Research isn’t linear – and neither are these optics!

Nonlinear materials are everywhere, even in your computer. They’re used in information processing and form the basis of electronic switches. Every letter in this article is a signal through a nonlinear material! This is because the way they respond to electrical signals depends on how strong the applied voltage is. In a similar way, the flow of light can be controlled with light using nonlinear optics. However, most optical materials are very weakly nonlinear, making it difficult to control light with light.

Perovskite breakthrough

Many researchers are interested in halide perovskites, a class of materials with unique optical and electrical properties. They’re being used to develop technology, including solar cells, LEDs, and sensors. At AMOLF, they’re a popular subject of study, and Professor Said Rodriguez’s research group uses them to explore how we can control light with light to process information in new and more efficient ways. Until now, halide perovskites have been shown to have the necessary nonlinear properties when laser light shines in fast pulses, but never in the case of continuous illumination.

The latest discovery from Said’s group changes that. Now published in Nature Photonics, this is the first time continuous-wave nonlinearity has been seen in a halide perovskite crystal. The phenomenon was not previously theorized.

Not just a phase

Not only is the halide perovskite crystal in question capable of nonlinear optics, it also does something special at a certain temperature. “We found that the nonlinear response depends on the temperature in a very pronounced way,” explains Said. “At a certain temperature it becomes extremely nonlinear, in a way that suggests a phase transition.” A phase transition, as with ice melting to become water, is when the material structure changes under certain conditions.

This possible phase transition in the halide perovskite crystal was, like the nonlinearity, also not predicted by theory. These phenomena go hand-in-hand – the nonlinear response was particularly strongest when the crystal is believed to be at the edge of a phase transition. That is: not ice, not water, but the moment in between.

What’s next?

With these new insights into the behavior of halide perovskites, researchers can explore new ways to use these properties in developing information processing technologies. Nonlinear optical materials are all around us, and now that halide perovskites have joined the line-up, they’ve shown an even greater promise for next-generation optical technologies.

For more details on how this discovery was made, you can read the publication here:

Continuous-wave nonlinear polarization control and signatures of criticality in a perovskite cavity

Reference

G. Keijsers, R.M. de Boer, B. Verdonschot, K.J.H. Peters, Z. Geng, and S.R.K. Rodriguez, Continuous-Wave Nonlinear Polarization Control and Signatures of Criticality in a Perovskite Cavity, Nature Photonics, May 27 (2025).