The band gap characteristics of one-dimensional and two-dimensional photonic crystals made of uniaxial anisotropic materials were analyzed with a focus on the band gap characteristics as a function of optical axis orientation in the aniostropic material. For one-dimensional case, with optical axis normal to periodicity axis, the two polarization of on axis light will experience different refractive indexes and thus the degeneracy in photonic band will disappear. Theoretically we show that in some nonlinear materials, with presence of certain symmetry, the band lines correspond to two polarizations will degenerate under a high electric field. It is also shown that the gap position and size varies as the position of the optical axis varies and the range is limited by the birefringence of the anisotropic material. In two dimensional photonic crystal, we showed that, changing the position of optical axis in the propagation plane is simply change of symmetry in photonic band structure. If the position of the optical axis is varied in the transversal direction, we can open or close the band gap. The characteristic of anisotropic material, the direction dependant refractive index can be used to improve the band structure of conventional isotropic photonic crystal.
We have developed a deep ultraviolet (DUV) lithography technique for fabricating super dense silicon based photonic crystals. Binary mask is used to create nano scale patterns of very high density. Based on the simulation, photonic crystals with both square and triangular lattice of air cylinders are designed and fabricated to work in communication frequency range (λ within 1.3 to 1.55μm) on amorphous silicon. In order to pattern circular hole we designed different kind of polygons on the mask and layout pattern was under sized at constant pitch. Bottom anti reflection coating (BARC) recipe was developed to improve circularity of the pattern and reduce interhole spacing.
To miniaturize optical passive components or to have optical interconnects replace the current copper/low k interconnects for clock distribution, super high index contrast optics are needed because they allow optical waveguides with small bending radius, ie. < 50um. Silicon nitride core on oxide cladding has loss of <0.1dB/180° for 20um bending radius. However, coupling loss from the fiber to SiN waveguides, with 0.7umx0.7um cross section for single mode, is very large, > 20dB. To reduce the coupling loss, our approach is to have a double-core architecture, where fiber is first coupled to fiber matched waveguide, and then coupling from fiber match waveguide to SiN waveguide through a spot size mode converter. We have found the mode converter loss is reduced by 8dB by reducing the tip of the taper from 0.35um to 0.15um. In this paper, we are reported results of tips with less than 0.1um. We also describe the fabrication technology that enables us to make such fine tip with smooth surfaces.