Scientific Method / Science & Exploration

Itty-bitty lasers hold hope for scalable optical processing

Researchers show that lasers that are just 1/10th the size of the wavelength …

One of the ever-present trends in electronics is that stuff gets smaller. Although it doesn't get much attention, the fact that electronic features can be scaled continuously in size down to something very close to the single atom level is an important reason why electronic devices are still king. The pretenders to the throne—optical devices—continue to sit off to one side while their supporters develop cunning plans for a takeover. All of those plans focus on making optical circuit elements smaller.

One of the main barriers to reducing the size of optical components is the wavelength of light. Visible light has a wavelength of around 500nm, so devices that manipulate light, like lenses and waveguides, must have comparable sizes. At least up until now—a long-awaited development has now provided a proof of principle, demonstrating that lasers can be made as small as 50nm, sizes that are comparable to current electronic features.

Normally, the absolute smallest length that laser hardware can have is a half wavelength, so the blue laser diode in your PS3 could be as short as 200nm (it's not though; it's considerably longer). Even worse, while the length may now be 200nm, the width and height have to be much bigger so that the end mirrors provide a good reflecting surface. These size issues have kept on-chip optics at something close to a stand-still for well over a decade.

In the last few years, scientists have begun to take fresh interest in the optical properties of metals. The way that light can cause the electrons in a metal oscillate coherently has led to new ideas for scaling down optical elements. These electron oscillations, called surface plasmon polaritons, have been shown to travel down wires just a few nanometers in diameter, providing the opportunity to scale things down.

These surface plasmon polaritons don't last very long though. The electrons in the metal quickly dissipate their energy by bouncing off the atoms in the wire, heating them up, while the remaining energy is re-radiated as light. To overcome this, you need an amplifier, and, as an initial source of light, a laser. Such a laser, called a spacer (surface plasmon laser), has been proposed on several occasions, but until now, nobody had actually produced one.

To create the spaser, researchers took gold nanospheres and coated them with a sodium glass. On top of that, they placed an outer shell of dye-impregnated glass. The inner gold sphere acts as the resonator for the light. The light remains on the outside of the gold sphere as the electrons inside slosh back and forth—the light field extends far enough that it passes through the dye-impregnated outer shell, which provides gain. 

The middle layer of sodium glass acts as a separator, preventing the dye molecules from interacting directly with the metal. Otherwise, the interaction would act to broaden the range of colors the dye molecule could emit and shorten the amount of time it spends in the excited state, where it can emit light.

The basic idea is that blue-green light is shone on the particles and absorbed by the dye. The excited dye molecules then begin to spontaneously emit green photons, some of which hit the chewy gold center, exciting a surface plasmon. The surface plasmon starts sloshing back and forth, and its field sweeps through the excited dye molecules, stimulating them to emit, adding to the surface plasmon. In the meantime, part of the energy stored in the surface plasmon radiates as coherent light.

The researchers observed that their spaser had all the characteristics expected of a laser: a threshold, narrow emission line, and relaxation oscillations. A threshold means that it requires a certain amount of input energy before a sufficient number of dye molecules contribute light to overcome the losses that occur as the electrons slosh about. Below this energy, a weak broad range of colors are emitted, while above, only a single color is emitted. 

Finally, after the energy is dumped into the dye, it takes some time for the spaser to start going but, once going, it quickly produces short, intense pulses of light, called relaxation oscillations—typically, we prefer to operate lasers so that only a single pulse is emitted on these occasions, but we can't always be choosy.

So, we now have 50nm laser hardware, which could conceivably be combined with nanowires to start developing optical circuits that really do look like electronic circuits (e.g., small and cheap). This laser was powered by a very powerful pump laser, meaning that it's only small if you ignore the enormous power supply. But that was a side effect of how the experiment was put together. A single spaser used about 20 microwatts of power, so much smaller pump sources are feasible. If they can achieve continuous wave operation, the researchers are on to a winner.

Nature, 2009, DOI: 10.1038/nature08318

Listing image by Patryk Buchcik

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