The team was not only able to successfully squeeze light into a remarkably tight space, but found a novel way to keep the light energy from dissipating as it moves along. "This work shatters traditional notions of laser limits," says Xiang Zhang, a professor of mechanical engineering and director of UC Berkeley's Nanoscale Science and Engineering Center. The research team recently published a study on their discovery.
While it is traditionally accepted that an electromagnetic wave--including laser light--can’t be focused beyond the size of half its wavelength, research teams around the world have found a way to compress light down to dozens of nanometers by binding it to the electrons that oscillate collectively at the surface of metals. This interaction between light and oscillating electrons is known as “surface plasmon resonance.”
Scientists have been racing to construct surface plasmon lasers that can sustain and utilize these tiny optical excitations. However, the resistance inherent in metals causes surface plasmons to dissipate almost immediately after being generated, posing a critical challenge to achieving the buildup of the electromagnetic field necessary for lasing.
Zhang and his research team took a novel approach to stem the loss of light energy by pairing a cadmium sulfide nanowire--1,000 times thinner than a human hair--with a silver surface separated by an insulating gap of only 5 nanometers, the size of a single protein molecule. In this structure, the gap region stores light within an area 20 times smaller than its wavelength. Because light energy is largely stored in this tiny non-metallic gap, loss is significantly diminished. With the loss finally under control through this unique "hybrid" design, the researchers could then work on amplifying the light.
"When you are working at such small scales, you do not have much space to play around with," says Rupert Oulton, the research associate in Zhang's lab who first theorized this approach last year and the study's co-lead author. "In our design, the nanowire acts as both a confinement mechanism and an amplifier,” Oulton says. “It's pulling double duty."
"What is particularly exciting about the plasmonic lasers we demonstrated ... is that they are solid-state and fully compatible with semiconductor manufacturing, so they can be electrically pumped and fully integrated at chip-scale," said Volker Sorger, a Ph.D. student in Zhang's lab and the study’s co-lead author.
