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THz Lasers Small Enough for Screening Devices

Photonics.com
Feb 2007
ALBUQUERQUE, N.M., Feb. 7, 2007 -- It might not be science fiction for long: Miniaturized terahertz laser technology may enable the use of a revolutionary new technique that detects dangerous materials through analysis of their unique molecular signatures.

Sandia National Laboratories, in collaboration with the Massachusetts Institute of Technology, has developed semiconductor lasers small enough to put into portable devices. Working in the underutilized terahertz (THz) portion of the electromagnetic spectrum that lies between microwaves and infrared, the scientists are building a highly integrated miniaturized THz transmitter-receiver (transceiver) that could make a number of applications possible. These could scan for items such as concealed weapons or materials, explosives and weapons of mass destruction. THz microelectronic transceivers may also find applications in advanced communication systems and high-resolution radars.
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Mike Wanke, principal investigator of the Terahertz Microelectronics Transceiver Grand Challenge, holds a miniaturized device that he said will eventually replace large pieces of equipment like those in the background. (Photo by Bill Doty)
The basic technique is now used by atmospheric scientists and astronomers, who use large-scale lasers to probe planetary atmospheres and to peer into the depths of nebulae by utilizing the THz portion of the spectrum. Sandia is developing the next generation of screening devices, which will identify hazardous and toxic materials even if concealed by clothing and packaging materials.

The project, the Terahertz Microelectronics Transceiver Grand Challenge, is in its second of three years of funding through Sandia’s Laboratory Directed Research and Development program.

Mike Wanke, principal investigator, said the infrastructure needed to move the terahertz technology from the laboratory to the field is unavailable right now, but "We want to develop that infrastructure and invent the necessary technologies.”

Wanke said that over the past three years, “The terahertz situation has begun to change dramatically, primarily due to the revolutionary development of terahertz quantum cascade lasers.”

These tiny lasers are semiconductor sources of terahertz radiation capable of output powers in excess of 100 mW. Previously, such powers could only be obtained by molecular gas lasers occupying cubic meters and weighing more than 100 kg, or free-electron lasers weighing tons and occupying entire buildings.
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Miniaturized device shown next to a dime. Miniaturized terahertz laser technology may enable the use of a revolutionary new technique that detects dangerous materials through analysis of their unique molecular signatures. (Photo courtesy Sandia National Laboratories)
Quantum cascade laser-based systems can be less than the size of a baseball and can be powered from a nine-volt battery. Sandia has been a leader in developing this new technology and in collaboration with MIT is responsible for several world-performance records for the lasers. Also, Sandia and its partners are the only US institutions that have demonstrated the ability to grow the unique semiconductor crystals such that they can be turned into operating terahertz quantum cascade lasers. The crystals are grown by Sandia research scientist John Reno, an expert in molecular beam epitaxy -- a method of laying down layers of materials with atomic thicknesses onto substrates.

Sandia researchers spent the first year of the Grand Challenge using Sandia’s unique strengths in integrated microelectronics and device physics to develop components that are now being combined to create an integrated THz microelectronic transceiver, a core enabling element.

The team is currently developing the receiver, doing systems tests and exploring packaging requirements. At the end of three years, the researchers expect to have an actual working prototype capable of detecting the materials and chemicals by reading distinctive molecular spectral “signatures.”

“Most materials and chemicals have their own unique terahertz spectral signatures,” Wanke says. “A terahertz transceiver system would be able to measure, for example, the signature of a gas and determine what it is.”

“Atmospheric scientists and radio astronomers have spent years developing terahertz spectral signature databases to identify chemicals in nebula and planetary atmospheres,” said Greg Hebner, program manager. “Even though the current devices are washing machine-sized, they are located in a few observatories, and one is even flying on a satellite. To address specific national security problems, we are working on reducing the size, weight and power requirement as well as expanding the existing spectral databases.”

In addition to monitoring for concealed hazardous materials, Mike believes a terahertz system can be used to monitor the air for toxic materials. Using air sampling technology developed at Sandia and other locations, hazardous vapors can be preconcentrated. Shining light from the quantum cascade laser through the concentrated sample provides a direct identification of the vapor. This technology can be used in conjunction with existing mass spectrometer-based systems to reduce false identifications.

“We are very optimistic about working in the terahertz electromagnetic spectrum,” Wanke says. “This is an unexplored area, and a lot of science can come out of it. We are just beginning to scratch the surface of what THz can do to improve national security.”

Sandia is a multiprogram laboratory operated by Sandia Corp., a Lockheed Martin company, for the US Department of Energy’s National Nuclear Security Administration. Sandia has R&D responsibilities in national security, energy and environmental technologies, and in economic competitiveness.

For more information, visit: www.sandia.gov


GLOSSARY
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
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