Researchers at the University of California, Berkeley, have reached a new milestone in laser physics by creating the world’s smallest semiconductor laser, capable of generating visible light in a space smaller than a single protein molecule.
This breakthrough, described in an advanced online publication of the journal Nature on Sunday, Aug. 30, breaks new ground in the field of optics. The UC Berkeley team not only successfully squeezed light into such a tight space, but found a novel way to keep that light energy from dissipating as it moved along, thereby achieving laser action.
“This work shatters traditional notions of laser limits, and makes a major advance toward applications in the biomedical, communications and computing fields,” said Xiang Zhang, professor of mechanical engineering and director of UC Berkeley’s Nanoscale Science and Engineering Center, which is funded by the National Science Foundation (NSF), and head of the research team behind this work.
The achievement helps enable the development of such innovations as nanolasers that can probe, manipulate and characterize DNA molecules; optics-based telecommunications many times faster than current technology; and optical computing in which light replaces electronic circuitry with a corresponding leap in speed and processing power.
While it is traditionally accepted that an electromagnetic wave – including laser light – cannot 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 plasmons.
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 these 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,” said 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. It’s pulling double duty.” …