We present a sidewall patterned shifted Bragg grating based on an add-drop filter in silicon-on-insulator platform with a coating of amorphous titanium dioxide. This particular waveguide grating is equivalent to two identical gratings written across either sides of a waveguide with a longitudinal offset of half of a period. The add-drop operation occurs on the basis of mode conversion due to shifted sidewall structure followed by mode splitting with asymmetric Y-junction. A signal launched through the wide arm (single mode) of an asymmetric Y-junction generates the fundamental mode at the stem of the Y-branch. First order mode is generated at the stem if the signal is launched through the narrow arm. Thus, an asymmetric Y-branch is used as a mode splitter fulfilling proper limiting condition for an adiabatic operation. A signal at the Bragg wavelength launched through the wide arm of asymmetric Y-junction generates fundamental mode at the stem. The fundamental mode converted to first order upon reflection from the shifted Bragg grating. The reflected mode couples into the narrow arm of the Y-junction. The bandwidth of the reflected signal depends on the grating strength. We used 80 nm grating amplitude for 800 nm wide waveguide. The height of the guiding layer is 220 nm. The TiO2 thickness is set to 180 nm. A reflection bandwidth of 2.2 nm with 14 dB extinction ratio is obtained at 1552.5 nm for 300 µm long grating. We further demonstrate the potential of TiO2 recoating with atomic layer deposition as a method of fine tuning the spectrum.
A polarization independent band-pass filter is created by combining a silicon cross-slot waveguide and a Bragg grating cavity. By theoretically investigating different types of cavities we show how the sensitivity to polarization of the device can vary, and how we can strongly confine light in a two-dimensional slot waveguide. This kind of structure, where a slot waveguide, a photonic crystal and a nanowire waveguide are merged together, may find applications in the field of sensing. Indeed, a slight variation in the surrounding refractive index breaks the device symmetry. One polarization can thus be used to monitor the fluctuation of the other one. We describe here the principle of a Bragg grating merged with a cross slot waveguide in which a cavity is placed. We discuss the advantage of using different geometries of cavity and how this choice may affect the response of the device.
We propose the Fourier Modal Method (FMM) as a convenient numerical tool for the design and analysis of nonlinear optical waveguides. The scope of this work includes the design of a polarization-independent nonlinear cross-slot waveguide for telecommunication applications at the wavelength of 1550 nm. The FMM method has been implemented, obeying the proper Fourier factorization rules, within a MATLAB<sup>TM</sup> environment. The influence of the modal field intensity on the transverse refractive index distribution due to the optical Kerr effect is modeled with FMM for a propagation invariant scheme of the waveguide. The waveguide is geometrically optimized for an enhanced nonlinear light matter interaction. A silicon-inorganic hybrid material platform based on hydrogenated amorphous silicon (a-Si:H) and amorphous titanium dioxide (TiO<sub>2</sub>) is considered for the mentioned waveguide. With the optimized design of the waveguide, the achieved value of the nonlinear waveguide parameter (γ) is 4.678 × 10<sup>4</sup> W<sup>-1</sup>Km<sup>-1</sup>.