Appl. Phys. Lett. 104, 091108 (2014)

Aluminium nitride (AlN) offers a broad electronic bandgap capable of servicing wavelength regions that lie beyond the reach of silicon-on-insulator technology. Matthias Stegmaier and co-workers at Karlsruhe Institute of Technology in Germany have now demonstrated waveguiding in AlN structures as narrow as 40 nm. The rib waveguides were made from 140-nm-thick polycrystalline AlN films sputtered on thermally oxidized silicon substrates with a 2.0-μm-thick oxide layer, which reduces leakage into the silicon substrate. Light with a wavelength of 400 nm was guided along the waveguides. The width of the waveguides is smaller than the diffraction limit (∼90 nm) for light in the core material with n ≈ 2.2. However, it should be noted that this is diffraction-limited guiding, and the optical mode width actually increases with decreasing waveguide width (and effective index) in this regime. When the waveguide becomes too narrow (∼30 nm in this case), the mode index decreases to that of SiO2, allowing light to leak (that is, the core mode is cut-off). The researchers noted that waveguides 170 nm or wider are preferable. Even though multiple modes are supported, surface roughness and propagation loss can be significantly improved.