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Physicists from Washington State University (WSU) have used lasers to draw conductive circuits into a crystal. The achievement demonstrates a new kind of transparent, three-dimensional electronics: circuits that can be erased and reconfigured, like the drawings of an Etch A Sketch.

Four years ago, a WSU doctoral student researcher stumbled upon a phenomenon: A crystal, when left exposed to light, increased its electrical conductivity 400-fold.

“The fluorescent room lights in the lab caused a crystal’s resistance to drop over the period of a couple weeks,” said Matt McCluskey, a WSU professor of physics and materials science, who spoke with Tech Briefs. “After we learned what was going on, we refined the technique so we can induce the conductivity effect quickly.”

And by quickly, McCluskey means a few seconds. In principle, the WSU light-writing method allows circuits to be defined “on the fly,” to adapt to changing needs.

Bringing the Heat

A crystal does not typically conduct electricity. When strontium titanate, however, is heated under precise conditions, the oxide is altered so that light will make the material conductive.

WSU researchers used light to write a highly conducting electric path in a crystal. On the left, a photograph of a sample with four metal contacts. On the right, an illustration of a laser drawing a conductive path between two contacts. (Credit: Washington State University)

Using a high temperature of 1200 °C, McCluskey and his student researchers created defects within the transparent crystal — modifications that support the channeling of current.

“We’re still trying to figure out exactly what happens,” said the professor, who believes that the alterations are strontium vacancies, or gaps where strontium atoms should exist. When light hits the sample, electrons are excited from the opening, which can then conduct electricity and reduce the crystal’s electrical resistance.

To induce the conductivity in a more sophisticated fashion, McCluskey used a laser to etch a line in the crystal. The violet solid-state laser featured a 405-nm wavelength and a 5 mW power — a strength comparable to a laser pointer.

The strontium titanate crystal was synthesized artificially and purchased from commercial vendors. The etching laser drew a conductive path between the electrical contacts at each end of the oxide.

Writing (and Re-Writing) Circuit Design

McCluskey recently increased the crystal’s conductivity 1,000-fold. The phenomenon, called “persistent photoconductivity,” occurs at room temperature and can last up to a year.

“We look at samples that we exposed to light a year ago and they’re still conducting,” said McCluskey in the university’s original press release. “It may not retain 100 percent of its conductivity, but it’s pretty big.”

The circuit can be erased by heating the component on a hot plate, and then recast with an optical pen. When heated, the electrons are trapped by the defects again, causing the sample to lose conductivity and return to pre-illumination conditions.

“It’s an Etch A Sketch,” said McCluskey.

To the Washington State researcher, the reconfigurable method opens up the possibility of a new type of electronics, where an engineer can define a circuit optically, erase it, and then define a new one.

“A person could write a circuit board with a laser pen, instead of having to deposit metal,” said McCluskey.” “When needs change, the circuit could be erased and then written again.”

Furthermore, the professor imagines the process being used to support “invisible electronics,” a transparent circuit board that could go on a window, for example.

The research was funded by the National Science Foundation. Co-authors on the paper are former students Violet Poole and Slade Jokela.

The work appeared in the online journal Scientific Reports.

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