The development of high-speed strobe-flash photography by the late MIT professor Harold “Doc” Edgerton in the 1960s allowed us to visualize events far too fast for the eye – a bullet piercing an apple, or a can of milk. A drop hitting the pool.
Now, using a suite of advanced spectroscopic instruments, scientists at MIT and the University of Texas at Austin have captured for the first time a snapshot of a light-induced metastable phase hidden from the equilibrium universe. Using single-shot spectroscopy techniques on 2D crystals with nanoscale modulation of electron density, they were able to observe this transition in real time.
“With this work, we are showing the birth and evolution of a hidden quantum phase induced by an ultrashort laser pulse in an electronically modified crystal,” said Frank Gao PhD ’22, co-author on a paper about the work. Says lead author who is currently a postdoc at UT Austin.
“Typically, shining a laser at materials is tantamount to heating them, but not in this case,” says Zhuquan Zhang, co-lead author and MIT graduate student in chemistry. “Here, the crystal’s radiation rearranges the electronic order, creating an entirely new phase from the higher-temperature one.”
A paper on this research was published today science advance. The project was jointly coordinated by Keith A. Nelson, the Haslam and Dewey Professor of Chemistry at MIT, and Edoardo Baldini, an assistant professor of physics at UT-Austin.
“Understanding the origin of such metastable quantum phases is critical to addressing long-standing fundamental questions in thermodynamics of any kind,” Nelson says.
“Key to this result was the development of a state-of-the-art laser method that can create ‘movies’ of irreversible processes in quantum materials, with a time resolution of 100 femtoseconds.” Baldini adds.
The material, tantalum disulfide, consists of covalently bound layers of tantalum and sulfur atoms stacked loosely on top of each other. Below a critical temperature, atomic and electron nanoscale “Star of David” structures of the material pattern – an unconventional distribution of electrons known as a “charge density wave”.
The formation of this new phase makes the material an insulator, but shining a single, intense light pulse pushes the material into the metastable hidden metal. “It’s a transient quantum state frozen in time,” Baldini says. “People have observed this light-induced hidden phase before, but the ultrafast quantum processes behind its origin were still unknown.”
“One of the major challenges is that observing an ultrafast transformation from an electronic order that can persist indefinitely is not practical with traditional time-resolving techniques,” Nelson says.
pulse of insight
Researchers developed a unique method that involved splitting a single probe laser pulse into several hundred separate probe pulses, which arrived at the sample at different times before and after being switched on by a separate, ultrafast excitation pulse . By measuring the change in each of these probe pulses or being transmitted through the sample and then stringing the measurement results like individual frames, they can produce a film that provides subtle insights into the mechanism. through which changes take place.
By capturing the dynamics of this complex phase change in a single-shot measurement, the authors demonstrated that the melting and recrystallization of the charge density wave leads to the formation of the hidden state. Theoretical calculations by Harvard Quantum Institute postdoc Zhiyuan Sun confirmed this interpretation.
While this study was done with a specific material, the researchers say that the same method can now be used to study other exotic phenomena in quantum materials. The discovery may also aid in the development of optoelectronic devices with on-demand photoreactions.
Other authors on the paper were chemistry graduate student Jack Liu, Department of Physics MRL Mitsui Career Development Associate Professor Joseph G. are Chekelsky; Linda Yeh PhD ’20, now a postdoc at Stanford University; and Yu-Siang Cheng PhD ’19, who is now an assistant professor at National Taiwan University.
This work was assisted by the US Department of Energy, Office of Basic Energy Sciences; The Gordon and Betty Moore Foundation EPiQS Initiative; and the Robert A. Welch Foundation.