EPFL forms photoconductive circuits directly onto glass

30 Jan 2024

Femtosecond laser writing could create energy harvesting devices more easily and cheaply.

Swiss research center EPFL has demonstrated a new route to manufacturing photoconductive circuits, by writing the conducting pathways directly onto a glass surface in a novel way.

Published in Physical Review Applied, the method employs a femtosecond laser as a surface modification method, a potentially more straightforward approach than current alternatives.

The study was carried out at EPFL's Galatea Laboratory, which specializes in surface modification using lasers.

Examples of its breakthroughs include the 2023 demonstration of how an all-glass femtosecond laser cavity can be manufactured by using a laser to both micromachine a holder for the cavity's optical components and to precisely adjust their position via a non-contact method.

In earlier work on tellurite glass, in which the main component in tellurium oxide, Galatea Lab studied how near-IR irradiation can locally create trigonal tellurium (t-Te) nanocrystals embedded in the tellurite glass matrix. The lab has now shown how a femtosecond source can fully transform the glass into a pure t-Te phase, without the need to add other materials.

This laser-created change offers a way to create pathways of nanoscale tellurium and tellurium oxide crystals, both semiconducting materials, etched into the glass where it has been exposed to laser illumination. Energy harvesting devices that are transparent and made from only one single material may then be possible using the technique.

"Tellurium being semiconducting, we wondered based on this finding if it would be possible to write durable patterns on the tellurite glass surface that could reliably induce electricity when exposed to light; and the answer is yes," said Yves Bellouard head of the Galatea Laboratory.

"An interesting twist to the technique is that no additional materials are needed in the process. All you need is tellurite glass and a femtosecond laser to make an active photoconductive material."

Light-sensing devices from single pieces of material

According to the team's paper, exposure to 270-femtosecond laser pulses at 1030 nanometers led to a lowering of measured resistivity by at least 10 orders of magnitude, compared to the unmodified tellurite glass surrounding the newly created patterns.

In EPFL's trails, exposing a simple line pattern on the surface of a tellurite glass material 1 centimeter in diameter allowed it to then generate current when exposed to both UV light and the visible spectrum, with the light harvesting behavior remaining present and reliable for some months after exposure.

Creating such patterns on a surface could lead to light-sensing devices of arbitrary sizes and shapes made by functionalizing a single piece of material, especially since the photoresponse appears to be highly reproducible under different illumination conditions across such a broad spectral range.

"It's fantastic, we're locally turning glass into a semiconductor using light,” enthused Yves Bellouard. "We are essentially transforming materials into something else, perhaps approaching the dream of the alchemist."