. Scientific Frontline: Highly reflective mirrors from the inkjet printer

Wednesday, September 7, 2022

Highly reflective mirrors from the inkjet printer

Colored, printed mirror layer on a film. Inkjet printing allows structuring so that large-scale logos can also be printed
Credit: Qihao Jin, KIT

Dielectric mirrors, also called Bragg mirrors, can almost completely reflect light. They are therefore suitable for countless applications, such as in camera systems, in microscopy, in medical technology or in sensor systems. So far, these mirrors had to be manufactured in expensive vacuum devices. Researchers at the Karlsruhe Institute of Technology (KIT) have now printed high quality Bragg mirrors with inkjet printers for the first time. The process could open the way to digital production of tailor-made mirrors. The results appeared in Advanced Materials.

For Bragg mirrors, several layers of material are applied thinly to a carrier. These mirrors, which consist of a large number of thin layers, form an optical mirror that ensures that light of a certain wavelength is specifically reflected. How strongly Bragg mirrors reflect depends on the materials, but also on how many layers you apply and how thick they are. So far, Bragg mirrors have had to be manufactured with expensive vacuum production systems. The Karlsruhe team succeeded for the first time in printing them on different carriers. This can considerably simplify production.

Inks made from nanoparticles

"It was a major challenge to develop suitable inks and to establish a reliable process for the production of several layers," said Professor Uli Lemmer from the Lighting Institute (LTI) at KIT, who led the project as part of the Cluster of Excellence "3D Matter Made to Order "leads. The components of the inks must have suitable optical properties and must also be soluble. In addition, each layer should be as even as possible to ensure a uniform stack of layers. In addition, the pressure must be controlled precisely and the results must be reproducible in order to reliably guarantee excellent optical properties, i.e. a high reflectivity of the Bragg mirrors. The research team relied on nanoparticles: "Due to the rapid development in nanochemistry, nanoparticles are becoming increasingly inexpensive and diverse," says Lemmer. His team used a mix of two different materials, titanium dioxide and polymethyl methacrylate, as optically active components of the inks. With these inks, they were able to produce the optical properties and the thickness of a single layer with extreme precision in inkjet printing. "We achieved an ultra-high level of reflection of 99 percent with only ten double layers," says Lemmer.

Print on large and small areas

The manufacturing method developed by the researchers at the LTI can be used on the one hand on very small areas down to areas of a few micrometers, so that, for example, optical components for microsystem technology or camera systems can be easily manufactured. On the other hand, large areas such as solar modules, facade elements or advertising displays of a few square meters can also be printed. The mirrors could even be printed on flexible plastic films. “The completely digital manufacturing process allows the production of mirror layers to be precisely adapted to the application. This is an immense advantage over the previous manufacturing processes,” says Lemmer.

Source/Credit: Karlsruhe Institute of Technology

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