Plasmon

Quantum photon properties revealed in another particle—the plasmon

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For years, researchers have been interested in developing quantum computers—the theoretical next generation of technology that will outperform conventional computers. Instead of holding data in bits, the digital units used by computers today, quantum computers store information in units called “qubits.” One approach for computing with qubits relies on the creation of two single photons that interfere with one another in a device called a waveguide. Results from a recent applied science study at Caltech support the idea that waveguides coupled with another quantum particle—the surface plasmon—could also become an important piece of the quantum computing puzzle.

As their name suggests,  plasmons exist on a surface—in this case the surface of a metal, at the point where the metal meets the air. Metals are conductive materials, which means that electrons within the metal are free to move around. On the surface of the metal, these free electrons move together, in a collective motion, creating waves of electrons. Plasmons—the quantum particles of these coordinated waves—are akin to photons, the  of light (and all other forms of electromagnetic radiation). 

“If you imagine the surface of a metal is like a sea of electrons, then surface plasmons are the ripples or waves on this sea,” says graduate student Jim Fakonas, first author on the study. 

These waves are especially interesting because they oscillate at optical frequencies. Therefore, if you shine a light at the metal surface, you can launch one of these plasmon waves, pushing the ripples of electrons across the surface of the metal. Because these plasmons directly couple with light, researchers have used them in photovoltaic cells and other applications for solar energy. In the future, they may also hold promise for applications in quantum computing. 

However, the plasmon’s odd behavior, which falls somewhere between that of an electron and that of a photon, makes it difficult to characterize. “According to quantum theory, it should be possible to analyze these plasmonic waves using quantum mechanics”—the physics that governs the behavior of matter and light at the atomic and subatomic scale—”in the same way that we can use it to study electromagnetic waves, like light,” Fakonas says. However, in the past, researchers were lacking the experimental evidence to support this theory. 

To find that evidence, Fakonas and his colleagues in the laboratory of Harry Atwater, Howard Hughes Professor of Applied Physics and Materials Science, looked at one particular phenomenon observed of photons——to see if plasmons also exhibit this effect. 

The applied scientists borrowed their experimental technique from a classic test of quantum interference in which two single, identical photons are launched at one another through opposite sides of a 50/50 , a device that acts as an imperfect mirror, reflecting half of the light that reaches its surface while allowing the the other half of the light to pass through. If quantum interference is observed, both identical photons must emerge together on the same side of the beam splitter, with their presence confirmed by photon detectors on both sides of the mirror. 

Since plasmons are not exactly like photons, they cannot be used in mirrored optical beam splitters. Therefore, to test for quantum interference in plasmons, Fakonas and his colleagues made two waveguide paths for the plasmons on the surface of a tiny silicon chip. Because plasmons are very lossy—that is, easily absorbed into materials that surround them—the path is kept short, contained within a 10-micron-square chip, which reduces absorption along the way. 

The waveguides, which together form a device called a directional coupler, act as a functional equivalent to a 50/50 beam splitter, directing the paths of the two plasmons to interfere with one another. The plasmons can exit the waveguides at one of two output paths that are each observed by a detector; if both plasmons exit the directional coupler together—meaning that quantum interference is observed—the pair of plasmons will only set off one of the two detectors. 

Indeed, the experiment confirmed that two indistinguishable photons can be converted into two indistinguishable surface plasmons that, like photons, display quantum interference. 

This finding could be important for the development of , says Atwater. “Remarkably, plasmons are coherent enough to exhibit quantum interference in waveguides,” he says. “These plasmon waveguides can be integrated in compact chip-based devices and circuits, which may one day enable computation and measurement schemes based on quantum interference.” 

Before this experiment, some researchers wondered if the photon–metal interaction necessary to create a  would prevent the plasmons from exhibiting quantum interference. “Our experiment shows this is not a concern,” Fakonas says. 

“We learned something new about the quantum mechanics of surface plasmons. The main thing is that we were able to validate the theoretical prediction; we showed that this type of interference is possible with plasmons, and we did a pretty clean measurement,” he says. “The quantum interference displayed by  appeared to be almost identical to that of , so I think it would be very difficult for someone to design a different structure that would improve upon this result.”

Lasing action in strongly coupled plasmonic nanocavity arrays.

