SiN is a popular material for use throughout the silicon-on-insulator platform. It is also a promising material for the fabrication of waveguide and resonators, owing to its low loss at visible and near-infrared wavelengths and compatibility with complementary metal–oxide–semiconductor processing. Generally, SiN is thought to be an amorphous, centrosymmetric material with no second-order harmonic bulk nonlinear response. However, Tingyin Ning and co-workers from the Tampere University of Technology in Finland have now observed strong second-harmonic generation from SiN films. They began by using plasma-enhanced chemically vapour deposition to produce an 800-nm-thick SiN film on a fused silica substrate. They then performed polarization-dependent measurements for various angles of incidence with light from a mode-locked YAG laser operating at the wavelength of 1,064 nm. Changing the polarization direction of the YAG laser light (and therefore that of the resulting second-harmonic wave) provided a measure of the second-harmonic generation intensity, which suggests that the SiN film exhibited in-plane isotropy. By using a simplified version of the Green's function formalism of nonlinear optics, the researchers obtained the non-vanishing tensor components of the second-order susceptibility from their experimental data. They then determined the absolute values of the tensor components by comparing the second-harmonic generation intensity to that through a Y-cut quartz crystal. The dominant component exhibited a magnitude of 2.5 pm V−1 — almost 60 times higher than that that of Si3N4, and three times larger than that of potassium dihydrogen phosphate.
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Horiuchi, N. Silicon nitride success. Nature Photon 6, 412 (2012). https://doi.org/10.1038/nphoton.2012.169
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DOI: https://doi.org/10.1038/nphoton.2012.169