16 Aug 2017
Professor Qing Hu’s tiny but high-efficiency terahertz laser suitable for imaging and chemical detection.Qing Hu’s group and colleagues at Sandia National Laboratories, US, and the University of Toronto, Canada, have developed a novel design of terahertz laser, which boosts the power output of the chip-mounted device by 80 percent. The breakthrough is described the latest issue of Nature Photonics.
In relation to this development, MIT comments: “Terahertz radiation has promising applications in medical and industrial imaging and chemical detection, among other uses. But many of those applications depend on small, power-efficient sources of terahertz rays, and the standard method for producing them involves a bulky, power-hungry, tabletop device.”
For more than 20 years, Prof Hu, a distinguished professor of electrical engineering and computer science at MIT, and his group have been working on sources of terahertz radiation that can be etched onto microchips.
As the best-performing chip-mounted terahertz source yet reported, the researchers’ device has been selected by NASA to provide terahertz emission for its Galactic/Extragalactic ULDB Spectroscopic Terahertz Observatory (GUSTO) mission.
The mission is intended to determine the composition of the interstellar medium, or the matter that fills the space between stars, and it will be using terahertz rays because they are uniquely well-suited to spectroscopic measurement of oxygen concentrations. Because the mission will deploy instrument-laden balloons to the Earth’s upper atmosphere, the terahertz emitter needs to be lightweight.
The Hu design is a new variation on a quantum cascade laser with distributed feedback. “We started with this because it was the best out there,” commented Ali Khalatpour, a graduate student in electrical engineering and computer science and first author on the paper. “It has the optimum performance for terahertz.”
Until now, however, the device has had a major drawback, which is that it naturally emits radiation in two opposed directions, wasting 50% of the energy required or a single-direction source. Khalatpour and his colleagues found a way to redirect 80 percent of the light that usually exits the back of the laser, so that it travels in the desired direction.
The researchers’ design is not restricted to a particular gain medium in the body of the laser. Khalatpour adde, “We increased power without designing a new active medium, which is pretty hard. If we come up with a better gain medium, we can double its output power, too.”
Bidirectional emission is a common feature of many laser designs. With conventional lasers, however, it is easily remedied by positioning a mirror over one end. But the wavelength of terahertz radiation is so long, and the researchers’ new lasers — known as photonic wire lasers — are so small, that much of the electromagnetic wave traveling the laser’s length actually lies outside the laser’s body. A mirror at one end of the laser would reflect back a tiny fraction of the wave’s total energy.Khalatpour and his colleagues’ solution to this problem exploits a peculiarity of the tiny laser’s design: a QCL consists of a long rectangular ridge called a waveguide. In the waveguide, materials are arranged so that the application of an electric field induces an electromagnetic wave along the length of the waveguide.
This wave, however, is a “standing wave,” which is essentially inert and will not radiate out of the waveguide. Therefore Hu’s group cut regularly placed slits into the waveguide, which allow terahertz rays to radiate. “Imagine that you have a pipe, and you make a hole, and the water gets out,” said Khalatpour says. The slits are spaced such that the emitted waves reinforce each other but only along the axis of the waveguide.
In the latest design, Khalatpour and Hu, along with John Reno of Sandia, and Nazir Kherani, a professor of materials science at the University of Toronto have placed reflectors behind each of the holes in the waveguide. The reflectors are wider than the waveguide, and they’re spaced so that the radiation they reflect reinforces the terahertz wave.
Ben Williams, an associate professor of electrical and computer engineering at UCLA, explained, “They have developed a particular type of terahertz quantum cascade laser, known as a third-order distributed-feedback laser, and this right now is one of the best ways of generating a high-quality output beam.
“Qing’s group have come up with a very elegant scheme to essentially force much more of the power to go in the forward direction. And it still has a good, high-quality beam, so it really opens the door to much more complicated antenna engineering to enhance the performance of these lasers.” The new work was funded by NASA, the National Science Foundation, and the U.S. Department of Energy.