Researchers develop technology to generate higher harmonics in nonlinear nanostructured metasurfaces

Natural and synthetic crystals can change the spectral color of light, a phenomenon known as the nonlinear optical effect. Color change is used for many applications, including nonlinear microscopy for biological structures and material examinations, LED light sources and lasers in optical communications, and photonics and resulting technologies such as quantum computing. Researchers at the University of Paderborn have now found a way to improve the physical process underlying this phenomenon. The results are published in the journal Light: Science & Applications.

„The process is based on the anharmonic energy of the crystal atoms and often induces a precise amplification of the light frequency, known as 'higher harmonics’ – similar to the overtones heard when the string of a musical instrument vibrates,” said Paderborn physicist Professor Cedric. Meyer explains.

Although the effect occurs naturally in many crystals, it is often very weak. With this in mind, there are various approaches to enhance the effect, for example, by combining different materials and their structures at the micro and nano scale. The University of Paderborn has conducted intensive, successful research in this area in recent decades.

A focal point of this research in photonics is metamaterials and in particular metasurfaces. It consists of structured elements applied in the nanometer range to a thin substrate that interacts with incoming light and creates, for example, optical resonances. By being longer and more focused, light can produce more harmonics more efficiently.

An interdisciplinary collaboration sees the research groups led by Prof. Cedric Meyer (Nanophotonics & Nanomaterials), Prof. Thomas Zentgraf (Ultrafast Nanophotonics) and Prof. Jens Forstner (Theoretical Electrical Engineering). Research Center/Transregio 142 to develop an innovative approach to generate high harmonics more efficiently. By using specific proportions of tiny elliptical cylinders made of silicon, they can take advantage of the Fano effect—a specific physical mechanism whereby multiple vibrations intensify each other.

The researchers initially used digital simulation to determine the ideal geometry parameters and explored the underlying physics. They then created the nanostructures using sophisticated lithography processes and conducted optical experiments. They were able to demonstrate through theory and experiment that this enables third harmonics—that is, light with three times the frequency of the incoming light—to be generated more efficiently than previously known structures.