In this work, a coupling strategy between mid-index SiNx and high-index active waveguides on the same silicon chip is proposed. To that aim, a sophisticated proof-of-concept integration between N-rich SiN and SOI micrometric waveguides is demonstrated achieving a <0.5 dB coupling for both TE/TM polarisations. The optical tunability of SiNx allows the mitigation of the mid-high refractive index discrepancy by interposing a SiO2/Si-rich SiN double-layer anti-reflective coating, attaining back-reflections close to −20 dB. On that basis, it is shown numerically that a sub-dB interconnection between multiple-quantum well/dot stratified stacks and a silicon nitride passive waveguide is achievable, while keeping the introduced back-reflection level below −30 dB.
Silicon nitride (SiNx), has been widely regarded as a CMOS photonics enabling material, facilitating the development of low-cost CMOS compatible waveguides and related photonic components. We have previously developed an NH3-free SiN PECVD platform in which its optical properties can be tailored. Here, we report on a new type of surface-emitting nitrogen-rich silicon nitride waveguide with antenna lengths of L < 5 mm. This is achieved by using a technique called small spot direct ultraviolet writing, capable of creating periodic refractive index changes ranging from -0.01 to -0.04. With this arrangement, a weak antenna radiation strength can be achieved, resulting in far-field beam widths < 0.0150, while maintaining a minimum feature size equal to 300 nm, which is compatible with DUV scanner lithography.
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