A Tiny Terahertz Laser
Terahertz radiation has captivated researchers, engineers, and security experts with its promise of sensitive chemical detection, ultrafast data transmission, and the ability to “see through” walls and clothing. Today, however, terahertz devices are in limited use. Part of the problem is that engineers are still developing cost-effective and portable ways to emit and control the radiation.
But now, researchers at Harvard University have developed a semiconductor-based terahertz laser that is smaller than a fingernail. And importantly, it works at room temperature, unlike other semiconductor terahertz lasers that need to be cooled with tanks of liquid nitrogen. When you shrink a laser to the size of a chip and make it work at high temperatures, it suddenly becomes much more useful, says Federico Capasso, a professor of physics at Harvard and lead researcher on the work. A room-temperature semiconductor terahertz laser is portable, and since it can be fabricated in bulk on a wafer, it would be more economical than other types of terahertz lasers. Removing the need for cryogenic cooling also makes it easier and less expensive to use.
Capasso is a leader in developing semiconductor lasers. In the 1990s, he was on the team of researchers at Bell Laboratories who developed a specific type of semiconductor laser called the quantum-cascade laser. In 2004, the first quantum-cascade lasers were commercialized, and now they’re used in medical diagnostic tests and for detecting certain types of chemicals and pollutants. “The next frontier,” Capasso says, “is finding a way to capture terahertz in a quantum-cascade laser.”
Quantum-cascade lasers use a series of differing energy gaps within an optical chip to produce mid-infrared light. Researchers believe that this design is the most practical one for a terahertz semiconductor laser, but it’s been difficult to make them work well. The size of the energy gaps, or quantum wells, within quantum-cascade lasers determines the frequency of light emitted; electrons are injected into the upper energy levels, and when they fall to the lower energy levels, they produce photons. But in order to produce terahertz frequencies, the energy gaps need to be extremely close together, and it’s difficult to selectively inject electrons in the right level. When the laser is cooled, the energy gaps spread out, and it’s not as difficult, but as the laser warms, the output power drops off dramatically.
While researchers at MIT and at universities in Italy and Switzerland are also developing terahertz quantum-cascade lasers, Capasso’s team is the first to show that a powerful beam of radiation, with a frequency of about five terahertz, is possible at room temperature.
The trick to getting a usable amount of terahertz light out at room temperature, says Capasso, was to take advantage of what the quantum-cascade laser does best: emit mid-infrared light. Thus, the laser was designed to emit two different wavelengths of infrared light into a specially designed region of semiconductor layers within the chip. Due to the properties of this special region, the incoming infrared photons mix with the atoms in the semiconductor layers and are converted into a single outgoing beam of terahertz radiation.
This light-mixing process is not a new phenomenon. In fact, it’s a well-established phenomenon in a branch of science called nonlinear optics. The same procedure–taking two incoming beams and producing a beam with a different wavelength–is actually an established method for producing terahertz radiation in larger systems. For instance, two mid-infrared lasers can shine light into a special nonlinear crystal, and the outcome is a terahertz beam. This setup, however, tends to take up as much space as a dining-room table. By building a region in the semiconductor laser that acts just like a larger nonlinear crystal, Capasso simply miniaturized an established yet bulky terahertz system.
“The significance [of the research] lies both in the fact that room-temperature operation has been achieved … and in the considerable power, up several orders of magnitude from earlier work,” says Claire Gmachl, a professor of electrical engineering at Princeton University. “Room-temperature operation is a big plus for essentially all applications,” she says, because it makes systems simpler to use and more cost effective.
Still, there is room for improvement, says Capasso. At room temperature, the terahertz laser currently emits a microwatt of power, but it would need to produce at least a milliwatt for practical applications. Capasso’s plan for more power is to modify the design. Currently, the laser shines light out of a narrow rectangular face on the side of the chip, which limits the total output. But by forcing light to come out the top of the chip, which has a much larger surface area than the side, the power could be boosted by an order of magnitude. A simple grating, added to the chip when it’s built, would channel the light out the wider surface, says Capasso. What’s more, he notes, adding a relatively small and inexpensive thermoelectric cooler to the laser could eke out even more power.
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