Grating couplers are ideal for coupling into the tightly confined propagation modes of semiconductor waveguides. In addition, nonlinear optics has benefited from the sub-diffraction limit confinement of horizontal slot waveguides. By combining these two advancements, slot-based nonlinear optics with mode areas less than 0.02 μm2 can become as routine as twisting fiber connectors together. Surface normal fiber alignment to a chip is also highly desirable from time, cost, and manufacturing considerations. To meet these considerable design challenges, a custom genetic algorithm is created which, starting from purely random designs, creates a unique four stage grating coupler for two novel horizontal slot waveguide platforms. For horizontal multiple-slot waveguides filled with silicon nanocrystal, a theoretical fiber-towaveguide coupling efficiency of 68% is obtained. For thin silicon waveguides clad with optically active silicon nanocrystal, known as cover-slot waveguides, a theoretical fiber-to-waveguide coupling efficiency of 47% is obtained, and 1 dB and 3 dB theoretical bandwidths of 70 nm and 150 nm are obtained, respectively. Both waveguide platforms are fabricated from scratch, and their respective on-chip grating couplers are experimentally measured from a standard single mode fiber array that is mounted surface normally. The horizontal multiple-slot grating coupler achieved an experimental 60% coupling efficiency, and the horizontal cover-slot grating coupler achieved an experimental 38.7% coupling efficiency, with an extrapolated 1 dB bandwidth of 66 nm. This report demonstrates the promise of genetic algorithm-based design by reducing to practice the first large bandwidth vertical grating coupler to a novel silicon nanocrystal horizontal cover-slot waveguide.
We demonstrate efficient coupling of light into silicon nanomembrane (SiNM) based photonic devices using a
subwavelength grating (SWG) coupler. Our designed 17.1 μm x 10 μm grating couplers are fabricated at the input
and the output of 8 mm x 2.5 μm SiNM waveguides on a glass substrate. A high coupling efficiency of 39.2% is
achieved at a wavelength of 1555.56 nm. The 1-dB and 3-dB bandwidths are measured to be 29 nm and 57 nm,
respectively. Peak efficiency variation of 0.26 dB is observed from a measurement of 5 grating pairs. Such grating
couplers open limitless possibilities for the development of high performance nanomembrane based photonic
devices.
We have investigated the feasibility of multimode polysilicon waveguides to demonstrate the suitability of polysilicon as
a candidate for multilayer photonic applications. Solid Phase Crystallization (SPC) with a maximum temperature of
1000°C is used to create polysilicon on thermally grown SiO2. We then measure the propagation losses for various
waveguide widths on both polysilicon and crystalline silicon platforms. We find that as the width increases for
polysilicon waveguides, the propagation loss decreases similar to crystalline silicon waveguides. The difference in loss
between the two platforms for a given waveguide width is due to the scattering from the polysilicon grain boundaries,
which excites higher order modes. Depending on the waveguide width, these modes either propagate as higher order
modes or are lost as radiation modes. Due to their different propagation constants, the presence of higher order modes is
confirmed using sub-wavelength grating couplers. At a waveguide width of 10μm, polysilicon and crystalline silicon
waveguides have propagation losses of 0.56dB/cm and 0.31dB/cm, respectively, indicating there is little bulk absorption
from the polysilicon. This propagation loss is the lowest for polysilicon demonstrated to date. Modal conversion in
multimode waveguides by polysilicon grain boundary scattering are investigated using a sub-wavelength grating coupler
and discussed. These results vindicate the use of polysilicon waveguides of varying widths in photonic integrated circuits.
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