Enhanced axial confinement in a monolithically integrated self-rolled-up SiN_x vertical microring photonic coupler

Journal Article · Wed Sep 14 00:00:00 EDT 2016 · Applied Physics Letters

DOI:https://doi.org/10.1063/1.4962901· OSTI ID:1468930

Yu, Xin ^[1]; Goddard, Lynford L. ^[1]; Li, Xiuling ^[1]; Chen, Xiaogang  ^[1]

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1. Univ. of Illinois, Urbana, IL (United States). Dept. of Electrical and Computer Engineering, Micro and Nanotechnology Lab.

We report an efficient method to introduce enhanced axial confinement in the self-rolled-up SiN_x vertical microtube coupler by depositing a thin layer of high refractive index material strip within the coupling section and effectively forming a vertical microring. Three times wider mode spacing is observed in such a vertical microring coupler monolithically integrated with a silicon nitride ridge waveguide as compared to the one without such axial confinement. More importantly, single mode operation within the telecomm C-band and S-band is achieved. The self-rolled-up microtube has been demonstrated to be a versatile device platform, which has found application in a number of areas including nanophotonics,1–7 microelectronics,8 chemical and biological sensing,9–11 and intelligent synthetic neural circuits.12 Because the microtube is a three dimensional (3D) structure readily fabricated using two dimensional processing technology, it may serve naturally as a component in 3D-integrated devices. We had previously demonstrated a vertical microtube coupler (VμTC) monolithically integrated with planar ridge waveguides in silicon photonic platform to achieve 3D photonic coupling and observed greatly enhanced coupling between the microtube and the underlying ridge waveguide.1,13 A similar result was reported independently by another research group shortly thereafter.2 One limitation of the VμTC used in previous experiments is that it supports many modes within the telecommunication C-band because of poor axial confinement. For example, we found multiple resonant coupling peaks, which is not desirable for wavelength division multiplexing (WDM) switches. In order to increase the mode spacing of this 3D photonic coupler and to achieve single mode operation within the C-band, we propose efficient axial confinement in the microtube to suppress the higher order modes. Axial confinement in self-rolled-up microtubes had been studied both theoretically and experimentally by a couple of research groups.14–17 In all these previous cases, the axial confinement was introduced by a specially designed geometrical shape of a section of the microtube. After being rolled up, the microtube wall will have different thicknesses at different axial locations. The different wall thicknesses result in a slightly varying effective refractive index of the supported whispering gallery modes (WGMs) along the axial direction of the microtube. When a parabolic shape is used, the effective index of the WGM at the center of the parabola will be the highest and decrease adiabatically along both sides. However, the refractive index change due to different wall thicknesses is very small; hence, the axial confinement is rather weak. Here in this paper, we present our experimental endeavor to incorporate a different axial confinement scheme in the monolithically integrated vertical microtube (VμT) to form a vertical microring (VμR) resonator and demonstrate 3 times wider mode spacing than the microtube based vertical photonic coupler and the potential to achieve a higher Q-factor.

• Research Organization: Univ. of Illinois, Urbana, IL (United States). Dept. of Electrical and Computer Engineering, Micro and Nanotechnology Lab.
• Sponsoring Organization: USDOE
• Grant/Contract Number: FG02-07ER46471
• OSTI ID: 1468930
• Alternate ID(s): OSTI ID: 1324485
• Journal Information: Applied Physics Letters, Vol. 109, Issue 11; ISSN 0003-6951
• Publisher: American Institute of Physics (AIP)
• Country of Publication: United States
• Language: English

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