The fin-shaped optical waveguide: for integrated photonics, this makes perfect sense
Fin waveguide is physically connected to the substrate -- a step forward for large-scale co-integration with electronics.
|FIGURE. An example of the fin waveguide in diamond-based integrated photonics shows the physical connection of the light-carrying portion of the diamond waveguide with the diamond substrate (fin proportions are not to scale). The waveguide by itself in air would be too thin to carry the fundamental mode, but with the addition of a Si3N4 confinement layer to its upper portion, the upper part of the waveguide can channel light. A low-index SiO2 buffer layer spanning most of the lower portion of the waveguide supports the waveguide but eliminates any light-carrying modes in the lower portion. (Drawing: John Wallace)|
The ability of an integrated-photonics optical waveguide to confine light is ordinarily created by building the waveguide atop a continuous layer of low-refractive-index material, allowing the waveguide to channel light via total internal reflection. For example, in silicon photonics, a continuous layer of silicon dioxide (SiO2) is ordinarily placed underneath the network of silicon waveguides to allow them to function.
However, in silicon photonics, the fact that the low-index layer is continuous creates problems for the integration of photonics with electronics. In other types of integrated photonics, such as those based on III-V semiconductors, silicon carbide (SiC), or diamond, the required buried layer of low-index material weakens the structure and complicates fabrication.
Strength and simplicity
To solve this problem, Richard Grote and Lee Bassett, researchers at the Quantum Engineering Laboratory, Department of Electrical and Systems Engineering, University of Pennsylvania (Philadelphia, PA), have come up with a waveguide geometry that simultaneously allows confinement of light and a physical connection with the substrate of the same material (see figure).1
The fin is spanned by a low-index material in its lower portion and a higher-index material at its top, creating an effective index high-enough to carry light at the top, but not in the lower portions. And the all-important physical connection to the substrate is maintained.
For silicon photonics, the structure is compatible with CMOS-compatible co-integration of the silicon photonic components with VLSI (very large scale integration) electronics such as those used in computer chips. For other types of integrated photonics, the arrangement provides strength and simplicity.
1. Richard R. Grote and Lee C. Bassett, arXiv:1601.01239v1 [physics.optics] 6 January, 2016.