Top science can also be done with ordinary potato starch. This is the finding of physicists of the Brno University of Technology and Palacký University in Olomouc, who investigated the optical properties of potato starch grains. They found that the starch grain, due to its unique structure, acts as a microlens, which can simultaneously focus the optical beam, change its polarization and spin it into a light vortex. From a physical point of view, starch performs spin-orbit conversion of light, so far only possible with advanced nanotechnology. The discovered mechanism expands the possibilities for light control and can be used for next-generation sensors, information transmission or quantum computers.
"Our interest in starch was aroused by commonly available images from a polarizing microscope, in which the images of individual grains resemble the shape of the Maltese Cross. It has long been known that these patterns are formed as a result of a special internal arrangement of starch grains. However, we focused on the full optical response of these grains and suspected that it may be related to an original way of changing the polarization rotation of the electric field into the swirling motion of electromagnetic energy. In optical terminology, this interaction represents a change in the spin of light to its orbital angular momentum and is currently widely studied and used in classical and quantum physics," recalls physicist Petr Bouchal, who initiated the research.
"The path to confirming our ideas was not easy. To fully understand the effects, it was necessary to make an optical description of the internal structure of the starch grain. It consists of approximately spherical skins like onions, but they contain regularly arranged and radially oriented lamellae. The grain affects light in a special way. It changes the optical path of light in a similar way to a conventional lens, but it also shapes it thanks to the effects related to the change in the polarization state of light caused by lamellas pointing towards the center of the grain. It was extremely difficult to experimentally demonstrate the interplay of both simultaneously occurring effects," adds Bouchal, who, together with Radim Chmelík, is behind the designs of special, previously patented microscopes used in measurements.
"A good understanding of the interaction of light with starch grains and its plausible description demonstrated the possibility to determine the polarization state of light falling on the grain directly from the focal trail that is captured by a conventional detector. We have experimentally verified this simple yet reliable method of polarization measurement and even tried to construct an original sensor that simultaneously measures the shape of the wavefront and the polarization state of light, variable at individual points of the wavefront. Although the shape disparity of natural starch grains made it impossible to carry out experiments in the originally intended scope, the functional principle of the sensor was clearly proven," adds physicist Petr Viewegh, who participated in these experiments with his colleague Petr Liška.
"Simultaneous imaging of the wavefront and spatially variable polarization of light is application-relevant and is a challenge for further research. Routine measurements would require creating an extensive matrix of grains of almost identical parameters, which is not possible with natural starch. However, a viable way is to create an artificial structure that imitates the optical response of starch grains in a more perfect way," believes Viewegh, who is involved in the preparation of such materials.
The research of scientists from the Faculty of Mechanical Engineering and CEITEC BUT, Petr Bouchal, Petr Viewegh, Petr Liška and Radim Chmelík, was joined by Zdeněk Bouchal of Palacký University in Olomouc, who created the theoretical framework of the experiments and performed the necessary calculations. The result of the collaboration is a paper in the prestigious journal Advanced Optical Materials entitled Spin-Orbit Photonics with Potato Starch Lenses.
Research on natural starch grains shows how perfect tools and structures nature has at its disposal. "The sophisticated optical effects revealed by our research are not limited to the starch under investigation, but are inherent in a wide range of structures known as spherulites. Spin-orbit effects, which we have theoretically investigated and experimentally verified step by step, promise for the design of new optical components, sensors and photonic chips and open the way to application in material diagnostics or optical communications," concludes Bouchal.
Bouchal, Petr & Viewegh, Petr & Liška, Petr & Chmelík, Radim & Bouchal, Zdeněk. (2025). Spin‐Orbit Photonics with Potato Starch Lenses. Advanced Optical Materials. 10.1002/adom.202500684.