New research from the Massachusetts Institute of Technology (MIT) demonstrates that light-generated phase changes occur differently than phase changes triggered by temperature. While optically induced phase changes have been observed before, the mechanism through which they proceeded was not known. Additional understanding of the process could lead to new types of optoelectronic devices, such as devices for data storage. To study light-induced phase change, the researchers used an electron density modulation frozen within a solid. This electronic analog, called a charge density wave, mimics the characteristics of a crystalline solid. 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. They found that when phase change is triggered by a pulse of laser light instead of by a temperature change, it unfolds differently, starting with a collection of whirlpool-like distortions, or topological defects. This illustration depicts one such defect disrupting the orderly pattern of parallel ripples. Courtesy of N. Gedik et al./MIT. The research team found that during optically induced melting, the phase change generated many topological defects in the material. These defects, which researchers compared to vortices on the surface of water, changed the dynamics of the electrons and lattice atoms in the material. The light-induced phase transition happened quickly, similar to the way a piece of semimolten red-hot iron cools almost instantly when it is quenched in cold water. The laser pulse used to simulate the phase change was less than 1 picosecond long. Using a combination of three techniques — ultrafast electron diffraction, transient reflectivity, and time- and angle-resolved photoemission spectroscopy — the researchers observed the changes to the electrons and atoms within the material in response to the laser pulse. “We can watch, and make a movie of, the electrons and the atoms as the charge density wave is melting,” said professor Nuh Gedik. The images revealed the formation and propagation of vortex-like topological defects during the phase change. The researchers were also able to observe the matter resolidifying once the laser pulses ceased. The time it took the material to resolidify was not uniform, but took place across multiple timescales. The intensity of the charge density wave recovered more rapidly than the orderliness of the lattice. Researcher Alfred Zong said that the next step in the research would attempt to determine how to engineer the topological defects in a controlled way for potential application in optoelectronics. By identifying the defect generation as a governing mechanism, the research provides a framework for understanding other photo-induced phase transitions. The research was published in Nature Physics (https://doi.org/10.1038/s41567-018-0311-9).
