Laser Breakthrough Opens Door To DNA Manipulation

Heralded as a breakthrough in laser technology that will benefit biomedicine by opening the door to DNA manipulation and other applications, scientists in the US have made the world's smallest semiconductor laser that can focus light in a space smaller than a single protein molecule.

The research that led to the breakthrough was done by Dr Xiang Zhang, professor of mechanical engineering and director of University of California Berkeley's Nanoscale Science and Engineering Center, and colleagues, and is described in the 30 August advanced online issue of the journal Nature.

The research breaks new ground in the field of optics because Zhang and colleagues have not only found a way to squeeze light into a tiny space but also found a way to stop it dissipating as it moves along, ie it maintains true laser properties.

For some time now optics experts have been experimenting with trying to make electromagnetic beams smaller and smaller, and it has been traditionally accepted that you can't compress such waves into spaces smaller than half their wavelength.

But as, Zhang said:

"This work shatters traditional notions of laser limits, and makes a major advance toward applications in the biomedical, communications and computing fields."

He said their achievement opens the door to the development of "nanolasers" that will be able to probe, manipulate and characterize DNA molecules. In other fields like telecommunications, it will enable data to be carried at speeds many times faster than current technology. And it will help the development of optical computers where light replaces electronic circuits, enabling huge leaps in speed and processing power.

Researchers have been trying to compress light down to the dozens of nanometers, reaching the traditionally held limit of half of its wavelenght, by binding the light to the electrons that oscillate together on the surface of metals, creating what are called surface plasmons: where light and oscillating electrons interact.

One of the problems of making lasers this small is that the natural electrical resistance of the metals cause the plasmons to dissipate very quickly, and the challenge is how to overcome this plus keep the integrity of the excitation between the light and oscillating electrons going continuously (amplification).

Zhang and colleagues created a structure capable of storing light energy in a non-metallic gap that was some 20 times smaller than its wavelength. The gap is about 5 nanometers, about the size of a single protein molecule. They made the structure out of cadmium sulfide nanowire (about 1,000 times thinner than human hair) with a silver surface.

The new structure overcame the problem of energy dissipation, now there remained the problem of amplification.

But what they found was the nanowire gap did both jobs: it was a confinement mechanism that stopped losses and it was an amplifier.

"It's pulling double duty," said lead author Dr Rupert Oulton, a research associate in Zhang's lab who first came up with the nanowire gap idea last year.

This was a real bonus because in such small spaces there is not much room to play with and by doing both jobs the new "hybrid" structure saved having to put another device in that space.

The authors wrote that holding and sustaining light in such a small space alters radically the way it interacts with matter and causes a significant increase in the spontaneous emission rate of light. Zhang and colleagues measured a six-fold increase in the spontaneous emission rate.

Biochemists are already using plasmons to look at protein to protein interaction. More conventional methods rely on labelling one protein with a fluorescent dye of one colour and another protein with a dye of another colour. This method has limits in that sometimes the dyes change the way the proteins behave, and also, when the labels sit on top of each other, you can't see if the proteins have really interacted.

So biochemists are really keen to have tools that allow them to see how proteins interact in their natural form, and devices that rely on plasmon resonance allow them to do that. They use the fact that plasmons require radiation of specific wavelengths to oscillate, and proteins attached to the surfaces that generate the plasmons change the frequency at which they resonate. By measuring changes in resonance, the scientists can tell what is happening at the protein-protein interaction level.

Now with this new breakthrough from Zhang and colleagues, the "probe" has got even smaller, making it more possible that one day, devices like plasmon nanolasers will be able to explore and manipulate at the DNA level.