Zenghu Chang, a researcher at the University of Central Florida (UCF; Orlando, FL), and his international team have developed a new ultrafast light source for observing electron motion in molecules at the point before the nuclei start to move; the technique can potentially lead to the generation of high-order harmonics and attosecond pulses at repetition rates of greater than 1 MHz.1
To do this, Chang and his team borrowed an idea from Chang's earlier work in the area of ultrafast lasers, in which 67-attosecond pulses of extreme ultraviolet light were created.
"The charge migration that theorists have been predicting since 1999 happens so quickly we haven't been able to observe it yet," Chang says. "It's very exciting, because we have found a new way to build light sources that may allow us to see it in the future."
Being able to see this superfast interaction between electrons will help scientists understand the rules that govern the quantum-mechanics world. By being able to observe what actually happens, scientists can also begin to understand interactions that help improve the efficiency of solar cells.
"We control the below-threshold harmonic light emission by using electromagnetic fields with time-dependent ellipticity, like we have done to the above-threshold high-order harmonics," says Chang. "We thought: Could we use the same gating fields to show the dependence of the below-threshold harmonic intensity on the carrier-envelope phase of the driving laser? It took us some time to find the right experimental parameters, but the answer is yes."
The team includes researchers from the Center for Quantum Science and Engineering and the Department of Physics at the National Taiwan University, the Materials Sciences Division at Lawrence Berkeley National Laboratory (Berkeley, CA), and the Department of Physics at St. Petersburg State University in Russia.
The Defense Advanced Research Projects Agency (DARPA), National Science Foundation (NSF), U.S. Department of Energy (DOE), National Science Council of Taiwan, and National Taiwan University funded the research.
REFERENCE:
1. Michael Chini et al., Nature Photonics (2014); doi:10.1038/nphoton.2014.83