Researchers in Japan have watched a hidden electronic state form inside a material in just 30 femtoseconds — a millionth of a billionth of a second — catching the fleeting intermediate stage that guides the transition. The work was published in Physical Review Letters.
The team, from the Institute of Science Tokyo, Tohoku University and Nagoya Institute of Technology, fired ultrafast laser pulses at a metal-organic framework (MOF), a material built by linking metal ions with organic molecules. Using time-resolved reflectance spectroscopy with pulses lasting only six femtoseconds, they observed the material's reflectance spectrum shift sharply and develop a new optical absorption band within 30 fs — the signature of a photoinduced hidden state.
'We found that the photoinduced hidden state forms within 30 fs through a previously unknown intermediate electronic state,' said Assistant Professor Tadahiko Ishikawa of Science Tokyo, who led the investigation with then-doctoral student Samiran Banu, now at RIKEN.
Theoretical calculations revealed the underlying sequence: immediately after absorbing light, the material briefly entered a bond-order wave state, in which bonds between neighboring sites alternated between stronger and weaker arrangements. Small atomic movements followed that electronic reorganization, together carrying the material into the hidden state. The calculations also suggest the final state may be polar — an uneven distribution of charge that could let light adjust a material's electronic behavior without permanently changing it.
Photoinduced states offer a way to alter a material's electronic and optical properties without relying only on heating or cooling. By revealing the intermediate steps, the method could help design photoresponsive materials for high-speed electronics, optoelectronic devices and other light-controlled systems — and, the authors say, similar hidden transitions may exist in other materials waiting to be found.
