New technology amplifies high harmonic generation in nanostructured metasurfaces

(Nanowerk News) Both natural and synthetic crystals possess an intriguing ability to modify the spectral hue of light through a phenomenon termed as the nonlinear optical effect. This colour conversion is instrumental in a myriad of applications such as nonlinear microscopy for the inspection of biological structures and materials, the operation of LED light sources and lasers, optical communication systems, and a plethora of technologies in the photonics domain, including quantum computing.
A team of researchers from Paderborn University has recently made strides in refining the physical process that underlies this intriguing phenomenon. Their discoveries have been disclosed in the scientific journal Light: Science & Applications ("A multi-mode super-fano mechanism for enhanced third harmonic generation in silicon metasurfaces").
Professor Cedrik Meier, a physicist at Paderborn, elaborates, "The mechanism is founded on the anharmonic potential of crystal atoms, frequently leading to an exact multiplication of light frequency, a process referred to as generating 'higher harmonics' – analogous to the overtones produced when a string of a musical instrument vibrates."
This effect, although naturally occurring in many crystals, is often markedly weak. This has triggered numerous attempts to amplify the effect, such as by ingeniously amalgamating diverse materials and their micro and nano structures. Over the past decades, Paderborn University has conducted extensive and successful research in this domain.
The exploration of photonics has been particularly focused on metamaterials, and more specifically, metasurfaces. This approach involves structuring elements in the nanometre range onto a thin substrate, which subsequently interacts with the incoming light to generate optical resonances, among other things. By lengthening the duration and augmenting the focus of light, the generation of higher harmonics can be made more efficient.
This interdisciplinary endeavour comprises research groups led by Professor Cedrik Meier (Nanophotonics & Nanomaterials), Professor Thomas Zentgraf (Ultrafast Nanophotonics) and Professor Jens Förstner (Theoretical Electrical Engineering) at Paderborn University, who have collaboratively been working as part of the 'Tailored Nonlinear Photonics' Collaborative Research Center/Transregio 142. Their innovative strategy to enhance the generation of higher harmonics involves using microscopically small elliptical silicon cylinders, capitalizing on the 'Fano' effect—a unique physical mechanism where multiple resonances reinforce each other.
The researchers employed digital simulation to identify the optimal geometric parameters and probe the underpinning physics. They created nanostructures using cutting-edge lithography processes and conducted optical examinations. The results conclusively demonstrated that their approach allows the generation of third harmonics—light with triple the frequency of the incoming light—more efficiently than any previously known structures, thereby validating their theory through experimentation.
Source: University of Paderborn (Note: Content may be edited for style and length)
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