Engineering Waveguide Nonlinear Effective Length via Low Index Thin Films
In nonlinear optics, we always have a trade-off present, where a stronger desired nonlinear response, e.g. a phase shift is also accompanied by one or more undesired nonlinear effects, such as additional absorption. Therefore we typically use a figure of merit that represents the ratio of the desired to undesired nonlinear response. In our case, we desire a strong phase shift and the undesired response is a reduction of the optical transmission.
This simple trade-off neglects a major factor, namely that nonlinear optics typically takes place over some finite propagation distance, which affects the maximum effect that can be achieved. Therefore, in this project, we define a new figure of merit for nonlinear optics that also includes these propagation effects. Using this figure, it becomes clear that there is a zone in the regions of 1 μm to 1 mm propagation length where none of the current nonlinear platforms, such as silicon, silicon nitride, etc., perform well. However, this is the propagation length typically used for photonic integrated circuits, identifying a need for a new, integrated photonics, nonlinear optical platform.
We present this platform in the same paper (now out in Advanced Optical Materials), consisting of silicon waveguides loaded with Indium Tin oxide (ITO), operated near the epsilon-near-zero (ENZ) point. ITO is known to be a highly nonlinear medium in this spectral region, but typically the transmission is too low. Here we reduce the transmission losses while maintaining the nonlinear properties through a spacing layer between the silicon waveguide core and ITO strip, allowing us to engineer the nonlinear strength of these devices.
The resulting platform shows the strongest nonlinear operation across the desired propagation length and represents a new paradigm for integrated nonlinear optics.
This work is part of a collaboration with the Synthetic Optics group here in St Andrews and the Advanced Structured Nanophotonics group at Heriot-Watt.