From reading DVDs to purifying drinking water, semiconductor lasers have found a remarkable array of uses in modern technology. Now, they could find even wider application thanks to researchers in the US who have developed a new type of device that could mean smaller, more powerful and cheaper ultraviolet lasers. The technology could lead to a CD that could store up to six hours of music, and might even provide a new way of probing single biological cells.
The amount of information stored on a CD depends on how finely the tracks are cut. But the tracks on a CD cannot be so fine that a laser cannot read them. This minimum size is called the diffraction limit and is about half the wavelength of the laser light.
Ultraviolet challenge
To cram more data into ever finer grooves, successive generations of technology have used lasers with ever shorter wavelengths. An audio CD player uses a 780 nm (near-infrared) laser, whereas a DVD player uses 657 nm red laser light and Blu-ray devices use 405 nm violet light. The obvious next step is an ultraviolet laser, but creating a suitable device has been a challenge.
A semiconductor laser contains a p–n junction: with the n-region containing free electrons and the p-region containing positive “holes”. When a voltage is applied, the electrons and holes move towards each other and combine to create light. The wavelength of the light depends on the semiconductor, with gallium nitride previously being considered the best candidate for ultraviolet lasers. However, gallium nitride does not make an efficient laser at room temperature because too much heat is released when the electrons and holes combine.
How to make a p-region?
To get round this problem Sheng Chu and colleagues led by Jianlin Liu at the University of California, Riverside have been working with zinc oxide instead. The challenge facing the team was how to make a p-region in zinc oxide – the n-region is easy. Previous work by the Riverside group and others had shown that doping the zinc oxide with small amounts of antimony will produce the necessary holes. The difficulty is producing a single crystal containing both an n-region and a p-region, so that electrons and holes can flow freely between the two.
Liu’s team did this by growing long, thin crystals of antimony-doped zinc oxide on a thin film of pure zinc oxide (see above). These nanowires have diameters of about 200 nm and are about 3 µm long. Just as the team had hoped, the ends of the nanowires fused into a single crystal with the thin film underneath. Tests revealed that the device worked extremely well as a prototype ultraviolet laser, producing light with a variety of wavelengths closely spaced around 385 nm.
Further development needed
Ritesh Agarwal of the University of Pennsylvania, who was not involved in the work, is impressed and feels that the technology should be further developed. “[The researchers] have demonstrated large-area lasing devices, but the true potential of nanowires will be realized when single-nanowire laser diodes can be fabricated with ease. This still remains a big challenge in this field,” he says.
Chu also feels the real significance of the research may lie in single-nanowire lasers that could be used to study living cells. “If we develop our method further, I hope we can insert this tiny laser into the cell or even smaller tissue inside the cell. If this technology were to be realized, then it would be a powerful tool for doing basic biological and biomedical research into the single cell and perhaps even for killing viruses,” he told physicsworld.com.
The work is reported in Nature Nanotechnology.
