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  • Quantum Plasmon Resonance Illuminated
Mar 2012
STANFORD, Calif., March 29, 2012 — Plasmon resonance at the nanoscale, the phenomenon responsible for the vibrant hues of stained-glass windows such as those at Notre Dame Cathedral in Paris, can kill cancer cells and has the potential to drive forward quantum optics and bioimaging, new research suggests.

The physical phenomenon of plasmon resonances in small metal particles has been used for centuries, most noticeably in the vibrant hues of the great stained-glass windows of the world. More recently, plasmon resonances have been used by engineers to develop new, light-activated cancer treatments and to enhance light absorption in photovoltaics and photocatalysis.

"The stained-glass windows of Notre Dame Cathedral and Stanford Chapel derive their color from metal nanoparticles embedded in the glass," said Jennifer Dionne, an assistant professor of materials science and engineering at Stanford University. "When the windows are illuminated, the nanoparticles scatter specific colors depending on the particle's size and geometry."

Plasmon resonance is generally understood at the classical level, but, as with so many other physical phenomena, it seems to break down at the quantum level. "For particles smaller than about 10 nanometers in diameter, plasmon resonances are poorly understood," said Jonathan Scholl, a doctoral candidate in Stanford's School of Engineering.

At the quantum level, metal nanoparticles exhibit unique physical and chemical properties that are not seen in larger amounts of the same material. The geometry of the nanoparticle also affects how it behaves, especially if the nanoparticle is confined. In that case, the effects of quantum confinement take over how a particle responds to electrons and photons.

Plasmon resonance in metal nanoparticles of varying sizes. This experiment is the first to resolve plasmon resonance on 1-nm particles. (Credit: Stanford University)

"Particles at this scale are more sensitive and more reactive than bulk materials," said Dionne,senior author of a paper on the research. "But we haven't been able to take full advantage of their optical and electronic properties without a complete picture of the science."

That complete picture is now available, thanks to the powerful environmental scanning transmission electron microscope (E-STEM), recently installed at Stanford's Center for Nanoscale Science and Engineering. The microscope, along with a technique called electron energy-loss spectroscopy (EELS), allowed the researchers to determine the shape of individual nanoparticles, and to observe plasmon resonance of individual metal particles at the atomic level. For the first time, researchers can directly correlate a nanoparticle’s geometry with its plasmon resonance.

The researchers have created a model for plasmon resonance’s transition from classical to quantum. "Technically speaking, we've created a relatively simple, computationally light model that describes plasmonic systems where classical theories have failed," Scholl said.

Medical science is already using plasmon resonance in the fight against cancer. Molecular appendages, called ligands, are attached to metal nanoparticles so that the particle will attach itself to chemical receptors on cancerous cells. When infrared light is shone on these nanoparticles, they burn away the cancerous cell without harming the surrounding tissue, a technique called photothermal ablation. Understanding how even smaller nanoparticles work would be hugely beneficial to this process, allowing for greater precision and effectiveness.

Researchers believe that understanding plasmon resonance at the quantum level will offer other benefits. "We might discover novel electronic or photonic devices based on excitation and detection of plasmons in quantum-sized particles. Alternatively, there could be opportunities in catalysis, quantum optics, and bioimaging and therapeutics," Dionne said.

The research is to be published in Nature.

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quantum optics
The area of optics in which quantum theory is used to describe light in discrete units or ‘quanta’ of energy known as photons. First observed by Albert Einstein’s photoelectric effect, this particle description of light is the foundation for describing the transfer of energy (i.e. absorption and emission) in light matter interaction.
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