Terahertz Laser With More Power

Microstructured reflectors boost the power output of tiny, chip-mounted terahertz lasers by 88 percent. (Source: MIT / D. Liu, Molgraphics)

Terahertz radiation has promising appli­cations in medical and indus­trial imaging and chemical detection. 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, Qing Hu, a 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. Now, members of Hu’s group and colleagues at Sandia National Labora­tories and the University of Toronto describe a novel design that boosts the power output of chip-mounted tera­hertz lasers by 80 percent.

The researchers’ design is a new variation on a quantum cascade laser with distri­buted feedback. “We started with this because it was the best out there,” says Ali Khalat­pour, a graduate student in electrical engi­neering and computer science. “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. Since most appli­cations of tera­hertz radiation require directed light, that means that the device squanders half of its energy output. 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.

As Khalat­pour explains, the researchers’ design is not tied to any particular gain medium in the body of the laser. “If we come up with a better gain medium, we can double its output power, too,” Khalatpour says. “We increased power without desig­ning a new active medium, which is pretty hard. Usually, even a 10 percent increase requires a lot of work in every aspect of the design.” In fact, bidi­rectional emission, or emission of light in opposed direc­tions, is a common feature of many laser designs. With conven­tional lasers, however, it’s easily remedied by putting a mirror over one end of the laser.

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 quantum cascade laser consists of a long rectan­gular 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.

A standing wave reflects back and forth in the waveguide in such a way that the crests and troughs of the reflec­tions perfectly coincide with those of the waves moving in the oppo­site direc­tion. A standing wave is essen­tially inert and will not radiate out of the waveguide. So Hu’s group cuts regu­larly spaced slits into the wave­guide, which allow terahertz rays to radiate out. “Imagine that you have a pipe, and you make a hole, and the water gets out,” Khalat­pour says. The slits are spaced so that the waves they emit reinforce each other only along the axis of the wave­guide. At more oblique angles from the waveguide, they cancel each other out.

In the new work, Khalat­pour and colleagues simply put reflectors behind each of the holes in the wave­guide, a step that can be seam­lessly incor­porated into the manu­facturing process that produces the waveguide itself. The reflec­tors are wider than the waveguide, and they’re spaced so that the radiation they reflect will reinforce the tera­hertz wave in one direction but cancel it out in the other. Some of the terahertz wave that lies outside the waveguide still makes it around the reflec­tors, but 80 percent of the energy that would have exited the wave­guide in the wrong direction is now redi­rected the other way.

“They have 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, which you need to be able to use the power that you’re generating, in combi­nation with a single frequency of laser operation, which is also desirable for spectro­scopy,” says Ben Williams, an associate professor of electrical and computer engi­neering at the Uni­versity of Cali­fornia at Berkeley. “This has been one of the most useful and popular ways to do this for maybe the past five, six years. But one of the problems is that in all the previous structures that either Qing’s group or other groups have done, the energy from the laser is going out in two direc­tions, both the forward direc­tion and the backward direc­tion.”

“It’s very difficult to generate this tera­hertz power, and then once you do, you’re throwing away half of it, so that’s not very good,” Williams says. “They’ve 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.” (Source: MIT)

Reference: A. Khalatpour et al.: Unidirectional photonic wire laser, Nat. Photonics, online 7 August 2017; DOI: 10.1038/nphoton.2017.129

Link: Millimeter-Wave and Terahertz Devices Group, Massachusetts Institute of Technology, Cambridge, USA

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