Nonlinear laser light at the nanoscale

STANFORD (US) — By harnessing plasmonics to intensify light, engineers have created an ultra-compact, nanoscale light source that could ultimately find applications in data communications.

Previous research showed that applying an electrical field to crystals produced a similar, though weaker, beam of light. This discovery, known as EFISH—for electric-field-induced second harmonic light generation—hasn’t amounted to more than an interesting bit of scientific knowledge, researchers say, because EFISH devices are big and demand high-powered lasers, large crystals, and thousands of volts of electricity to produce the effect. As a result, they are impractical for all but a few applications.

A paper published in Science details a new device that shrinks EFISH devices to the nanoscale, resulting in an ultra-compact light source with both optical and electrical functions.


Spring-loaded electrons

The device is based on the physical forces that bind electrons in orbit around the nucleus of an atom.

“It’s like a spring,” says Mark Brongersma, associate professor of materials science and engineering at Stanford University.

In most cases, when you shine a light on an atom, the added energy will predictably pull the electron away from the positively charged nucleus in a linear fashion, so that when the light is turned off and the electron springs back to its original orbit, the energy released is the same as the light that displaced it.

The key phrase here being “in most cases.” When the light source is a high-intensity laser shining on a solid, researchers discovered that the farther the electrons are pulled away from the nuclei the less linearly the light interacts with the atoms.

“In other words, the light-matter interaction becomes nonlinear,” says Alok Vasudev, a graduate student and co-author of the paper. “The light you get out is different from the light you put in.”

Engineering possibilities

“Now, Alok and I have taken this knowledge and reduced it to the nanoscale,” says the paper’s first author, Wenshan Cai, a postdoctoral researcher in Brongersma’s lab. “For the first time we have a nonlinear optical device at the nanoscale that has both optical and electrical functionality. And this offers some interesting engineering possibilities.”

For many photonic applications, including signal and information processing, it is desirable to electrically manipulate nonlinear light generation. The new device resembles a nanoscale bowtie with two halves of symmetrical gold leaf approaching, but not quite touching, in the center. This thin slit between the two halves is filled with a nonlinear material. The narrowness—it’s just 100 nanometers across, is critical.

“If you have two electrodes placed extremely close together, as we do in our experiment, it doesn’t take many volts to produce a giant electrical field. In fact, it takes just a single volt.

“It is this fundamental science that allows us to shrink the device by orders of magnitude from the human scale to the nanoscale,” says Cai.

Enter plasmonics

Plasmonics, the study of a curious physical phenomenon that occurs when light and metal interact, then enters the scene. As photons strike metal they produce waves of energy coursing outward over the surface of the metal, like the ripples when a pebble is dropped in a pond.

Engineers have learned to control the direction of the ripples by patterning the surface of the metal in such a way that almost all of the energy waves are funneled inward toward the slit between the two metallic electrodes.

The light pours into the crevice as if over the edge of a waterfall and there it intensifies, producing light some 80 times stronger than the already intense laser levels from which it came. The researchers next apply a modest voltage to the metal—resulting in the tremendous electrical field necessary to produce an EFISH beam.

Practical applications

“This type of device may one day find application in the communications industry,” says Brongersma. “Most of the masses of information and social media interaction we send through our data centers, and the future data we will someday create, are saved and transmitted as electrical energy—ones and zeros.”

“Those ones and zeros are just a switch; one is on, zero is off,” says Cai. “As more energy-efficient optical information transport is rapidly gaining in importance, it is not a great leap to see why devices that can convert electrical to optical signals and back are of great value.”

For the time being, however, the researchers caution that practical applications remain down the road, but they have created something new.

“It’s a great piece of basic science,” says Brongersma. “It is work that combines several disciplines—nonlinear optics, electronics, plasmonics, and nanoscale engineering—into a really interesting device that could keep us busy for a while.”

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