The development of high-speed strobe-flash photography in the 1960s by the
late MIT professor Harold "Doc" Edgerton allowed us to visualize events too
fast for the eye—a bullet piercing an apple, or a droplet hitting a pool of
milk.
Now, by using a suite of advanced spectroscopic tools, scientists at MIT and
University of Texas at Austin have for the first time captured snapshots of
a light-induced metastable phase hidden from the equilibrium universe. By
using single-shot spectroscopy techniques on a 2D crystal with nanoscale
modulations of electron density, they were able to view 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 modulated
crystal," says Frank Gao Ph.D. '22, co-lead author on a paper about the work
who is currently a postdoc at UT Austin.
"Usually, shining lasers on materials is the same as heating them, but not
in this case," adds Zhuquan Zhang, co-lead author and current MIT graduate
student in chemistry. "Here, irradiation of the crystal rearranges the
electronic order, creating an entirely new phase different from the
high-temperature one."
A paper on this research was published today in Science Advances. The
project was jointly coordinated by Keith A. Nelson, the Haslam and Dewey
Professor of Chemistry at MIT, and by Edoardo Baldini, an assistant
professor of physics at UT-Austin.
Laser shows
"Understanding the origin of such metastable quantum phases is important to
address long-standing fundamental questions in nonequilibrium
thermodynamics," says Nelson.
"The key to this result was the development of a state-of-the-art laser
method that can 'make movies' of irreversible processes in quantum materials
with a time resolution of 100 femtoseconds." adds Baldini.
The material, tantalum disulfide, consists of covalently bound layers of
tantalum and sulfur atoms stacked loosely on top of one another. Below a
critical temperature, the atoms and electrons of the material pattern into
nanoscale "Star of David" structures—an unconventional distribution of
electrons known as a "charge density wave."
The formation of this new phase makes the material an insulator, but shining
one single, intense light pulse pushes the material into a metastable hidden
metal. "It is a transient quantum state frozen in time," says Baldini.
"People have observed this light-induced hidden phase before, but the
ultrafast quantum processes behind its genesis were still unknown."
Adds Nelson, "One of the key challenges is that observing an ultrafast
transformation from one electronic order to one that may persist
indefinitely is not practical with conventional time-resolved techniques."
Pulses of insight
The researchers developed a unique method that involved splitting a single
probe laser pulse into several hundred distinct probe pulses that all
arrived at the sample at different times before and after switching was
initiated by a separate, ultrafast excitation pulse. By measuring changes in
each of these probe pulses after they were reflected from or transmitted
through the sample and then stringing the measurement results together like
individual frames, they could construct a movie that provides microscopic
insights into the mechanisms through which transformations occur.
By capturing the dynamics of this complex phase transformation in a
single-shot measurement, the authors demonstrated that the melting and the
reordering of the charge density wave leads to the formation of the hidden
state. Theoretical calculations by Zhiyuan Sun, a Harvard Quantum Institute
postdoc, confirmed this interpretation.
While this study was carried out with one specific material, the researchers
say the same methodology can now be used to study other exotic phenomena in
quantum materials. This discovery may also help with the development of
optoelectronic devices with on-demand photoresponses.
Reference:
Frank Y. Gao et al, Snapshots of a light-induced metastable hidden phase
driven by the collapse of charge order, Science Advances (2022).
DOI: 10.1126/sciadv.abp9076
Tags:
Physics