9 August 2010


Harvard–Leeds team boost THz laser collimation and power collection efficiency

A team of applied scientists at the USA’s Harvard University and the UK’s University of Leeds have demonstrated a new terahertz (THz) semiconductor laser that emits beams with a much smaller divergence than conventional THz laser sources (Nanfang Yu et al, ‘Designer spoof surface plasmon structures collimate terahertz laser beams’, Nature Materials, doi:10.1038/nmat2822). The advance opens the door to a wide range of applications in terahertz science and technology, it is claimed. Harvard has filed a broad patent on the invention.

The finding was spearheaded by postdoctoral fellow Nanfang Yu and Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering, both of Harvard’s School of Engineering and Applied Sciences (SEAS), and by a team led by Edmund Linfield at the University of Leeds’ School of Electronic and Electrical Engineering.

Quantum cascade lasers (QCLs) emitting at terhaertz frequencies in the mid-infrared region of the spectrum were first demonstrated by Capasso and his team at Bell Labs in 1994. The compact millimeter-long lasers can operate at room temperature with high optical powers and are increasingly used in the commercial sector for a wide range of military and civilian applications in infrared countermeasures and chemical sensing. By varying the thickness of the device’s layers, energy levels in the structure can be adjusted, allowing tunability for various applications.

In particular, terahertz rays (T-rays) can penetrate efficiently through paper, clothing, plastic, and many other materials, making them suitable for detecting concealed weapons and biological agents, imaging tumors without harmful side effects, and spotting defects (such as cracks) within materials. THz radiation is also used for high-sensitivity detection of tiny concentrations of interstellar chemicals.

“Unfortunately, present THz semiconductor lasers are not suitable for many of these applications because their beam is widely divergent,” says Capasso. “By creating an artificial optical structure on the facet of the laser, we were able to generate highly collimated (i.e. tightly bound) rays from the device,” he adds. “This leads to the efficient collection and high concentration of power without the need for conventional, expensive, and bulky lenses.”

Specifically, to get around the conventional limitations, the researchers fabricated an array of sub-wavelength-wide grooves (using a metamaterial) directly on the facet of quantum cascade lasers. The devices emit at a frequency of 3THz (a wavelength of 100 microns) in the far-infrared part of the spectrum.

Picture (right): Deep ‘pink’ grooves form an effective grating that coherently scatters the energy of the surface waves into the far-field.




Picture (left): Metamaterial patterns are fabricated directly on the highly doped GaAs facet of the device. Artificial coloring shows deep and shallow micron-scale grooves. Shallow ‘blue’ grooves efficiently couple laser output into surface electromagnetic waves on the facet and confine waves to the facet.

Credit: Courtesy of the laboratory of Federico Capasso, Harvard School of Engineering and Applied Sciences.

“Our team was able to reduce the divergence angle of the beam emerging from these semiconductor lasers dramatically, whilst maintaining the high output optical power of identical unpatterned devices,” says Linfield. Using a simple one-dimensional grating design, the lasers' beam divergence was reduced from about 180° to about 10°, the directivity was improved by over 10dB and the power collection efficiency was boosted about six-fold compared with the unpatterned devices, without compromising the lasers' high-temperature performance.

The use of metamaterials (artificial materials engineered to provide properties which may not be readily available in nature) was critical. While metamaterials have potential use in novel applications such as cloaking, negative refraction and high-resolution imaging, their use in semiconductor devices has been very limited to date.

“In our case, the metamaterial serves a dual function: strongly confining the THz light emerging from the device to the laser facet and collimating the beam,” explains Yu. “The ability of metamaterials to confine strongly THz waves to surfaces makes it possible to manipulate them efficiently for applications such as sensing and THz optical circuits,” he adds. “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,” comments Linfield.

Co-authors of the study also include Qi Jie Wang (formerly of Harvard University and now with the Nanyang Technological University in Singapore); graduate student Mikhail A. Kats and postdoctoral fellow Jonathan A. Fan (both of Harvard University); and postdoctoral fellows Suraj P. Khanna and Lianhe Li and faculty member A. Giles Davies (all from the University of Leeds).

The research was partially supported by the Air Force Office of Scientific Research. The Harvard-based authors also acknowledge the support 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 Engineering and Physical Sciences Research Council.

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