Dielectric mirrors, also referred to as Bragg mirrors, reflect light nearly completely. Hence, they are suited for various appli­cations, such as camera systems and sensor systems for micro­scopy and medical techno­logies. So far, such mirrors have been produced by complex processes in expensive vacuum devices. Researchers from Karlsruhe Institute of Techno­logy (KIT) now are the first to print Bragg mirrors of high quality with inkjet printers. This may pave the way towards the digital manu­facture of customized mirrors. 

Bragg mirrors are produced by applying several thin layers of materials onto a carrier. The resulting optical mirror specifically reflects the light of a certain wavelength. Reflec­tivity of a Bragg mirror depends on the materials, the number of layers applied, and their thicknesses. So far, Bragg mirrors have been produced in expensive vacuum production faci­lities. KIT researchers now were the first to print them on different carriers. This largely faci­litates production. “It was a big challenge to develop suitable inks and to establish a reliable process for the production of several thin layers,” says Uli Lemmer from KIT’s Light Techno­logy Institute (LTI), who heads the project that is part of the “3D Matter Made to Order“ cluster of excellence.

The constituents of the inks must have suitable optical properties and be soluble. Moreover, every layer should be as homogeneous as possible in order to obtain a consistent stack of layers. Pressure must be controlled precisely and results must be repro­ducible to guarantee excellent optical properties and a high reflec­tivity of the Bragg mirrors. The research team decided to use nanoparticles. “Thanks to the rapid development of nano­chemistry, nanoparticles are getting cheaper and more diverse,” Lemmer says. His team used a mix of two different materials, titanium oxide and polymethyl methacrylate, as optical components of the inks. Using the inks, researchers succeeded in producing extremely precise optical properties and thicknesses of the layers. “We reached a very high reflec­tivity of 99 % with ten double layers only,” Lemmer says. 

The method developed by the researchers may be used on very small areas down to ranges of a few micrometers, enabling production of optical components for microsystems techno­logies or camera systems. On the other hand, large areas of some square meters can be printed for solar modules, facades, and adver­tising displays. Such mirrors have even been printed on flexible plastic foils. “The completely digital process enables fabri­cation of mirror layers that are precisely adjusted to the appli­cation. This is a great advantage compared to conventional fabri­cation methods,” Lemmer points out. (Source: KIT)

Reference: Q. Zhang et al.: Fabrication of Bragg Mirrors by Multilayer Inkjet Printing, Adv. Mat. 34, 2201348 (2022); DOI: 10.1002/adma.202201348

Link: Light Technology Institute, Karlsruhe Institute of Technology KIT, Karlsruhe, Germany