Far-Infrared Now Near: Researchers Debut Compact Terahertz Laser

Terahertz rays could find use in wireless networks and detect cancers and bombs

2 min read
A terahertz laser chip with thermoelectric cooler
Terabit wireless backbones, cancer scanners, bomb detectors: Many possible applications await for portable terahertz lasers. Here, a prototype laser chip with thermoelectric cooler.
Photos: MIT

Terahertz rays could have a dizzying array of applications, from high-speed wireless networks to detecting cancers and bombs. Now researchers say they may finally have created a portable, high-powered terahertz laser.

Terahertz waves (also called submillimeter radiation or far-infrared lightlie between optical waves and microwaves on the electromagnetic spectrum. Ranging in frequency from 0.1 to 10 terahertz, terahertz rays could find many applications in imaging, such as detecting many explosives and illegal drugs, scanning for cancers, identifying protein structures, non-destructive testing and quality control. They could also be key to future high-speed wireless networks, which will transmit data at terabits (trillions of bits) per second.

However, terahertz rays are largely restricted to laboratory settings due to a lack of powerful and compact terahertz sources. Conventional semiconductor devices can generate terahertz waves ranging either below 1 terahertz or above 10 terahertz in frequency. The range of frequencies in the middle, known as the terahertz gap, might prove especially valuable for imaging, bomb detection, cancer detection and chemical analysis applications, says Qing Hu, an electrical engineer at MIT.

Previous research had suggested that so-called quantum cascade lasers might bridge this gap. At the heart of these devices are wafers consisting of alternating layers of semiconducting material with different levels of electrical conductivity. When electrically charged, electrons remain confined in the more electrically conductive layers between the more electrically resistant layers. Eventually these electrons emit light in a "quantum cascade."

However, quantum cascade lasers designed to emit light within the terahertz gap long required temperatures below -63 degrees C. The problem was that at higher temperatures, the confined electrons leaked through the more electrically resistant layers.

A laser chip with thermoelectric cooler on a block, compared with a coffee mug. The background is a big cryo-cooler The chip that launched a trillion bits: Terahertz laser chip prototype with cooling apparatus beneath and behind it. Photo: MIT

Now scientists at MIT and the University of Waterloo in Canada have devised a high-power quantum cascade laser that emits roughly 4-terahertz light with a maximum operating temperature of -23 degrees C. This suggests that it could work using compact thermoelectric coolers instead of bulky cryogenics, enabling portable applications.

The wafer in this laser consisted of a more electrically conductive gallium arsenide sheet sandwiched between more electrically resistant aluminum gallium arsenide layers. The scientists doubled the concentration of aluminum in the aluminum gallium arsenide layers to better confine the electrons within the gallium arsenide sheet. They also carefully tuned the thicknesses of these layers to help ensure these electrons emitted light in a quantum cascade.

Hu says they can likely adjust the laser to also emit between 2 to 7 terahertz in frequency. Between 7 and 10 terahertz, a quantum cascade laser based on a different material such as gallium nitride might be necessary, he says.

As to whether lasers could operate within the terahertz gap at even higher temperatures, "I would almost unequivocally say terahertz operations at room temperature or above can be achieved," Hu says. "This means we could even have handheld terahertz lasers."

The scientists detailed their findings online Nov. 2 in the journal Nature Photonics.

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This CAD Program Can Design New Organisms

Genetic engineers have a powerful new tool to write and edit DNA code

11 min read
A photo showing machinery in a lab

Foundries such as the Edinburgh Genome Foundry assemble fragments of synthetic DNA and send them to labs for testing in cells.

Edinburgh Genome Foundry, University of Edinburgh

In the next decade, medical science may finally advance cures for some of the most complex diseases that plague humanity. Many diseases are caused by mutations in the human genome, which can either be inherited from our parents (such as in cystic fibrosis), or acquired during life, such as most types of cancer. For some of these conditions, medical researchers have identified the exact mutations that lead to disease; but in many more, they're still seeking answers. And without understanding the cause of a problem, it's pretty tough to find a cure.

We believe that a key enabling technology in this quest is a computer-aided design (CAD) program for genome editing, which our organization is launching this week at the Genome Project-write (GP-write) conference.

With this CAD program, medical researchers will be able to quickly design hundreds of different genomes with any combination of mutations and send the genetic code to a company that manufactures strings of DNA. Those fragments of synthesized DNA can then be sent to a foundry for assembly, and finally to a lab where the designed genomes can be tested in cells. Based on how the cells grow, researchers can use the CAD program to iterate with a new batch of redesigned genomes, sharing data for collaborative efforts. Enabling fast redesign of thousands of variants can only be achieved through automation; at that scale, researchers just might identify the combinations of mutations that are causing genetic diseases. This is the first critical R&D step toward finding cures.

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