Abstract
Silicon nitride waveguides provide low propagation loss but weak mode confinement due to the relatively small refractive index contrast between the Si₃N₄ core and the SiO2 cladding. On the other hand, metal-insulator-metal (MIM) plasmonic waveguides offer strong mode confinement but large propagation loss. In this work, MIM-like plasmonic waveguides and passive devices based on horizontal Cu-Si₃N₄-Cu or Cu-SiO₂-Si₃N₄-SiO₂-Cu structures are integrated in the conventional Si₃N₄ waveguide circuits using standard CMOS backend processes, and are characterized around 1550-nm telecom wavelengths using the conventional fiber-waveguide-fiber method. The Cu-Si₃N₄(~100 nm)-Cu devices exhibit ~0.78-dB/μm propagation loss for straight waveguides, ~38% coupling efficiency with the conventional 1-μm-wide Si₃N₄ waveguide through a 2-μm-long taper coupler, ~0.2-dB bending loss for sharp 90° bends, and ~0.1-dB excess loss for ultracompact 1 × 2 and 1 × 4 power splitters. Inserting a ~10-nm SiO₂ layer between the Si3N4 core and the Cu cover (i.e., the Cu-SiO2(~10 nm)-Si₃N₄(~100 nm)-SiO2(~10 nm)-Cu devices), the propagation loss and the coupling efficiency are improved to ~0.37 dB/μm and ~52% while the bending loss and the excess loss are degraded to ~3.2 dB and ~2.1 dB, respectively. These experimental results are roughly consistent with the numerical simulation results after taking the influence of possible imperfect fabrication into account. Ultracompact plasmonic ring resonators with 1-μm radius are demonstrated with an extinction ratio of ~18 dB and a quality factor of ~84, close to the theoretical prediction.
Highlights
Silicon nitride (Si3N4) is transparent in both the visible and infrared spectrum and can be deposited on almost any substrate using mature technologies such as low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD), it has become one of the promising materials for integrated photonics applications, especially for three-dimensional (3D) integration of multiple photonics above the processed microelectronics [1, 2]
The electrical field (|Ex|) distribution of the fundamental 1550-nm TE mode is depicted in Fig. 3(b), calculated using the eigen-mode expansion (EME) method [22]
The propagation loss measured by the standard cutback method is plotted in Fig. 3(c) as a function of wavelength ranging from visible to infrared, using different laser sources which are available in our laboratory
Summary
Silicon nitride (Si3N4) is transparent in both the visible and infrared spectrum and can be deposited on almost any substrate using mature technologies such as low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD), it has become one of the promising materials for integrated photonics applications, especially for three-dimensional (3D) integration of multiple photonics above the processed microelectronics [1, 2]. Propagation loss as low as 0.1 dB/cm and intrinsic quality factor of ring resonators as high as 3×106 have been demonstrated on the Si3N4 platform [3]. Active functions such as parametric amplification [4] and broadband supercontiniuum generation [5] have been realized using the nonlinear properties of Si3N4. In silicon-oninsulator (SOI) platform, CMOS-compatible photonic and plasmonic integrated circuits have been demonstrated based on horizontal Cu-insulator-Si-insulator-Cu nanoplasmonic waveguides [9,10,11] or vertical Cu-insulator-Si hybrid plasmonic waveguides [12]. The horizontal MIM-like waveguides developed in this paper provide an alternative way to realize various proposed MIM plasmonic devices cost-effectively
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