Integrated nanophotonic structures for mode conversion mode coupling and filtering
Creators: name: Abbaslou, Siamak
Publisher: name: No Publisher Supplied
Year: 2017

description: Silicon, which is the second element of group IV of the periodic table, forms the backbone of the semiconductor industry, similar to how carbon is the pillar of the living world. Silicon processing has gradually become a mature technology in electronics industry. Therefore, the most economically viable way of achieving optoelectronic integration is to adopt the complementary metal oxide semiconductor (CMOS) infrastructure and use silicon technology for optical devices applications. Silicon photonics furnishes a great opportunity for reducing energy consumption in communication, at the same time resolving interconnect bandwidth density, which is becoming the performance bottleneck in integrated electronics.     A variety of fascinating physical phenomena and practical devices can be implemented by manipulating light in the nanoscale, where the decrease in waveguide dimension to subwavelength scale and precise refractive index engineering enable new phenomena to emerge. However, the very first challenge of nanophotonic devices is coupling light in and out of a nanophotonic chip. Mode matched coupling devices are required to reduce the transmission losses between the optical fiber and the nanophotonic waveguides. We present a robust design for low loss, wide band grating coupler. The optimized fully etched two dimensional grating coupler can effectively couple the fundamental mode from a single mode silica optical fiber to a single mode silicon waveguide with 500-fold mode size difference. The device is fabricated through a single step E-beam lithography process on a silicon-on-insulator wafer. We characterize the mode converter device through the V-groove fiber array surface grating coupler measurement setup. The developed measurement setup and grating couplers serve as a platform to characterize optical and electro-optical integrated devices in our lab.    As a solution to different challenges, photonic crystals and grating structures comprise an important part of this thesis. Photonic crystal nanocavities can strongly confine photons in a small cavity on the wavelength scale. Exciting pure symmetrical modes in nanocavities is challenging. The second part of my thesis is dedicated to designing a high extinction ratio symmetrical-mode filter based on photonic crystal nanobeam cavities. Through carefully designing the cavity mode and the distributed bragg reflectors we have achieved high extinction ratio for the desired mode against the background transmission in the stop band region. The excitation of symmetrical modes with an even or odd symmetry is based on the design principles for a mode symmetry transforming Mach-Zender coupler. The designed device is fabricated and tested with the surface coupling setup.    Beam expanders or mode converters are widely employed in matching the modes of waveguides of different widths. Here we are interested in a beam expander that can effectively couple the fundamental mode from a narrow waveguide to that of a wide waveguide in a short distance. We find that as the taper length is shorter than the final waveguide width, the insertion loss increases almost exponentially. Designing a compact beam expander with low insertion loss is challenging. In the last part of this thesis, our goal is to design an ultra-compact on-chip mode size converter. This can be achieved through incorporating a composite adiabatic and non-adiabatic structure. We have utilized an evolutionary algorithm to design a beam expander. The fabricated device is characterized with our developed surface coupling setup.