Materials can behave in unexpected ways after absorbing light. In some cases, light pushes them into photoinduced states with properties that are very different from those they display under ordinary conditions. These states give researchers another way to alter material behavior beyond conventional approaches such as heating or cooling. Learning exactly how they form could be important for developing future photoresponsive materials and advanced optical technologies.
Capturing a Transformation in Femtoseconds
One of the biggest challenges is speed. The first steps in forming a photoinduced state can unfold on the femtosecond (fs) timescale (a millionth of a billionth of a second), making them exceptionally difficult to observe.
To investigate these earliest moments, a team led by Assistant Professor Tadahiko Ishikawa from the Department of Chemistry, School of Science, Institute of Science Tokyo (Science Tokyo), Japan, worked with then doctoral student Samiran Banu (currently a Special Postdoctoral Researcher at RIKEN) and collaborators at Tohoku University and Nagoya Institute of Technology, Japan.
The researchers focused on a metal-organic framework (MOF), a material constructed by connecting metal ions with organic molecules. Their goal was to determine exactly how a photoinduced hidden state develops. The findings were published in the journal Physical Review Letters.
“We found that the photoinduced hidden state forms within 30 fs through a previously unknown intermediate electronic state,” says Ishikawa.
Ultrafast Lasers Reveal a Hidden State
To follow the transformation, the researchers used time-resolved reflectance spectroscopy and ultrashort laser pulses lasting only six fs. The method allowed them to measure how the light reflected by the material changed almost immediately after the MOF absorbed a laser pulse.
With this extremely fine time resolution, the team tracked rapid changes in the material’s electronic behavior. Within 30 fs, its reflectance spectrum shifted dramatically and developed features linked to the appearance of a new optical absorption band. Those changes indicated that the photoinduced hidden state had formed.
Experiments alone, however, could not fully explain what was happening. The researchers therefore combined their measurements with theoretical calculations to reconstruct the sequence of events inside the material.
A Fleeting Electronic State Comes First
The analysis showed that immediately after absorbing light, the material briefly entered an intermediate electronic state. During this moment, electronic bonds between neighboring sites alternated between stronger and weaker in a repeating pattern. This configuration is known as a bond-order wave state.
The state existed only briefly. It was followed by small movements in the positions of atoms within the material, and those structural changes ultimately produced the photoinduced hidden state.
Theoretical calculations also suggested that the newly formed state may be polar. In such a state, positive and negative electrical charges are distributed unevenly across the material. If these photoinduced polar states can be reliably created and controlled, they could provide new ways to manipulate electronic properties using light.
“By revealing intermediate states, our method could help design materials that can be efficiently controlled using light,” explains Ishikawa.
Toward Materials Controlled by Light
Beyond showing how a photoinduced hidden state develops, the research points to a possible strategy for manipulating material properties with extremely short pulses of light. Being able to create and control these temporary states could contribute to new photoresponsive materials designed for high-speed electronics, optoelectronic devices, and other technologies that require precise control over how materials behave.
Future research could extend the same experimental and theoretical approach to other types of materials. By exposing the previously invisible steps that occur during ultrafast transformations, scientists may move closer to designing materials whose properties can be deliberately and efficiently controlled with light.
