In the 1960s, the late MIT professor Harold “Doc” Edgerton developed high-speed strobe-flash photography, which enabled humans to view events too quick for the human eye, such as a bullet penetrating an apple or a droplet striking a pool of milk.

Scientists from the Massachusetts Institute of Technology (MIT) and the University of Texas at Austin have for the first time acquired images of a light-induced metastable phase concealed from the universe’s equilibrium. They were able to observe this transition in real-time by employing single-shot spectroscopic techniques on a 2D crystal with nanoscale electron density modulations.

“With this study, we demonstrate the creation and evolution of a hidden quantum phase caused by an ultrashort laser pulse in an electronically controlled crystal,” says UT Austin postdoc and Ph.D. ’22 graduate Frank Gao.

“Generally, shining lasers on materials is equivalent to heating them, but not in this situation,” says Zhuquan Zhang, co-author and current chemistry graduate student at MIT. “In this instance, irradiation of the crystal rearranges the electronic order, resulting in a whole new phase distinct from the high-temperature phase.”

Science Advances has just published a study on this topic. Both MIT’s Professor of Chemistry Keith A. Nelson, together with UT’s Assistant Professor of Physics Edoardo Baldini, led the research. There are many fundamental questions in thermodynamics that can only be answered if we understand how these metastable quantum phases occur, says Nelson.

“The key to this discovery was the invention of a cutting-edge laser method with a time resolution of 100 femtoseconds,” explains Baldini. The material, tantalum disulfide, is composed of loosely stacked layers of covalently bound tantalum and sulfur atoms. Below a specific temperature, the atoms and electrons of the material arrange themselves into tiny “Star of David” shapes, an unorthodox electron distribution known as a “charge density wave.”

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The creation of this new phase transforms the material into an insulator, yet a single powerful light pulse transforms it into a metastable metal. “It is a quantum state that is fixed in time,” explains Baldini. People have previously observed this light-induced hidden phase, but the ultrafast quantum mechanisms that led to its formation were unknown.

Nelson adds, “Observing an ultrafast transition from one electrical order to one that can stay indefinitely is not possible with traditional time-resolved techniques, which is one of the greatest obstacles.”

One probing light beam was split into several hundred distinct probing beams that landed at the sample at various times before and after switching was activated by a separate, ultrafast excitation pulse. Scientists were able to generate a movie that provides microscopic insight into transformation mechanisms by measuring the difference in each of these probing pulse after they were mirrored from or passed thru the sample.

The authors demonstrated that the melting and reordering of the charge density wave result in the development of the hidden state by capturing the dynamics of this complicated phase shift in a single measurement. The theoretical computations of Harvard Quantum Institute postdoc Zhiyuan Sun verified this view.

While this research was conducted on a specific material, the same methodology can now be applied to investigate additional strange occurrences in quantum materials, according to the researchers. This discovery may potentially aid in the creation of optoelectronic devices with photoresponses on demand.

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