First Random Laser Made of Paper-based Ceramics

New biotemplating process makes it possible to create lasers out of cellulose paper

The team used conventional laboratory filter paper as a structural template due to its long fibers and the stable structure. (Photo: Institute for Complex Systems /Rome)

Straubing, November 10, 2016 (sl) – Material synthesis that is inspired by biology is an important area of research at the Chair of Biogenic Polymers of the Technical University of Munich at the Straubing Center of Science. It utilizes models from nature and biogenic materials to develop new materials and technologies. In a joint research project with physicists from the Institute for Complex Systems in Rome, a team led by Professor Cordt Zollfrank built the first controllable random laser out of cellulose paper in Straubing. 

The basis for the development of the laser was a refined biotemplating process developed by the team, which uses natural materials as templates. This process allowed for complex hierarchical structures, such as those of whole pine cones, to be transformed into silica glass. This essentially produced artificially petrified pine cones, which not only had the exact structure of the original biological object down to the nanometer level—they also behaved in the same way: When they absorbed moisture, their scales opened up, and when they were dry, the structure closed back up again.

“Chaos is part of the functional principle”

In the now-completed project, which is presented in the current issue of the publication “Advanced Optical Materials“, the researchers once again succeeded in transforming a natural structure into an artificial one down to the last detail: “The prerequisite for a random laser is a defined degree of structural chaos,” scientist Dr Daniel Van Opdenbosch explained. This is where it differs from a classical laser, which requires a very precisely defined cavity. Instead, in a random laser, light propagates along random paths which result from the irregular structure of the medium. This is because the structure provides points, at which the incident light is scattered.

“The laser is ‘random’ because the photons deflected in various directions can be scattered in the opposite direction at other points, which means that, if the interference points are ordered correctly in three-dimensional space, a feedback effect results that allows for lasing, i.e. the amplification of light through stimulated emission,” said Daniel Van Opdenbosch, describing the principle, which is similar to that of a chain reaction.

“Successful misuse of laboratory materials”

For their random lasers, the TUM researchers used normal laboratory filter paper. “Due to its long fibers and the resulting stable structure, it appeared to be a suitable medium for this purpose,” said Van Opdenbosch. In the laboratory, the paper was impregnated with tetraethyl orthotitanate, an organometallic compound. When it dries and the cellulose is burned off at 500 degrees Celsius, it leaves a titanium dioxide residue—the same substance used in sunblock to provide protection from the sun. “This effect in sunblock is based on titanium dioxide’s strong light scattering effect, which we also needed for our random laser,” said Van Opdenbosch.

“Random laser not that random after all”

Despite the randomness, the light waves can still be controlled, as his colleagues Dr Neda Ghofraniha, Dr Luca La Volpe and Prof. Dr Claudio Conti at the Institute for Complex Systems of the Italian National Research Council found out, with whom Daniel Van Opdenbosch and Cordt Zollfrank collaborated for this interdisciplinary project. With the help of a spectrometer, they were able to differentiate the various laser wavelengths generated in the material and detect them separately from one another.

“The test setup which was used to map the samples consisted of a green laser whose energy could be adjusted, microscope lenses, and a mobile table which allowed the sample to be moved past,” said Van Opdenbosch, explaining the procedure. “That way, my colleagues were able to find out that at different excitation energy levels, different areas of the material radiate different laser waves.” In light of this analysis, it is possible to configure the laser in any number of ways and to determine in which direction and with what intensity it radiates or to move its ray in three-dimensional space through a chronological sequence of different excitation patterns.

With this knowledge, potential practical applications are within reach. “Such materials can, for example, be highly useful as micro switches or detectors for structural changes,” said Van Opdenbosch.

“Ordered chaos”

But for him, the true success of this project lies in its contribution to basic research. “For the very first time, we were able to use a biological structure as a template for a technical random laser,” Van Opdenbosch said with visible pleasure when speaking about the successful conclusion of the project, which identified yet another innovative property of artificial structures based on natural templates. The fact that it is an optical property is of particular importance for the researchers at the Chair.

This is because, in parallel to their work, ongoing intensive research is being conducted on the relationship between structure and color perception. “Unlike human technology, Mother Nature prefers using structures instead of pigments to create an impression of color. For example, certain beetles have, over geological time scales, developed a ‘customized chaos’ in the structures of their shells which they use to create color effects,” said Professor Zollfrank.

 As part of a focal project of the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), the researchers at the Chair of Biogenic Polymers in Straubing are now working on using these templates from nature to create plastics that leverage customized chaos. This will also be done using titanium dioxide.

(Original publication: Ghofraniha, Neda, Luca La Volpe, Daniel Van Opdenbosch, Cordt Zollfrank, and Claudio Conti. 2016. “Biomimetic Random Lasers with Tunable Spatial and Temporal Coherence.” Advanced Optical Materials, September. doi:10.1002/adom.201600649.)