Quasi-phase-matching has long been a widely used approach in nonlinear photonics. It enables efficient parametric frequency conversions such as second-harmonic generation.
However, in silicon photonics, the task remains challenging.
In a new study, scientists from Photonic Systems Laboratory have introduced second-order optical Nonlinearity into silicon nitride chips.
Camille Brès said, “When using a green laser pointer, for example, the laser itself is not green because these are particularly difficult to manufacture. So we change the frequency of an existing laser. It emits at a frequency half that of green; then, we double it by using Nonlinearity in a crystal that gives us green. Our study integrates this functionality but on chips that can be manufactured with standard techniques developed for electronics (CMOS). Thanks to this, we will be able to generate different colors of light on a chip efficiently.”
Existing photonic chips compatible with CMOS processes use standard photonic materials. These materials do not possess second-order Nonlinearity and therefore are not inherently capable of transforming light in this way. This becomes a barrier to the advancement of technology.
Nonlinearity is used to convert light where it is not usually possible. Scientists developed a technique to induce Nonlinearity. They also used a resonator- a ring-shaped structure that magnifies the nonlinear processes experienced by light. Silicon nitride resonators, the technology of which was established at EPFL and now commercialized by Ligentec SA, exhibit very low losses so that light circulates in resonators for a very long time.
Edgars Nitiss, Ph.D. and co-first author, said, “Nonlinearity comes from the interaction between light and matter. This exchange must be long if the process is functional and efficient. However, the chip is a small object we do not benefit from long distances. The light introduced into the resonator is captured and travels the time necessary for the nonlinear interaction to be increased.”
Camille Brès said, “Thanks to this technique, the chip’s efficiency is significantly improved. But a new constraint is imposed. When using a resonator, we are limited in terms of the colors available.”
Jianqi Hu, Ph.D., and co-first author said, “Indeed, the effectiveness of a nonlinear effect also depends on the phase agreement between the different interacting colors, whereas they inevitably have different propagation speeds. Just like two cars on the highway. We want the one in the fast lane to slow down while the other accelerates so that they can roll next to each other and thus interact.”
Usually, this is achieved in very constrained cases in a resonator. To avoid these constraints, scientists found a solution that simultaneously offered access to a range of several colors despite using the resonator.
In the resonator, light waves propagate, producing a coherent interaction that changes the material’s properties. Self-organization of the structure is achieved in a completely all-optical manner, which automatically compensates for the phase mismatch regardless of the input color.
Scientists noted, “As such, we circumvent critical limitations of resonators while still benefiting from their strong efficiency enhancement.”
- Nitiss, E., Hu, J., Stroganov, A. et al. Optically reconfigurable quasi-phase-matching in silicon nitride microresonators. Nat. Photon. (2022). DOI: 10.1038/s41566-021-00925-5