Researchers at Imperial College London, working with the University of Exeter, have investigated how a 310-nm-thick layer of indium tin oxide (ITO) changes its optical response when struck by intense femtosecond laser pulses. The study follows the material from strong optical switching, through saturation, and into a distinct high-intensity regime.
ITO is a transparent conducting material widely used as a see-through electrical contact in displays and other optoelectronic devices. Near its epsilon-near-zero (ENZ) wavelength, small changes in its electronic state can produce large changes in how it reflects and transmits light.
By combining time- and wavelength-resolved measurements with a microscopic model, the team identified where the useful switching response stops increasing, explained the electronic origin of that limit, and found signs that a different process takes over when the material is driven harder.
The research is published in Light: Science & Applications.
From strong switching to saturation
At lower intensities, the pump laser pulse heats free electrons and drives them to higher-energy states in ITO's non-parabolic conduction band. The electrons then behave as though they are heavier, lowering the material's plasma frequency and shifting the balance between reflected and transmitted light.
The measurements showed large changes in both transmission and reflection: transmitted intensity increased by up to 280%, while reflected intensity fell by as much as 74%. At higher pump intensities, the response saturated. The team's model traces this ceiling to the depletion of electrons below the Fermi level, where changes in effective mass have the strongest optical effect. Once most of these electrons have been excited, adding more energy produces progressively less change. The model also indicates that, once enough energy has been absorbed, the underlying switch can occur within a few femtoseconds – faster than the 44-femtosecond pump pulse.
"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."
Yan Li, first author and PhD student, Department of Physics, Imperial College London
The microscopic model was developed by Professor Sir John Pendry at Imperial and Professor Simon Horsley at Exeter. It accurately fits the measured plasma-frequency shift up to the saturation region with the high-wavevector effective mass as the only free parameter.
A distinct high-intensity regime
At the highest intensities studied, the reflected and transmitted signals partially recovered, and the probe spectrum shifted towards the blue. These changes appeared on a faster timescale and could not be captured by the standard two-temperature model, which treats the hot electrons and the atomic lattice as two coupled thermal populations.
The authors propose that Auger-type electron-electron interactions lift additional electrons from the valence band into the conduction band. These extra carriers would counteract the initial effective-mass change and help to explain the partial recovery.
This interpretation is consistent with the observations, but it remains a proposed mechanism. More detailed multiband calculations and direct time-resolved measurements of the carrier populations will be needed to test it.
"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."
Sir John Pendry, senior author and Professor of Solid-State Physics, Imperial College London
Why the limit matters
Most photonic technologies control light using structures that are fixed in space. Time-varying photonics adds another design route: changing a material while light is passing through it. Done quickly enough, this can reshape optical pulses and shift their frequency, with potential uses in ultrafast modulation, frequency conversion and light synthesis.
A particularly ambitious goal is to develop photonic time crystals, in which optical properties repeat in time rather than structure repeating in space. This study does not create such a crystal. Instead, it provides information relevant to designing time-varying systems: how large and how fast the optical modulation can be, and where it saturates.
Beyond ITO
The study's wider contribution is a way to connect the energy deposited by a laser to electronic band structure and then to the optical signal that emerges. This makes saturation a predictable consequence of the material physics, rather than simply an empirical ceiling.
The model is especially suited to transparent conducting oxides and could be extended to aluminium zinc oxide, other heavily doped semiconductors and potentially a broader range of ENZ materials. Further work is needed to verify the proposed high-field carrier process directly and to explore repeated temporal modulation.
"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."
Professor John Tisch, senior author and Professor of Laser Physics, Imperial College London
