Third-­harmonic generation of ultraviolet light in silicon nitride metasurfaces

Ultraviolet (UV) light is of significant interest for a variety of applications in science and technology. However, it still comprises a challenge to generate UV radiation in a cheap and compact manner. One of the recently proposed solutions suggests the use of nonlinear optical effects, such as second­ and third­harmonic generation (THG), in thin nanostructured dielectric films. The yield of these phenomena in thin films can be greatly improved by imposing a pattern and making use of the resulting lattice resonances. This work exploits the third­harmonic generation of UV light in nanostructured silicon nitride (SiNx) membranes. By means of numerical analysis, it is shown that a patterned metasurface generates a THG signal by up to 5­6 orders of magnitude more efficiently than a flat unpatterned film of the same thickness. The exact enhancement factor is determined by the time duration of an excitation pulse and the exact employed resonance of the structure. Furthermore, the present work considers the nanofabrication aspect of the problem. Thus, the manufacturing methodology of such SiNx membranes is rendered with regard to a specific environment of the DTU Nanolab cleanroom. Special attention is paid to the nanofabrication challenges that arise and the measures taken in an attempt to resolve them. The structures fabricated during this project could be used in several further planned works involving, for example, z­scan and sensing experiments. Overall, the conducted study, which exploits the phenomenon of THG in thin dielectric films, contributes to the development of compact and effective UV light sources.