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, director at UC Berkeley of a National Science Foundation (NSF) Nanoscale Science and Engineering Center, and head of the research team behind this work.
A bright point of light from a single plasmon laser emanates from the optical setup used by UC Berkeley researchers (enlarged closeup at right). These semiconductor lasers — the world’s smallest — are extremely efficient, so the small amount of scattered light is clearly visible, even in ambient room lighting. Camera saturation of the bright laser light gives the impression of a larger spot. (Courtesy of Xiang Zhang Lab/UC Berkeley)
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.
DEFECT IN THIS PRINCIPLE:
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.
STEPS TO IMPROVISE THE DEFECT:
Zhang, a professor of mechanical engineering, 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.
IMPLEMENTING THE IMPROVISED PRINCIPLE IN SEMICONDUCTOR LASER:
The UC Berkeley researchers used semiconductor materials and fabrication technologies that are commonly employed in modern electronics manufacturing. By engineering hybrid surface plasmons in the tiny gap between semiconductors and metals, they were able to sustain the strongly confined light long enough that its oscillations stabilized into the coherent state that is a key characteristic of a laser.
COMMENTS OF SCIENTISTS:
What is particularly exciting about the plasmonic lasers we demonstrated here 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 study co-lead author.
“Plasmon lasers represent an exciting class of coherent light sources capable of extremely small confinement,” said Zhang. “This work can bridge the worlds of electronics and optics at truly molecular length scales.”
Scientists hope to eventually shrink light down to the size of an electron’s wavelength, which is about a nanometer, or one-billionth of a meter, so that the two can work together on equal footing.
“The advantages of optics over electronics are multifold,” added Thomas Zentgraf, a post-doctoral fellow in Zhang’s lab and another co-lead author of the Nature paper. “For example, devices will be more power efficient at the same time they offer increased speed or bandwidth.”