Scientists Catch a Hidden Electronic State Forming in Just 30 Femtoseconds

Researchers in Japan have captured an extraordinarily fast electronic transformation inside a metal-organic framework, observing a brief intermediate state and the hidden state that followed it within just 30 femtoseconds — a millionth of a billionth of a second. The team, from Science Tokyo, Tohoku University and the Nagoya Institute of Technology, combined ultrafast laser spectroscopy with theoretical calculations to uncover a previously unknown intermediate electronic state that helps drive the transformation.

Materials can behave in unexpected ways after absorbing light, entering photoinduced states with properties very different from those under ordinary conditions — offering another way to alter material behavior beyond conventional methods such as heating or cooling. But the first steps in forming such a state can unfold on the femtosecond timescale, making them exceptionally difficult to observe.

A team led by Assistant Professor Tadahiko Ishikawa at the Institute of Science Tokyo, working with then-doctoral student Samiran Banu (now a postdoctoral researcher at RIKEN) and collaborators at Tohoku University and the Nagoya Institute of Technology, studied a metal-organic framework — a material built by connecting metal ions with organic molecules. Using time-resolved reflectance spectroscopy with ultrashort laser pulses lasting only six femtoseconds, in a study published in Physical Review Letters, the team found that within 30 femtoseconds the material's reflectance spectrum shifted dramatically and developed a new optical absorption band, signaling that a photoinduced hidden state had formed.

Combining the measurements with theoretical calculations, the researchers found that immediately after absorbing light, the material briefly entered an intermediate electronic state in which electronic bonds between neighboring sites alternated between stronger and weaker in a repeating pattern, known as a bond-order wave state. This state existed only briefly before small shifts in atomic positions produced the photoinduced hidden state. The calculations also suggested the newly formed state may be polar, with positive and negative charges distributed unevenly across the material.

"By revealing intermediate states, our method could help design materials that can be efficiently controlled using light," said Ishikawa. The researchers say the approach could contribute to future photoresponsive materials for high-speed electronics and optoelectronic devices, and plan to extend the same experimental and theoretical method to other materials.