Researchers demonstrate highly directional Terahertz laser rays

August 8, 2010, Harvard University
The metamaterial patterns are directly sculpted on the highly doped GaAs facet of the device. Artificial coloring in the figure indicates deep and shallow micron scale grooves, which have different functions. The shallow "blue" grooves efficiently couple laser output into surface electromagnetic waves on the facet and confine the waves to the facet. Credit: Courtesy of the laboratory of Federico Capasso, Harvard School of Engineering and Applied Sciences

A collaborative team of applied scientists from Harvard University 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 the August 8th issue of Nature Materials, 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 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 School of Electronic and Electrical Engineering, University of Leeds.

Terahertz rays () can penetrate efficiently through paper, clothing, plastic, and many other materials, making them ideal 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—similar to how light is emitted from a lamp" 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. 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 sculpted an array of sub-wavelength-wide grooves, dubbed a metamaterial, directly on the facet of quantum cascade lasers. The devices emit at a frequency of 3 THz (or a wavelength of one hundred microns), in the invisible part of the spectrum known as the far-infrared.

The deep "pink" grooves form an effective grating that coherently scatters the energy of the surface waves into the far-field. 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 dramatically, whilst maintaining the high output optical power of identical unpatterned devices," says Linfield. "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."

The use of metamaterials, artificial materials engineered to provide properties which may not be readily available in Nature, was critical to the researchers' successful demonstration. 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."

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not rated yet Aug 08, 2010
Wonder if this could be used in metal detecting? To see a sharp image of a detected object would save a lot of digging.
3 / 5 (2) Aug 08, 2010
Not sure if it can penetrate deep enough to be worthwhile. They talk about penetrating clothing, skin and the like, they don't talk about penetrating ground. I imagine it would penetrate a couple of inches but a couple of feet? Another story. My guess is such a development would have to wait for ultrasonic audio transceivers working say at 100 khz in the audio band or even higher frequencies, megahertz band maybe.
1 / 5 (3) Aug 08, 2010
I expect it means even more details of air traveller's bodies when they are scanned. Just a matter of time before these intrusive images pollute the internet, celebrities, children, adults.
4.5 / 5 (2) Aug 08, 2010
there already is such a thing as ground penetrating radar...
not rated yet Aug 08, 2010
Yes there is but a lot lower frequency, Ghz, not Thz. They are talking wavelength of 1/10th of a millimeter.
2 / 5 (1) Aug 08, 2010
That and the power of those ground penetrating radars are on the order of thousands of watts or pulse peak power in the megawatts. We are not anywhere near able to generate that kind of energy at 3 Terahertz.
not rated yet Sep 05, 2010
THz radiation is absorbed very strongly by water. Also, the diameter of particles in soil is quite similar to THz wavelengths (~100 microns) so there would be a lot of scattering. I guess this might make it a quite difficult to do ground-probing THz reflection imaging.

It might be possible if the QCL emission frequency was chosen to lie within a water transmission window.

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