On Tuesday, October 3, 2023, the Nobel Prize in Physics was awarded to three physicists for their pioneering creation of extremely short-duration pulses of light that can be used to investigate the dynamic processes that happen with molecules and atoms.
The Nobel Committee awarded the prize “for experimental methods that generate attosecond pulses of light for the study of electron dynamics in matter” to Pierre Agostini, Ferenc Krausz, and Anne L’Huillier. The work of this trio continues with teams around the globe attempting to perfect this Nobel-winning technique.
“Attosecond pulses are increasingly being used by various research groups to investigate dynamics in matter that occur on unimaginably short time scales,” said Jan Vogelsang, a physicist at the University of Oldenburg in an email. “The primary aim is to understand how charge carriers in a wide variety of structures interact directly with the electric field of visible light.”
Vogelsang explained that attosecond pulses of light can be used to probe matter because an initial light pulse is used to excite the dynamics to be investigated, elevating the energy states of electrons, and before these electrons release photons to drop to lower energy states, an attosecond pulse probes the dynamic state after a fixed waiting time.
“This is much like a flash in photography seems to freeze a fast movement,” Vogelsang said. “Attosecond pulses advance into a completely new time range that could not be investigated before. Visible light oscillates too slowly to produce such short flashes of light. For this reason, one moves into the ultraviolet range or even the soft X-ray range, where the oscillation period of light is shorter, and thus shorter pulses can be generated.”
This allows the elementary processes in atoms, such as electrons emitting photons of light as they change energy levels at ultra fast speeds, to be studied. This ability could lead to a better understanding of conductivity and chemical reactions, both influenced by this energy transition.
“Such time-resolved attosecond experiments already require relatively complex experimental set-ups. There are various approaches to combine them with the high spatial resolution of other methods, such as scanning probe techniques, coherent diffractive imaging, or electron microscopy,” Vogelsang continued. “One problem here is that two technologically demanding techniques must be combined without sacrificing resolution.”
He explained that this inevitably leads to technological challenges that can only be overcome with creativity and significant improvements in lasers, microscopes, and measurement concepts.
Vogelsang is one of the authors of a new study published in Advanced Physics Research, along with 2023 Nobel Laureate, L’Huillier, in which they attempt to further the use of attosecond pulses and overcome some of these issues by uniting the technique with electron microscopy, which uses electrons emitted by a sample and their wave-like characteristics to build an image of an object in the microscope.
Improving atomic dynamics with attosecond pulses
Improving on a Nobel-winning idea is no mean feat and requires the accumulation of knowledge gathered over the last 15 years.
“In this study, we pursued the combination of a subspecies of electron microscopy with attosecond spectroscopy,” said Vogelsang. “This has not yet been achieved with the time resolution now demonstrated. One reason for this is certainly that attosecond pulses are often not available in the quantity — pulses per second — that is essential for such experiments.”
Vogelsang said that improving attosecond pulse matter investigations hinges on several developments. For instance, the more pulses of light per second that are generated, the easier it is to get a small measurement signal from a large data set.
Once a measurement signal is obtained, it is then more straightforward for experimenters to try out new paths and thus change the sample — with sample selection and sample preparation important for these statistical-heavy investigations. Also, the stability and reproducibility of the lasers are vital for these attosecond pulse experiments.
For their research, the team chose zinc oxide as a substance to investigate because the electrons in its atoms have sharply defined energy states with a low binding energy — the amount of energy needed to completely liberate an electron from an atom.
“We were able to use these states for electron emission, so we could still observe a clear structure in the electron spectrum from the interaction with the electric field of another laser pulse,” Vogelsang explained. “Zinc oxide is also a widely studied material, so there is a lot of background information on its properties.”
A winning combination
The research showed that attosecond time-resolved photoemission electron microscopy (PEEM) is possible with current technology. This opens up the possibility of conducting interesting experiments, studying how electrons interact with each other and how they interact with optical fields on surfaces and nanostructures.
“The major breakthrough of this work is the demonstration of a time-resolved experiment with attosecond pulses in an electron microscope. This has been pursued for over 15 years but had not been achieved previously,” Vogelsang added. “In general, one can say that there is no “magic trick” here; many improvements had to be combined with hard work over a number of years to move this experiment forward.”
In the future and for the development of full-resolution PEEM, the physicist said that attosecond pulses must be focused on smaller areas without deteriorating the gathering of good statistical data. Additionally, the team will have to select the nanostructured samples for these studies carefully.
“In the past, whenever researchers developed new devices to get to the bottom of things even more precisely, they made new discoveries,” Vogelsang said. “We see here the possibility of exploring new areas of simultaneously high spatial and temporal resolution to investigate ultrafast dynamics on the nanoscale.
“On the one hand, it is basic research. On the other hand, you never know what exciting discovery you might make the next day,” he concluded.
Reference: Jan Vogelsang, et al., Time-resolved photoemission electron microscopy on a ZnO surface using an extreme ultraviolet attosecond pulse pair, Advanced Physics Research (2023). DOI: 10.1002/apxr.202300122
Feature image credit: Clyde He on Unsplash