Ultrafast microscopy sheds light on metallic nanoframe behavior

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Argonne and Northwestern University scientists teamed up to understand how light interacts with metallic nanoframes, with implications for biosensing, quantum information science and beyond.

Imagine using light to control chemical reactions that could break down pollutants or diagnose diseases. To bring about these potentially transformative advancements in catalysis, biosensing and related fields, scientists are focusing on an incredibly small matter: how light interacts with the individual molecules—even atoms—of tiny, custom-made metallic scaffolds, known as nanoframes.

At these intimate scales, packets of light—or photons—can trigger tiny electron "oscillations" in parts of these scaffolds. Knowing the precise location, size, orientation and evolution of these oscillations could allow researchers to design metallic nanoframes that can be controlled by photons—an essential step in realizing those paradigm-shifting applications.

Scientists at the U.S. Department of Energy's (DOE) Argonne National Laboratory partnered with a team at Northwestern University to visualize these oscillations in a class of metallic nanoframes that are promising candidates for applications in light-driven catalysis and biosensing. The team used advanced ultrafast electron microscopy techniques at Argonne's Center for Nanoscale Materials (CNM) to visualize and analyze the electron oscillations in nanoframes of various shapes made from gold and platinum.

Watching plasmon oscillations in motion

The team discovered that, when "excited" by ultrashort optical pulses, the electron oscillations—known as localized surface plasmon resonances—shift in space and time depending on the nanoframe's shape and size. They also showed that coupling between multiple nanoframes can influence the behavior of these oscillations, creating new opportunities for energy transfer and field enhancement.

"By capturing how light interacts with nanostructures in both space and time, we've opened a new window into the nanoscale world," said co-senior author Koray Aydin, associate professor of electrical and computer engineering at Northwestern University.

"Our work reveals how the shape and arrangement of metallic nanoframes can be harnessed to control energy flow, paving the way for advances in sensing, catalysis and quantum information sciences."

Probing nanoframes with ultrafast tools

At Northwestern, the team synthesized nanoframes of various shapes, including triangles and hexagons. They brought the nanoframes to the CNM and used photon-induced near-field electron microscopy (PINEM)—a variant of ultrafast electron microscopy—to probe the light-matter interactions within these nanostructures.

PINEM allowed the researchers to capture the spatial and temporal dynamics of the plasmonic fields with nanometer-scale resolution and femtosecond-scale precision.

The study also employed advanced computational simulations to model the electric field distributions and other optical properties of the nanoframes. These simulations complemented the experimental observations, providing deeper insights into the structure-function relationships of the nanoframes.

"This research demonstrates the power of ultrafast electron microscopy in revealing the intricate dynamics of plasmonic nanostructures," said co-senior author Haihua Liu, an electron microscopy scientist at Argonne.

"By combining experimental and computational approaches, we've gained a comprehensive understanding of how these nanoframes interact with light, which is critical for designing next-generation technologies in biosensing and energy."

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Potential impacts in sensing and energy

Nanoframes of this class are already being explored for their potential in biosensing, where their ability to amplify localized electric fields could lead to highly sensitive diagnostic tools.

In catalysis, these nanostructures could enable more efficient chemical reactions by concentrating energy at specific sites. Their unique optical properties also make them promising candidates for applications in certain cancer treatments and quantum information processing.

The study also shed light on a special type of coupling between nanoframes, plasmonic coupling, which could be leveraged to design more complex systems for energy harvesting and nanophotonic devices. For example, coupling between nanoframes can create "hotspots" of electric fields, which are critical for making light-driven processes more efficient.

"There are many different future directions for this line of research," said Liu. "It touches on so many different applications."

Key concepts

Quasiparticles & collective excitationsNanostructuresElectron techniques

Provided by Argonne National Laboratory