'Spoof' structures provide laser beam strength
Researchers from Harvard University and the University of Leeds have shown how to tighten up the beam of terahertz rays (T-rays) produced from semiconductor lasers. This work is an important step towards making cheaper, compact T-ray sources that could be used for applications such as security screening, drug quality control and materials testing.
T-rays can penetrate efficiently through paper, clothing, plastic, and many other materials, making them ideal for detecting concealed illicit substances, imaging tumours without harmful side effects, and spotting defects such as cracks within materials. However, traditional systems for generating and detecting T-Rays are expensive, often costing in excess of £100,000. This has greatly limited the extent to which the terahertz (THz) part of the electromagnetic spectrum has been exploited.
To address this, semiconductor lasers, which can be mass-produced cheaply, have been designed for the THz frequency range. But up until now, the radiation from such lasers has not been well suited for any of these applications because the beam of T-rays spreads out too much.
A team of researchers led by Dr Nanfang Yu and Professor Federico Capasso (Harvard) and Professors Edmund Linfield and Giles Davies (Leeds), has now shown how semiconductor lasers can generate narrow beams of T-rays if the laser's edge is patterned with a 'spoof' optical structure. In this case, the spoof structure consisted of a series of tiny grooves of varying depths, with each groove being less than one hundredth of a millimetre in size.
The team produced a tightly bound, high powered beam of T-rays from the patterned semiconductor laser, with a frequency of 3 THz, without having to use any lenses.
"Our team was able to reduce the spread of the beam emerging from these semiconductor lasers dramatically," said Professor Linfield from the School of Electronic and Electrical Engineering, University of Leeds. "This was achieved without compromising the high output power that you get from unpatterned THz semiconductor lasers."
"This type of laser could be used by customs officials to detect illicit substances and by pharmaceutical manufacturers to check the quality of drugs being produced and stored," he said.
Spoof structures - or metamaterials - have not been used very much in semiconductor lasers up until now, but this was a critical part of the work, according to the researchers.
"The metamaterial serves a dual function: strongly confining the THz light emerging from the device to the laser facet [edge] and collimating the beam," said Nanfang Yu, postdoctoral fellow at Harvard's School of Engineering and Applied Sciences (SEAS). "The ability of metamaterials to confine THz waves strongly to surfaces makes it possible to manipulate them efficiently for applications such as on-chip sensing and THz optical circuits."
The advance is published in Nature Materials. Harvard has filed a broad patent on the invention.
The research was partially supported by the US Air Force Office of Scientific Research and the UK Engineering and Physical Sciences Research Council (EPSRC). The Harvard-based authors also acknowledge the support of the Centre for Nanoscale Systems (CNS) at Harvard University, a member of the National Nanotechnology Infrastructure Network (NNIN).
