Design, Simulation and Fabrication of Photonic Crystal Slab Waveguide Based Polarization Processors

Abstract

The Photonic Crystal (PC) is a potential candidate for a compact optical integrated circuit on a solid state platform. The fabrication process of a PC is compatible with CMOS technology; thus, it could be potentially employed in hybrid optical and electrical integrated circuits. One of the main obstacles in the implementation of an integrated optical circuit is the polarization dependence of wave propagation. Our goal is to overcome this obstacle by implementing PC based polarization controlling devices. One of the crucial elements of polarization controlling devices is the polarization rotator. The polarization rotator is utilized to manipulate and rotate the polarization of light. In this thesis, we have proposed, designed and implemented an ultra-compact passive PC based polarization rotator. Passive polarization rotator structures are mostly composed of geometrically asymmetric structures. The polarization rotator structure consists of a single defect line PC slab waveguide. The geometrical asymmetry has been introduced on top of the defect line as an asymmetric loaded layer. The top loaded layer is asymmetric with respect to the z-axis propagation direction. To synchronize the power conversion and avoid power conversion reversal, the top loaded layer is alternated around the z-axis periodically. The structure is called periodic asymmetric loaded PC slab waveguide. Due to the compactness of the proposed structure, a rigorous numerical method, 3D-FDTD can be employed to analyze and simulate the final designed structure. For the quick preliminary design, an analytical method that provides good approximate values of the structural parameters is preferred. Coupled-mode theory is a robust and well-known method for such analyses of perturbed waveguide structures. Thus, a coupled-mode theory based on semi-vectorial modes was developed for propagation modeling on square hole PC structures. In essence, we wish to develop a simple yet closed form method to carry out the initial design of the device of interest. In the next step, we refined the design by using rigorous but numerically expensive 3D-FDTD simulations. We believe this approach leads to optimization of the device parameters easily, if desired. To extend the design to a more general shape PC based polarization rotator, a design methodology based on hybrid modes of asymmetric loaded PC slab waveguide was introduced. The hybrid modes of the structure were calculated utilizing the 3D-FDTD method combined with the Spatial Fourier Transform (SFT). The propagation constants and profile of the slow and fast modes of an asymmetric loaded PC slab waveguide were extracted from the 3D-FDTD simulation results. The half-beat length, which is the length of each loaded layer, and total number of the loaded layers are calculated using the aforementioned data. This method provides the exact values of the polarization rotator structure’s parameter. The square hole PC based polarization rotator was designed employing both coupled-mode theory and normal modal analysis for THz frequency applications. Both design methods led to the same results. The design was verified by the 3D-FDTD simulation of the polarization rotator structure. For a square hole PC polarization rotator, a polarization conversion efficiency higher than 90% over the propagation distance of 12 λ was achieved within the frequency band of 586.4-604.5 GHz corresponding to the normalized frequency of 0.258-0.267. The design was extended to a circular hole PC based polarization rotator. A polarization conversion efficiency higher than 75% was achieved within the frequency band of 600-604.5 GHz. The circular hole PC polarization rotator is more compact than the square-hole PC structure. On the other hand, the circular hole PC polarization rotator is narrow band in comparison with the square hole PC polarization rotator. In a circular hole PC slab structure, the Bloch modes (fast and slow modes) couple energy to the TM-like PC slab modes. In both square and circular hole PC slab structures with finite number of rows, and the TM-like PC slab modes are extended to the lower edge of the bandgap. In bandgap calculation using PWEM, it is assumed that the PC structure is extended to infinity, however in practice the number of rows is limited, which is the source of discrepancy between the bandgap calculation using PWEM and 3D-FDTD. In an asymmetric loaded circular hole PC slab waveguide, the leaky TM-like PC slab modes are extended deep inside the bandgap and overlapped with both the slow and fast Bloch modes; whereas, in an asymmetric loaded square hole PC slab waveguide, the leaky TM-like PC slab modes are below the frequency band of slow and fast modes. Therefore, TM-like PC slab modes have significantly more adverse effect on the performance of the circular-hole based polarization rotator leading to a narrow band structure. SOI based PC membrane technology for THz application was developed. The device layer is made of highly resistive silicon to maintain low loss propagation for THz wave. The PC slab waveguide and polarization rotators were fabricated employing this technology. Finally, an a-SiON PC slab waveguide structures were also fabricated at low temperature for optical applications. This technology has the potential to be implemented on any substrate or CMOS chips.