficiently control THz waves.

According to Chen, "devices that generate and detect THz radiation are already in development, but techniques to actually control the waves are lagging behind. This is the next logical step in the development of terahertz technologies for wider electronics and photonics applications."

Like microwaves, terahertz radiation has the ability to penetrate a wide variety of non-conducting materials like paper, plastics, wood, and ceramics. Because it can "see" through plastics and cardboard, it might also be used in manufacturing to inspect packaged objects for quality control or process monitoring. THz radiation is sensitive to the water content, which means it might be used to detect differences in body tissue density. Because terahertz radiation is non-ionizing, it does not damage DNA like X-rays and might someday be used as a safer alternative for certain types of medical and dental imaging. Non-ionizing means the radiation does not have enough energy to convert electrically neutral atoms into ions by knocking an atom's electron from its orbit.

To create their device, Chen and his colleagues used micro-fabrication processes to lay down an array of gold metamaterial structures over a semiconductor substrate. An applied voltage between the substrate and the metamaterial enables the device to modulate the intensity of THz waves by up to 50 percent. The experimental demonstration of the device exceeds the performance of existing electrical THz modulators and the team hopes to further improve the device's performance in coming months. news, tec,

In addition to Chen, other members of the THz device development team include Willie Padilla, formerly of Los Alamos and now with Boston College, Richard Averitt, formerly of Los Alamos and now with Boston University, Antoinette Taylor from Los Alamos, and Joshua Zide and Arthur Gossard from the University of California, Santa Barbara. The research was supported by Laboratory Directed Research and Development funds and the Center for Integrated Nanotechnologies, a DOE/Office of Science Nanoscale Research Center.

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