Light Sparks New Material State in 30 Femtoseconds

The properties of a material depend not only on how its atoms are arranged, but also on how its electrons are arranged and move. So what happens inside a material when light changes its properties?

Previous studies have often examined how light breaks down ordered electronic states. Much less is known, however, about how new ordered states begin to form, because these changes happen too quickly to observe in detail.

A research team led by Assistant Professor Tadahiko Ishikawa at Institute of Science Tokyo (Science Tokyo) investigated this question using a metal-organic framework (MOF), a material in which metal ions and organic molecules are connected in an ordered structure. The researchers shone light on the material and tracked what happened immediately afterward on the femtosecond timescale. One femtosecond is one quadrillionth of a second.

Learn more about the features and potential of MOFs

The observations showed that the electronic state changed first after the material was exposed to light. Changes related to the material's structure followed, and within about 30 femtoseconds, the material had transformed into a new state that was not present before the light was applied. Calculations also suggest that this new state may exhibit electric polarization.

Together, the experiments and calculations indicate that the new state forms through a close connection between changes in the electronic state and changes in the material's structure.

A major achievement of this study is that it captured the very early stage in which a transient state created immediately after light excitation develops into a different state. Researchers have extensively studied how light disrupts an existing state, but the process by which a different state forms afterward has remained difficult to understand because it happens so quickly.

The calculations revealed another intriguing feature. The MOF studied here normally has a relatively rigid structure, making changes in atomic positions difficult. After light excitation, however, the link between the electronic and structural changes becomes much stronger. As a result, even a very small structural change can help stabilize the new state.

The state examined in this study cannot be reached simply by heating or cooling the material. Because the structure of MOFs can be tuned by changing their combinations of metal ions and organic components, future studies may be able to design materials that can produce and stabilize a variety of light-induced states.

Combining this approach with techniques that can directly observe atomic motion could also reveal, in greater detail, how a material changes after absorbing light and how a new state takes shape.

Thirty femtoseconds is about one thirty-three-trillionth of a second. It is such a short period of time that even atoms in a water molecule would vibrate only a few times. Yet on a timescale far too short for us to perceive, a material's state is already changing.

In this study, we were able to capture some of what happens during that brief moment through changes in the material's optical spectrum. But we have not yet directly observed every change taking place. I would like to look more closely at that moment to understand what is happening inside the material and how the new state takes shape.

(Tadahiko Ishikawa, Assistant Professor, Department of Chemistry, School of Science, Institute of Science Tokyo)

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