Nanoscale lasers are about to get even smaller
August 4, 2009
In a collaborative effort between the Arizona State University and Technical University of Eindhoven in the Netherlands, researchers have found a way to make optical lasers much smaller than it was previously thought was possible, making dreams of speedier computers and faster Internet access closer to reality than ever before.
During the past few decades, laser technology has become ubiquitous in the world of electronics: from CD/DVD players to barcode scanners, from bloodless surgery to laser printers, the impact of this technology on our everyday life is growing at an outstanding pace. Scientists have also long realized their potential in obtaining significantly faster communication. However, in order to achieve this, we need the technology to miniaturize lasers and integrate as many as possible on a single chip.
Pushing the limits of laser miniaturization
Only a few years ago, scientists believed the minimal dimension for a laser to be exactly one half of the wavelength involved, thus putting a strict, unbreakable 750 nanometers barrier for lasers used in optical communication, where it is common to employ a 1,500 nanometer wavelength.
But when light travels in a denser medium such as a semiconductor this limit can be further reduced to account for the material's refraction index. A refraction index expresses the relationship between the angle that a beam forms before and after propagating from one material to the other, with an effect you can easily witness whenever you put a spoon behind an half-empty glass of water.
With a refraction index of about 3.0, the minimum dimension for an optical laser can therefore be reduced from the previous 750 nanometers to about 250 nanometers. The reason that makes this possible is that light is slowed down by a factor of three when propagating within a semiconductor.
A widespread theory among scientists states that it is not possible to break the diffraction barrier, thus putting a severe limit to dreams of downscaling and integration. But researchers have recently found an interesting workaround that has disproved this previously widely accepted theory.
A metal-semiconductor sandwich will do the trick
One way that the researchers found to effectively reduce the size of optical lasers is to employ a combination of semiconductors and metals like gold and silver. Once hit by a beam of light, in fact, the electrons that have reached an 'excited state' in metals can help confine light in a laser, further reducing the refraction limit.
The structure used by the researchers was described as a 'metal-semiconductor-metal sandwich' in which the semiconductor is as thin as 80 nanometers and lays between 20-nanometer dielectric layers. This layer, the researchers demonstrated, can emit a laser beam with the smallest thickness ever produced.
The structure, however, has worked only at cryogenic temperatures. Part of the system, though, is already fully functional at room temperatures and the next step for the team will be to achieve the same kind of light emission under normal operating temperatures.
This research also represents a major advance in nanophotonics, which entails studying the behavior of light on the nanoscale level and the building of structures and devices at that scale. Nanolasers can be used for many applications, the most promising of which are for communications on a central processing unit (CPU) of a computer chip.