Complete photonic bandgaps in 12-fold symmetric quasicrystals

Photonic crystals are attracting current interest for a variety of reasons, such as their ability to inhibit the spontaneous emission of light. This and related properties arise from the formation of photonic bandgaps, whereby multiple scattering of photons by lattices of periodically varying refractive indices acts to prevent the propagation of electromagnetic waves having certain wavelengths. One route to forming photonic crystals is to etch two-dimensional periodic lattices of vertical air holes into dielectric slab waveguides. Such structures can show complete photonic bandgaps, but only for large-diameter air holes in materials of high refractive index (such as gallium arsenide, n = 3.69), which unfortunately leads to significantly reduced optical transmission when combined with optical fibres of low refractive index. It has been suggested that quasicrystalline (rather than periodic) lattices can also possess photonic bandgaps. Here we demonstrate this concept experimentally and show that it enables complete photonic bandgaps—non-directional and for any polarization—to be realized with small air holes in silicon nitride (n = 2.02), and even glass (n = 1.45). These properties make photonic quasicrystals promising for application in a range of optical devices

Photonic crystal and quasi-crystals providing simultaneous light coupling and beam splitting within a low refractive-index slab waveguide

© 2017 The Author(s). Coupling between free space components and slab waveguides is a common requirement for integrated optical devices, and is typically achieved by end-fire or grating coupling. Power splitting and distribution requires additional components. Usually grating couplers are used in combination with MMI/Y-splitters to do this task. In this paper, we present a photonic crystal device which performs both tasks simultaneously and is able to couple light at normal incidence and near normal incidence. Our approach is scalable to large channel counts with little impact on device footprint. We demonstrate in normal incidence coupling with multi-channel splitting for 785 nm light. Photonic crystals are etched into single mode low refractive index SiON film on both SiO2/Si and borosilicate glass substrate. Triangular lattices are shown to provide coupling to 6 beams with equal included angle (60°), while a quasi-crystal lattice with 12-fold rotational symmetry yields coupling to 12 beams with equal included angle (30°). We show how to optimize the lattice constant to achieve efficient phase matching between incident and coupled mode wave vectors, and how to adjust operating wavelength from visible to infrared wavelengths