The laser whisperer

OUT OF THE DEEP FREEZE: Mechanical engineering professor Xiang Zhang and his team of researchers discovered how to successfully operate plasmon lasers at room temperature, opening the door for further scientific achievements using laser technology. (Photo by Peg Skorpinski.)

The U.S. Capitol’s Statuary Hall. Grand Central Terminal in New York. St. Paul’s Cathedral in London. The rotunda at San Francisco’s City Hall. Visitors to these landmarks have long been fascinated by a remarkable acoustical feature within their walls: the whispering gallery. Stand at one point in the room—beneath a dome or arch—and speak softly, and that sound will emanate across the room to another distant point, where it can be heard clearly.

A research team led by Xiang Zhang, professor of mechanical engineering and faculty scientist at Lawrence Berkeley Laboratory, has taken inspiration from the phenomena of whispering galleries to achieve a major scientific breakthrough in the use of plasmon lasers.

By creating a technique to bounce surface plasmons inside of a nanosquare device, much in the way sound waves reflect back and forth in a whispering gallery, the team was able to operate plasmon lasers at room temperature, overcoming what had been a major barrier to practical utilization of the technology.

“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,” says Zhang, who also directs UC Berkeley’s Center for Scalable and Integrated Nanomanufacturing.

In recent years, scientists have turned to plasmon lasers, which can generate light well below its natural diffraction limit of half a wavelength. Plasmon lasers work by coupling electromagnetic waves with the electrons that oscillate at the surface of metals, allowing them to squeeze light into nanoscale spaces. Last year, Zhang’s team reported a plasmon laser that generated visible light in a space only 5 nanometers wide—about the size of a single protein molecule.

But the efforts to utilize such advancements for commercial purposes had hit a wall of ice.

“To operate properly, plasmon lasers need to be sealed in a vacuum chamber cooled to cryogenic temperatures as low as 10 Kelvin, or minus 441 degrees Fahrenheit, so they have not been usable for practical applications,” says Renmin Ma, a post-doctoral researcher in Zhang’s lab and co-lead author of the paper.

MIND THE GAP: 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 most intense electric fields of the device reside in the magnesium fluoride gap. (Photo by Renmin Ma and Rupert Oulton.)

In previous designs, most of the light produced by the laser had leaked out. To sustain the laser operation, researchers had to increase amplification of the remaining light energy, which meant putting the materials into a deep freeze.

But in taking a cue from the design of whispering galleries, the team developed a total internal reflection technique. The configuration was made out of a cadmium sulfide square, measuring 45 nanometers thick and 1 micrometer long, which was placed on top of a silver surface and separated by a 5-nanometer gap of magnesium fluoride.

With this new design, the team was able to enhance the emission rate of light 18-fold, yet confine the light to a space of about 5 nanometers. Because they were able to control the loss of light, there was no need to amplify the remaining light energy, and the laser could be used at room temperature.

Rupert Oulton, a former post-doctoral researcher in Zhang’s lab and now a lecturer at Imperial College London, is the other co-lead author of the paper. Other co-authors are Volker Sorger (Ph.D.’12 ME) and Guy Bartal, a former research scientist in Zhang’s lab.

Their groundbreaking achievement, described in the December 2010 online edition of Nature Materials, is a “major step towards applications” for plasmon lasers, says Zhang.