Modal analysis of silicon nanostructured waveguide with holey cladding in 2-D isosceles triangular lattice

Silicon photonics, either in the form of integrated optical chips or fiber, has attracted much interest due to their small foot-print, high refractive-index, high thermal conductivity, high non-linear-optical coefficient, and compatibility with CMOS and fiber-drawing process technology. Recently, fabrication of well-arranged deeply etched nanometric holes on silicon has also been reported. This structure is potential for a.o. sensing applications due to the wide surface provided by the nano-holes. In this work, we report a modal analysis on a nanostructured silicon waveguide with holey cladding in 2-D isosceles lattice running parallel with the propagation axis. For this purpose, we used our finite-element method leaky mode solver. The diameter of the holes is 326 nm, and the lattice parameters are a = 680 nm and c = 481 nm. Computations were carried out for wavelengths λ between 1.1-1.55 μm. Although an inverse woodpile bulk photonic crystal with similar lattice parameters (which is the 3-D form of the isosceles triangular lattice used in the cladding of our structure) was reported to exhibit bandgap around λ=1.21 μm, the waveguide under study shows no effect of such bandgap as this photonic-crystal-fiber-like waveguide relies on the leaky defect-resonance instead of photonic bandgap as its waveguiding mechanism. Although the hole’s dimension is considerably smaller than the wavelength, the waveguide still shows extremely low confinement loss due to the high index contrast provided by the silicon-air interface. This fact implies that the dimension of the waveguide is still far from waveguiding limit, and the total losses are expected to be limited by the imperfection of the fabrication technology. At around λ=1.375 μm, we observed anti-crossing between q-HE21 and q-TM01 modes. We anticipate that the abrupt change of modal characteristics around this point might be useful for sensing applications. Such change does not only occur in the attenuation, but also in the group index. Additionally, we also observed the non-degeneracy of the two q-HE11 modes as results of symmetry breaking of the structure.