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Periodic dielectric structures are typically integrated with a planar waveguide to create photonic band-edge modes for feedback in one-dimensional distributed feedback lasers and two-dimensional photonic-crystal lasers1, 2, 3, 4. Although photonic band-edge lasers are widely used in optics and biological applications, drawbacks include low modulation speeds and diffraction-limited mode confinement5, 6. In contrast, plasmonic nanolasers can support ultrafast dynamics and ultrasmall mode volumes7, 8, 9. However, because of the large momentum mismatch between their nanolocalized lasing fields and free-space light, they suffer from large radiative losses and lack beam directionality. Here, we report lasing action from band-edge lattice plasmons in arrays of plasmonic nanocavities in a homogeneous dielectric environment. We find that optically pumped, two-dimensional arrays of plasmonic Au or Ag nanoparticles surrounded by an organic gain medium show directional beam emission (divergence angle <1.5° and linewidth <1.3 nm) characteristic of lasing action in the far-field, and behave as arrays of nanoscale light sources in the near-field. Using a semi-quantum electromagnetic approach to simulate the active optical responses, we show that lasing is achieved through stimulated energy transfer from the gain to the band-edge lattice plasmons in the deep subwavelength vicinity of the individual nanoparticles. Using femtosecond-transient absorption spectroscopy, we verified that lattice plasmons in plasmonic nanoparticle arrays could reach a 200-fold enhancement of the spontaneous emission rate of the dye because of their large local density of optical states.

Source: http://www.nature.com

 

is a �x o �&� �t� mprove the delivery rate, Anderson says.

“We believe that these particles can be made more efficient. They’re already very efficient, to the point where micrograms of drug per kilogram of animal can work, but these types of studies give us clues as to how to improve performance,” Anderson says.

Molecular traffic jam

The researchers found that once cells absorb the lipid-RNA nanoparticles, they are broken down within about an hour and excreted from the cells.

They also identified a protein called Niemann Pick type C1 (NPC1) as one of the major factors in the nanoparticle-recycling process. Without this protein, the particles could not be excreted from the cells, giving the siRNA more time to reach its targets. “In the absence of the NPC1, there’s a traffic jam, and siRNA gets more time to escape from that traffic jam because there is a backlog,” says Gaurav Sahay, an MIT postdoc and lead author of the Nature Biotechnology paper.

In studies of cells grown in the lab without NPC1, the researchers found that the level of gene silencing achieved with RNA interference was 10 to 15 times greater than that in normal cells.

Lack of NPC1 also causes a rare lysosomal storage disorder that is usually fatal in childhood. The findings suggest that patients with this disorder might benefit greatly from potential RNA interference therapy delivered by this type of nanoparticle, the researchers say. They are now planning to study the effects of knocking out the NPC1 gene on siRNA delivery in animals, with an eye toward testing possible siRNA treatments for the disorder.

The researchers are also looking for other factors involved in nanoparticle recycling that could make good targets for possibly slowing down or blocking the recycling process, which they believe could help make RNA interference drugs much more potent. Possible ways to do that could include giving a drug that interferes with nanoparticle recycling, or creating nanoparticle materials that can more effectively evade the recycling process.

“This paper describes a new and very important way to improve the potency of siRNA delivery systems by inhibiting proteins that recycle imported material back out of the cell,” says Pieter Cullis, a professor of biochemistry and molecular biology at the University of British Columbia who was not part of the research team. “It is possible that this approach will give rise to the order-of-magnitude improvements in potency required for siRNA-based therapeutics to be more generally effective agents to treat disease.”

The research was funded by Alnylam Pharmaceuticals and the National Heart, Lung, and Blood Institute.

 

Source: http://web.mit.edu

 

Z�htX1� @�� elial-mesenchymal transition — a process that allows cancer cells to lose their adhesion and become mobile, helping them metastasize.

Other authors of the paper are MIT postdoc Sungmin Son; Stanford University postdoc Dario Amodei; MIT grad students Nathan Cermak, Joon Ho Kang and Vivian Hecht; former MIT postdoc Monte Winslow; Tyler Jacks, the David H. Koch Professor of Biology at MIT and director of the Koch Institute; and Parag Mallick, an assistant professor of radiology at Stanford.

The research was funded by the National Cancer Institute, through MIT’s Physical Sciences Oncology Center and Stanford’s Center for Cancer Nanotechnology Excellence and Translation, and Stand Up to Cancer.

 

Source: http://web.mit.edu