By Josh Perry, Editor
[email protected]

Researchers at the Massachusetts Institute of Technology (MIT) in Cambridge, Mass., along with partners at Stanford University (Palo Alto, Calif.) and Skolkovo Institute of Science and Technology (Skoltech) in Russia, have demonstrated that intense laser pulses can be used to create light-induced phase changes.

To study phase changes in materials, such as freezing and thawing, researchers used charge density waves — electronic ripples that are analogous to the crystal structure of a solid. (MIT)

According to a report from MIT, this new finding could lead to the creation of novel optoelectronic or data storage devices.

“For this study, instead of using an actual crystal such as ice, the team used an electronic analog called a charge density wave — a frozen electron density modulation within a solid — that closely mimics the characteristics of a crystalline solid,” the article explained.

Melting typically occurs in a uniform manner throughout the material but light-induced melting created numerous topological defects that created tiny vortices in the material. This rapid phase-change was created by laser pulses of less than a trillionth of a second, which does not allow the materials to reach equilibrium the way that gradual heating or cooling would.

“The team used a combination of three techniques, known as ultrafast electron diffraction, transient reflectivity, and time- and angle-resolved photoemission spectroscopy, to simultaneously observe the response to the laser pulse,” the article continued. “For their study, they used a compound of lanthanum and tellurium, LaTe₃, which is known to host charge density waves.”

Researchers could visualize the electrons moving and were able to demonstrate the presence of topological defects. They also saw that the time for re-solidifying was not uniform and took place slower than it took the charge density wave to recover its initial intensity.

The research was recently published in Nature Physics. The abstract stated:

“Upon excitation with an intense laser pulse, a symmetry-broken ground state can undergo a non-equilibrium phase transition through pathways different from those in thermal equilibrium. The mechanism underlying these photoinduced phase transitions has long been researched in the study of condensed matter systems, but many details in this ultrafast, non-adiabatic regime still remain to be clarified.

“To this end, we investigate the light-induced melting of a unidirectional charge density wave (CDW) in LaTe₃. Using a suite of time-resolved probes, we independently track the amplitude and phase dynamics of the CDW. We find that a fast (approximately 1 picosecond) recovery of the CDW amplitude is followed by a slower re-establishment of phase coherence.

“This longer timescale is dictated by the presence of topological defects: long-range order is inhibited and is only restored when the defects annihilate.

“Our results provide a framework for understanding other photoinduced phase transitions by identifying the generation of defects as a governing mechanism.”