In its search for materials and technologies that could help push the speed and storage limits of silicon-based computer technology, a team of researchers looked at a category of materials known as “strongly correlated systems,” so called because of the strong interactions between the electrons in such materials. The vertical red line shows when the laser electric field (yellow oscillating curve) crosses the threshold field, destroying the insulating phase of the material. The top panel shows the average number of doublon-hole pairs per site (blue) and the decay of the insulating field-free ground state (red). Courtesy of MBI Berlin. In Mott insulators, for example, although the electrons should flow freely, the mutual interactions between the electrons in this “strongly correlated system” impede the flow of electrons, causing the material to behave as an insulator instead of a conductor. Laser pulses can be used to disrupt the order of behavior in a strongly correlated material, changing its physical properties. The external laser pulse can force a phase transition in the structural order of the electrons in the material. In theory, such phase transitions should allow researchers to develop new switching elements for next-generation electronics that are faster and potentially more energy efficient than present-day transistors. However, these phase transitions are extremely fast and therefore difficult to study. Until now, scientists have only been able to determine the state of the material before, and after, such a phase transition. Researchers from the Max Born Institute and Russian Quantum Center devised a method for studying this phase transition process. The researchers irradiated the material with short, tailored laser pulses. They observed the material’s reaction to the pulses, noting how the electrons in the material were excited into motion. At the same time, researchers noted that the electrons emitted resonant vibrations at specific frequencies. When an intense light field excited a highly nonequilibrium electronic response in a semiconductor or a dielectric, the vibrations occurred as harmonics of the incident light. “By analyzing this high harmonic spectrum, we can observe the change in the structural order in these strongly correlated materials ‘live’ for the first time,” said researcher Rui Silva. Researchers showed that high-harmonic generation could be used to time-resolve ultrafast many-body dynamics associated with an optically driven phase transition with accuracy exceeding one cycle of the driving light field. In some cases, say the researchers, it takes only a single light oscillation to spin the order of the electrons in the material and turn an insulator into a metal-like conductor. Only recently have laser sources been made available that can trigger such transitions. The laser pulses must be strong enough and also extremely short and in the femtosecond range. This work could pave the way for time-resolving highly nonequilibrium many-body dynamics in strongly correlated systems, with few femtosecond accuracy. “If we want to use light to control the properties of electrons in a material, then we need to know exactly how the electrons will react to light pulses,” professor Misha Ivanov said. With the latest-generation laser sources, which allow full control over the electromagnetic field even down to a single oscillation, the new method could further scientific investigation into the materials of the future. The research was published in Nature Photonics (doi:10.1038/s41566-018-0129-0).
