Imperial College London and University of Exeter map saturation limits in ultrafast ITO optical switching with a 44 fs pump pulse

Researchers from Imperial College London and the University of Exeter published a combined experimental and theoretical study in Light: Science & Applications in August 2026 that maps the full response of an indium tin oxide (ITO) thin film when driven by a 44 fs near-infrared pump pulse - including the saturation regime that caps how far the material's optical properties can be shifted^[1].

What the experiment measured

The team used pump-probe spectroscopy to track how ITO's optical properties change across a range of pump intensities^[1]. ITO is a doped semiconductor that sits near its epsilon-near-zero (ENZ) point in the near-infrared - the spectral region where its dielectric permittivity approaches zero. In that ENZ region, propagating light attains a very high phase velocity, and the material exhibits strong optical nonlinearity. That combination makes ITO one of the most studied candidates for all-optical modulators in silicon photonic platforms.

The researchers found three distinct regimes as pump intensity increases:

• Low intensity: a two-temperature model - tracking electron and lattice temperatures separately - accurately describes the response.
• High intensity: the model breaks down; the pump heavily depopulates electrons from below the Fermi level, causing response saturation^[1].
• Very high intensity: Auger transitions from the valence band introduce additional, complex structure into the optical response due to non-equilibrium rearrangement of energy between electrons and holes^[1].

The physical mechanism

The team modelled the switching as a change in plasma frequency^[1]. When the femtosecond pulse excites hot electrons into a non-parabolic conduction band, their effective mass increases - which shifts the plasma frequency and, with it, the material's refractive index^[1]. Saturation at high pump intensities arises because the pump heavily depopulates electrons from below the Fermi level, a mechanism the standard two-temperature model does not capture^[1].

Transparent conducting oxides near their epsilon-near-zero frequency exhibit near-unity ultrafast modulations of the refractive index, which have enabled the field of time-varying metamaterials, yet the underlying carrier dynamics at high driving fluences have remained poorly understood. The Imperial/Exeter paper directly addresses that gap by providing a validated model that holds across all intensity regimes.

Why it matters for photonic device design

ITO has attracted sustained interest as a modulator material because it can be integrated with silicon-on-insulator waveguides. Leveraging the ENZ effect, researchers have demonstrated 3 dB modulation depth in a non-resonant waveguide structure with a length of 20 µm, providing insight into the design of very compact modulators for chip-scale optical links. Knowing where the material saturates - and what physical process causes that saturation - is directly relevant to setting the operating point of any ITO-based modulator.

The paper was published in Light: Science & Applications, volume 15, article 336, in August 2026, under a Creative Commons licence^[1].

The work also has implications for the broader field of time-varying metamaterials, where ITO thin films are used as the switching element. Time-switched and time-varying metamaterials whose properties are modulated in time have emerged as a new paradigm for wave manipulation and information processing, enabling control of wave momentum, frequency, and digital information, with applications in nonreciprocity, smart reconfigurable response, and next-generation wireless communications.

The next step to watch is whether the validated model can be incorporated into simulation tools used to design ITO-integrated photonic circuits - particularly whether the Auger-transition regime can be avoided by design, or whether it can be exploited to extend the modulation range beyond what the two-temperature model predicts.