The new technology strengthens the generation of high harmonics in nanostructured metasurfaces

(Nanowerk News) Both natural and synthetic crystals have the interesting ability to modify the spectral hue of light through a phenomenon known as the nonlinear optical effect. This color conversion is critical in a variety of applications such as nonlinear microscopy for the examination of biological structures and materials, operation of LED light sources and lasers, optical communication systems, and a large number of technologies in the photonics domain, including quantum computing.

A team of researchers from the University of Paderborn have recently taken steps in refining the physical processes that underlie this intriguing phenomenon. Their discoveries have been disclosed in scientific journals Light: Science & Applications (“Multi-mode super-fano mechanism to enhance third harmonic generation in silicon metasurfaces”).

Professor Cedrik Meier, a physicist at Paderborn, explains, “This mechanism is based on the anharmonic potentials of crystal atoms, which often lead to a doubling of the precise frequency of light, a process referred to as producing ‘higher harmonics’ – analogous to the resulting overtones when the strings of a musical instrument vibrate.”

This effect, although it occurs naturally in many crystals, is often very weak. This has sparked many attempts to amplify its effects, such as by cleverly combining various materials and their micro- and nano-structures. Over the last decades, the University of Paderborn has conducted extensive and successful research in this area.

Photonic exploration is specifically focused on metamaterials, and more specifically, metasurfaces. This approach involves structuring elements in the nanometer range onto a thin substrate, which then interacts with incoming light to generate optical resonances, among other things. By extending the duration and increasing the focus of the light, the generation of higher harmonics can be made more efficient.

This interdisciplinary effort comprises a research group led by Professor Cedrik Meier (Nanophotonics & Nanomaterials), Professor Thomas Zentgraf (Ultrafast Nanophotonics) and Professor Jens Förstner (Theoretical Electrical Engineering) at the University of Paderborn, who have worked together as part of the ‘Adapted Nonlinear Photonics ‘ Collaborative Research Center/Transregio 142. Their innovative strategy for enhancing the generation of higher harmonics involves the use of microscopically tiny elliptical silicon cylinders, making use of the ‘Fano’ effect—a unique physical mechanism in which multiple resonances amplify one another.

The researchers used digital simulations to identify optimal geometric parameters and investigate the underlying physics. They created the nanostructures using a state-of-the-art lithography process and carried out an optical inspection. The results conclusively showed that their approach allowed the generation of third harmonics — light with a frequency three times that of the incident light — more efficiently than previously known structures, validating their theory through experimentation.