physicists at washington state university (pullman, wash.) have developed a proof of concept from four-year-old research by demonstrating the ability to write an electrical circuit into a crystal using a laser, according to a report on the school website.

this new technique allows the circuit to be removed and redrawn
like an etch-sketch. (wikimedia commons)

the scientists used a laser to etch a line into the crystal and placed electrical contacts on each end of the line to demonstrate that it would carry a current. this was proposed four years ago when a student saw a 400-fold increase in the electrical conductivity of a crystal when it was exposed to light.

researchers believe that this breakthrough could lead to a new class of transparent, 3-d electronics that can be erased and reconfigured.

“ordinarily, a crystal does not conduct electricity,” the article explained. “but when the crystal strontium titanate is heated under the right conductions, it is altered so light will make it conductive. the phenomenon, called ‘persistent photoconductivity,’ also occurs at room temperature. this makes it easier to improve conductivity than cooling materials with liquid nitrogen, another focus of research.”

although the exact reason for this change is not yet known, researchers hypothesize that heat pushes strontium atoms off the materials, which creates light-sensitive defects that lead to persistent photoconductivity.

according to the report, the recent study increased the crystal’s conductivity by 1,000-fold and the phenomenon can last as long as a year, although not necessarily at 100 percent. also, heating the crystal on a hot plate and recasting it with an optical pen erased the circuit.

the research was recently published in scientific reports. the abstract stated:

“materials with persistent photoconductivity (ppc) experience an increase in conductivity upon exposure to light that persists after the light is turned off. although researchers have shown that this phenomenon could be exploited for novel memory storage devices, low temperatures (below 180 k) were required.

“in the present work, two-point resistance measurements were performed on annealed strontium titanate (srtio, or sto) single crystals at room temperature. after illumination with sub-gap light, the resistance decreased by three orders of magnitude. this markedly enhanced conductivity persisted for several days in the dark.

“results from ir spectroscopy, electrical measurements, and exposure to a 405 nm laser suggest that contact resistance plays an important role. the laser was then used as an ‘optical pen’ to write a low-resistance path between two contacts, demonstrating the feasibility of optically defined, transparent electronics.”