An optical chip improved by light

 

Miniaturization and energy efficiency are becoming increasingly important in technology. This holds true for electronic chips as well. Light, and optics in general, play a role in the development of small and portable chips. Professor Camille Brès' Photonic Systems Laboratory has effectively implemented a revolutionary principle for introducing second-order optical nonlinearity onto silicon nitride chips. The discovery was first published in the journal Nature Photonics.

Different colors of light

"When utilising a green laser pointer, for example, the laser is not green because these are extremely difficult to produce." As a result, we alter the frequency of a laser that already exists. It emits at half the frequency of green, which we double by utilising nonlinearity in a crystal to produce green. Our research focuses on putting this capabilities into chips that can be produced using regular electronics manufacturing procedures (CMOS). "We will be able to efficiently generate multiple colours of light on a chip as a result of this," Camille Brès explains. The approach that was demonstrated had never been used before. Standard photonic materials, such as silicon, do not have second-order nonlinearity and so are not intrinsically capable of converting light in this way in current photonic circuits compatible with CMOS processes. "This turns out to be a roadblock to technological growth," the professor continues.

An amplifier ring

Scientists from the School of Engineering have devised a method for inducing nonlinearity, which can be used to convert light in situations where it would otherwise be impossible. They also employed a resonator—a ring-shaped structure that intensifies the nonlinear processes experienced by light—to make this conversion efficient. Silicon nitride resonators, whose technology was developed at EPFL and is now commercialised by Ligentec SA, have very low losses, allowing light to circulate through them for an extended period of time. "The interaction of light and matter causes non-linearity. If the procedure is to be functional and efficient, this interchange must be lengthy. The chip, on the other hand, is a small thing that does not benefit from long distances "Edgars Nitiss, Ph.D., and co-first author, explains. The light that enters the resonator is caught and travels the distance required to increase the nonlinear interaction.

Two cars on the highway

The chip's efficiency is greatly improved as a result of this technology. However, a new restriction is placed. We are limited in terms of colour choices when employing a resonator "Camille Brès agrees. Indeed, the effectiveness of a nonlinear effect is determined by the phase agreement between the many interacting hues, despite the fact that their propagation rates are invariably different. As if they were two cars driving along the highway. We want the one in the fast lane to slow down while the other speeds up so they can roll alongside each other and interact "Jianqi Hu, Ph.D., and co-first author, explains. In a resonator, this is usually only possible in extremely limited situations. Despite the use of the resonator, the researchers were able to find a way to get around this limitation and provide access to a variety of colours. Light waves propagate in the resonator, causing a coherent interaction that affects the material's characteristics. A totally all-optical self-organization of the structure is accomplished, which automatically compensates for phase mismatch independent of the input colour. As a result, we get around one of resonators' major drawbacks while still reaping the benefits of their high efficiency," the researcher says.

References:

Edgars Nitiss et al, Optically reconfigurable quasi-phase-matching in silicon nitride microresonators, Nature Photonics (2022). DOI: 10.1038/s41566-021-00925-5