Engineers take plasmon lasers out of deep freeze

December 20, 2010 By Sarah Yang, University of California - Berkeley

Schematic of a plasmon laser showing a cadmium sulfide (CdS) square atop a silver (Ag) substrate separated by a 5 nanometer gap of magnesium fluoride (MgF2). The cadmium sulfide square measures 45 nanometers thick and 1 micrometer long. The most intense electric fields of the device reside in the magnesium fluorid gap. (Renmin Ma and Rupert Oulton)
( -- Researchers at the University of California, Berkeley, have developed a new technique that allows plasmon lasers to operate at room temperature, overcoming a major barrier to practical utilization of the technology.

The achievement, described Dec. 19 in an advanced online publication of the journal , is a "major step towards applications" for plasmon lasers, said the research team's principal investigator, Xiang Zhang, UC Berkeley professor of mechanical engineering and faculty scientist at Lawrence Berkeley National Laboratory.

"Plasmon lasers can make possible single-molecule biodetectors, photonic circuits and high-speed optical communication systems, but for that to become reality, we needed to find a way to operate them at room temperature," said Zhang, who also directs at UC Berkeley the Center for Scalable and Integrated Nanomanufacturing, established through the National Science Foundation's (NSF) Nano-scale Science and Engineering Centers program.

In recent years, scientists have turned to plasmon lasers, which work by coupling with the electrons that oscillate at the surface of metals to squeeze into nanoscale spaces far past its natural diffraction limit of half a wavelength. Last year, Zhang's team reported a plasmon laser that generated visible light in a space only 5 nanometers wide, or about the size of a single protein molecule.

But efforts to exploit such advancements for commercial devices had hit a wall of ice.

"To operate properly, plasmon lasers need to be sealed in a cooled to cryogenic temperatures as low as 10 kelvins, or minus 441 degrees Fahrenheit, so they have not been usable for practical applications," said Renmin Ma, a post-doctoral researcher in Zhang's lab and co-lead author of the Nature Materials paper.

Electron microscope image of the plasmon laser. (Renmin Ma and Rupert Oulton)

In previous designs, most of the light produced by the laser leaked out, which required researchers to increase amplification of the remaining light energy to sustain the laser operation. To accomplish this amplification, or gain increase, researchers put the materials into a deep freeze.

To plug the light leak, the scientists took inspiration from a whispering gallery, typically an enclosed oval-shaped room located beneath a dome in which sound waves from one side are reflected back to the other. This reflection allows people on opposite sides of the gallery to talk to each other as if they were standing side by side. (Some notable examples of whispering galleries include the U.S. Capitol's Statuary Hall, New York's Grand Central Terminal, and the rotunda at San Francisco's city hall.)

Instead of bouncing back sound waves, the researchers used a total internal reflection technique to bounce surface plasmons back inside a nano-square device. The configuration was made out of a cadmium sulfide square measuring 45 nanometers thick and 1 micrometer long placed on top of a silver surface and separated by a 5 nanometer gap of magnesium fluoride.

The scientists were able to enhance by 18-fold the emission rate of light, and confine the light to a space of about 20 nanometers, or one-twentieth the size of its wavelength. By controlling the loss of radiation, it was no longer necessary to encase the device in a vacuum cooled with liquid helium. The laser functioned at room temperature.

"The greatly enhanced light matter interaction rates means that very weak signals might be observable," said Ma. "Lasers with a mode size of a single protein are a key milestone toward applications in ultra-compact light source in communications and biomedical diagnostics. The present square plasmon cavities not only can serve as compact light sources, but also can be the key components of other functional building-blocks in integrated circuits, such as add-drop filters, direction couplers and modulators."

Explore further: Plasmonic whispering gallery microcavity paves the way to future nanolasers

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5 / 5 (1) Dec 20, 2010
Omega Point, here we come.
5 / 5 (1) Dec 20, 2010
Wow, can you say tricorder Mccoy.
not rated yet Dec 21, 2010

given the "folding" method I proposed as a first step to translating our existing technologies to optical technologies, it should be possible to see computers thousands, even millions of times more powerful than anything in existence within just a few years.

It's hard to even comprehend the amount of space you can save at every level of design of a computer, by the combination of getting into 3-d, and the optical circuitry cutting out most of the space wasted by wires.

This news comes faster and faster. We are seeing breakthroughs that are an order of magnitude more potential every year or two.

Now they need to get to work actually designing a 3-d chip architecture, and the 3-d tools that will be needed to build them.

It might take several years before engineers can figure out what manner of design allows the greatest density of processors and ram in 3-d.
4 / 5 (1) Dec 21, 2010
Do I understand that this device is 1 micrometer * 5nm *45nm?

The scale in the photo does not seem consistent with the description of the device, since that appears to be about 1 micron * 1 micron square device...
not rated yet Dec 21, 2010
NO, it's 1 um on a side by 45nm high. They don't say what the other dimension is. 5nm is the thickness of the MgF2 layer, and gets added to the 45nm of CdS. Even though they repeatedly call it a square, the object in the picture is not square.
not rated yet Dec 21, 2010

Hmmm...seems too big to be useful then?

I mean if that's 1 micron * 0.75micron * 50nm, that's completely huge compared to existing transistors.

One of the other optical circuits on another article is like 7000 atoms. By comparison, this device is several million atoms.

Not quite what we had hoped...

It still be used to replace electronic buses, but seems far too big to be particularly useful at the chip scale...
not rated yet Dec 21, 2010

I'm wrong, I'm forgetting how primitive our 2-d architecture is compared to what optics and 3-d allows.

It would still be worth it to use these, in spite of their size, even in individual chips, because it still facilitates the "folding" into 3d, which saves so much space even if the only thing you count is wires that you no longer need, or when you still need wires they can run in one straight line or groups of "almost straight lines," instead of circumventing everything...

It's not nearly as good as it would be if it were the same size as an intel transistor, but it is still worth it at that scale, even though I was initially pissed when I realized how big it was...
not rated yet Dec 21, 2010
When they get the manufacturing technology to fully make use of this in 3-d architecture, and by the time they fully miniaturize all components and find the most efficient design for space and power needs, it's going to be possible to make a hand held computer, about the size of a smart phone, which has about as much total computing power as an entire cloud computing service currently has.

Further, it will be so efficient on power usage, that you'll probably be able to power it with piezo-electric devices just by shaking it a few times, etc.

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