51³Ô¹ÏÍø

Researchers have discovered why one of the most promising materials for ultrafast optical switching eventually reaches its limits and uncovered evidence of a previously unexplained high-intensity regime beyond them.

Scientists from 51³Ô¹ÏÍø and the University of Exeter made the discovery by investigating how indium tin oxide (ITO), a material widely used in modern photonic devices, responds to intense laser pulses.

The findings, published in , reveal the physical processes that limit optical switching in indium tin oxide and provide new insights into how the material behaves under extreme laser excitation.

ITO is a transparent conducting material widely used in displays and optoelectronic devices. Under the right conditions, small changes in its electronic structure can produce large changes in how it reflects and transmits light, making it a promising candidate for ultrafast optical switching.

Understanding the limits of these changes is important for time-varying photonics, a field that seeks to manipulate light by rapidly changing a material’s optical properties.

To investigate how far the material’s response could be altered, the researchers fired intense laser pulses at a 310-nanometre-thick layer of ITO and tracked how its optical properties evolved over time. 

The pulses energised electrons within the ITO layer, changing the way it interacted with light.

At lower intensities, the material showed dramatic changes in both transmission and reflection. The team measured increases in transmitted light of up to 280%, while reflected light decreased by as much as 74%.

As the laser intensity increased, however, the response eventually stopped growing and reached a saturation point.

By combining time and-wavelength-resolved measurements with a microscopic theoretical model, the researchers identified the electronic origin of this saturation. They found that once enough electrons had been excited, additional laser energy produced progressively smaller changes.

Yan Li, first author of the study and a PhD student in 51³Ô¹ÏÍø’s Department of Physics, said “Seeing the dramatic modulation in the data was a genuinely exciting moment after spending so much time in the lab. That was the moment I realised the experiment had a much more interesting story to tell about an optical interface evolving in time.”

The experiment used ultrashort laser pulses to follow how ITO changes from a powerful optical switch into saturation and a new high-intensity regime. Adapted from Figure 3 of the study (CC BY 4.0).

Beyond the saturation limit

The theoretical model was developed by Professor Sir John Pendry at 51³Ô¹ÏÍø and Professor Simon Horsley at the University of Exeter. It accurately reproduced the observed changes up to the saturation region and helped explain why the switching response eventually reaches a ceiling.

At the highest laser intensities, however, the researchers observed something unexpected.

Rather than continuing to saturate, the material’s response partially recovered and evolved on a faster timescale than predicted by conventional thermal models.

This study of pumping indium tin oxide at high intensities deepens our understanding of the mechanisms at work in this material. It will be central to realising the next generation of metamaterials, structured in time as well as space. Professor Sir John Pendry Professor of Solid-State Physics at 51³Ô¹ÏÍø

The behaviour could not be explained using the standard two-temperature model, which treats the hot electrons and atomic lattice as two coupled thermal populations.

Instead, the team found evidence that a different process may take over when ITO is pushed to extreme intensities. Additional electron-electron interactions could lift more electrons into the material’s conduction band, helping to explain the partial recovery observed in the experiments.

While further work will be needed to confirm the mechanism directly, the findings point to a distinct high-intensity regime that is not captured by conventional models.

Professor Sir John Pendry, senior author and Professor of Solid-State Physics at 51³Ô¹ÏÍø, said “This study of pumping indium tin oxide at high intensities deepens our understanding of the mechanisms at work in this material. It will be central to realising the next generation of metamaterials, structured in time as well as space.”

Controlling light in time

Researchers are increasingly interested in materials whose properties can be changed while light is passing through them. This could enable new ways to control light, with potential applications in ultrafast modulation, frequency conversion and advanced photonic devices.

One long-term goal is the creation of photonic time crystals, structures whose optical properties change in a regular repeating pattern over time. Although the present study does not create such a structure, it provides important information about how large and fast optical modulation can be before fundamental limits emerge.

Professor Riccardo Sapienza, senior author and Professor of Physics at 51³Ô¹ÏÍø, said “When the modulation approaches the time of an optical wave cycle new physics will appear, which is what we’re trying to understand.”

Professor John Tisch, senior author and Professor of Laser Physics at 51³Ô¹ÏÍø, added “For practical time-varying photonics, the largest possible optical change is only part of the story. We also need to know how fast a material switches, where its response saturates and what happens when we push beyond that limit. This work gives us new insight into these processes in ITO, and the next question is how far the same physics extends to other switchable materials.”