A collaborative team of scientists at Harvard and the University of Leeds have demonstrated a new terahertz (THz) semiconductor laser that emits beams with a much smaller divergence than conventional THz laser sources. The advance, published in NatureMaterials, opens the door to a wide range of applications in terahertz science and technology. Harvard has filed a broad patent on the invention.The finding was spearheaded by postdoctoral fellow Nanfang Yu andFederico Capasso, the Robert L. Wallace Professor of Applied Physics andVinton Hayes Senior Research Fellow in Electrical Engineering, both ofHarvard’s School of Engineering and Applied Sciences (SEAS), and by ateam led by Edmund Linfield at the School of Electronic and ElectricalEngineering, University of Leeds.Terahertz rays (T-rays) can penetrate efficiently through paper,clothing, plastic, and many other materials, making them ideal fordetecting concealed weapons and biological agents, imaging tumorswithout harmful side effects, and spotting defects, such as cracks,within materials. THz radiation is also used for high-sensitivitydetection of tiny concentrations of interstellar chemicals.“Unfortunately, present THz semiconductor lasers are not suitable formany of these applications because their beam is widelydivergent—similar to how light is emitted from a lamp” says Capasso. “Bycreating an artificial optical structure on the facet of the laser, wewere able to generate highly collimated (i.e., tightly bound) rays fromthe device. This leads to the efficient collection and highconcentration of power without the need for conventional, expensive, andbulky lenses.”Specifically, to get around the conventional limitations, theresearchers sculpted an array of sub-wavelength-wide grooves, dubbed ametamaterial, directly on the facet of quantum cascade lasers. Thedevices emit at a frequency of 3 THz (or a wavelength of one hundredmicrons), in the invisible part of the spectrum known as the far-infrared.“Our team was able to reduce the divergence angle of the beam emergingfrom these semiconductor lasers dramatically, whilst maintaining thehigh output optical power of identical unpatterned devices,” saysLinfield. “This type of laser could be used by customs officials todetect illicit substances and by pharmaceutical manufacturers to checkthe quality of drugs being produced and stored.”The use of metamaterials, artificial materials engineered to provideproperties which may not be readily available in Nature, was critical tothe researchers’ successful demonstration. While metamaterials havepotential use in novel applications such as cloaking, negativerefraction and high resolution imaging, their use in semiconductordevices has been very limited to date.“In our case, the metamaterial serves a dual function: stronglyconfining the THz light emerging from the device to the laser facet andcollimating the beam,” explains Yu. “The ability of metamaterials toconfine strongly THz waves to surfaces makes it possible to manipulatethem efficiently for applications such as sensing and THz optical circuits.”Additional co-authors of the study included Qi Jie Wang, formerly ofHarvard University and now with the Nanyang Technological University inSingapore; graduate student Mikhail A. Kats and postdoctoral fellowJonathan A. Fan, both of Harvard University; and postdoctoral fellowsSuraj P. Khanna and Lianhe Li and faculty member A. Giles Davies, allfrom the University of Leeds.The research was partially supported by the Air Force Office ofScientific Research. The Harvard-based authors also acknowledge thesupport of the Center for Nanoscale Systems (CNS) at Harvard University,a member of the National Nanotechnology Infrastructure Network (NNIN).The Leeds-based authors acknowledge support from the UK’s Engineeringand Physical Sciences Research Council.