Phase-matched Metamaterials for Second-harmonic Generation

Abstract

Metamaterials exhibit unconventional electromagnetic properties that cannot be found in nature, such as negative index of refraction or strong optical activity. Moreover, they show promise for enabling nanoscale nonlinear optics. Current nonlinear optical interactions of practical use rely on phase matching combined with long propagation lengths, which are not compatible with the size requirements of miniaturized systems. In order to be able to improve the realizable conversion efficiencies of nonlinear processes and discover novel functionalities at the nanoscale, new kinds of nonlinear metamaterials need to be investigated.

By utilizing local-field enhancements and the phase engineering of localized surface plasmon resonances, it is possible to construct metamaterials which generate nonlinear frequencies into the direction where the fundamental light came from. In this Thesis, we demonstrate how phase matching is achieved in nanoscale nonlinear materials. Especially, we fabricate three-dimensional plasmonic metamaterial devices that were phase matched for back-propagating second harmonic-generation. Our samples consist of one to five metasurfaces stacked on top of each other and the aim was to observe how the intensity of the second-harmonic field varies with the number of metasurfaces stacked in a backward phase-matched metamaterial.

The results show that the second harmonic signal depends quadratically on the number of metasurfaces, which confirms that the sample was successfully phase-matched by controlling the dimensions of the nanoparticles and the separation between the metasurfaces. This provides insight into how the performance of nonlinear metamaterials can be increased considerably by stacking metasurfaces inside of a three-dimensional metamaterial. Furthermore, the measurements provided experimental confirmation for backward phase matching, where the fundamental and second-harmonic waves were counter-propagating. These results demonstrate a completely novel principle for improving the conversion efficiencies of nanoscale nonlinear materials.