Coherent Tunneling by Adiabatic Passage in Silicon Nitride based Integrated Waveguide Structures

TL;DR

The study addresses robust, compact light routing in silicon nitride photonic integrated circuits via coherent tunneling by adiabatic passage (CTAP) at telecom wavelengths ($1.55~\mu m$). It develops a three-waveguide Si$_3$N$_4$ CTAP scheme and analyzes both high- and low-confinement regimes using bidirectional eigenmode propagation, identifying optimal geometries and the trade-offs between fabrication feasibility and coupling length. The work also proposes a hybrid CTAP design with a central silicon waveguide to suppress central excitation and mitigate two-photon absorption, expanding control over light routing. The findings offer practical design guidelines for robust interconnects in PICs and suggest avenues for dynamic control in photonic molecules and microring-based nonlinear devices.

Abstract

Nowadays silicon nitride photonic integrated circuits serve as a mature platform for numerous applications. Planar waveguides and directional couplers made by CMOS-compatible technology are its basic elements. Here we demonstrate the possibilities of efficient light routing and transfer provided by the integrated planar Si3N4 waveguides structure based on the coherent tunneling by adiabatic passage (CTAP) at the 1.55um telecom band. We addressed both high- and low-confinement silicon nitride CTAP structures and proved high efficiency of light routing in them. The mechanisms that limit the light control efficiency have been revealed. The accessible parameters of such structures have been determined. Besides that, there was proposed the original hybrid Si3N4 Si - Si3N4 waveguides structure providing the enhanced efficiency and flexibility of the CTAP in comparison with the single-material Si3N4 waveguide structures.

Figures (9)

• Figure 1: Scheme of the addressed CTAP structure. Gray areas refer to the fused silica substrate, and blue color denote the silicon nitride waveguides. The inset shows the structure cross section. Horizontal and vertical scales are different.
• Figure 2: Intensity $(|E|^2)$ distribution in the silicon nitride waveguides for $p$-polarized input waveguide mode at the wavelength of $1.55\mu m$. White dotted lines indicate the contours of Si$_3$N$_4$ waveguides. Horizontal and vertical scales are different. White arrows denote the light propagation direction. The inset shows the light intensity distribution in the structure cross section according to the white dashed line. The colormap in the inset is the same as in the main plot.
• Figure 3: Intensity $(|E|^2)$ distribution in the cross-section of the silicon nitride waveguides with the width $w=0.55 \mu m$ and height a) $h=1.0 \mu m$, b) $h=0.55 \mu m$ and c) $h=0.3 \mu m$ at $x=57.6 \mu m$ (corresponds to the cross-section in the Fig. \ref{['fig:OptParam']}) for $p$-polarized input waveguide mode at the wavelength of $1.55\mu m$. Black lines indicate the contours of Si$_3$N$_4$ waveguides in the fused silica.
• Figure 4: Dependence of minimal gap, $d$, versus the thickness of the waveguides, $h$ providing the maximum efficiency of the CTAP for different values of waveguide width, $w$. The dependence was obtained for $p$-polarized input waveguide mode at the wavelength of $1.55\mu m$.
• Figure 5: Intensity $|E|^2$ at the center of the middle waveguide ($y=0~\mu m$, $z=0~\mu m$) versus the longitudinal coordinate $x$ in the low-confinement (solid red line) and high-confinement (dashed blue line) silicon nitride waveguides for $p$-polarized input waveguide mode at the wavelength of $1.55~\mu m$. Other parameters are $w=0.55~\mu m$, $d=1.38~\mu m$, and $R=2.5~mm$.