Terahertz radiation — the band of the electromagnetic spectrum between microwaves and visible light — 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.
MIT researchers have developed a new version of a chip-scale quantum cascade terahertz laser with distributed feedback. Until now, however, the device has had a major drawback: 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. The researchers have now 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.
Bidirectional emission is a common feature of many laser designs. With conventional lasers, however, it's easily remedied by putting 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 only a tiny fraction of the wave's total energy. The researchers’ solution to this problem exploits a peculiarity of the tiny laser's design. A quantum cascade laser consists of a long rectangular ridge called a waveguide. In the waveguide, materials are arranged so that the application of an electric field induces a standing electromagnetic wave along its length. The standing wave is essentially inert and will not radiate out of the waveguide. To address that problem, the researchers cut regularly spaced slits into the waveguide, allowing the terahertz rays to radiate out. The slits are spaced so that the waves they emit reinforce each other — their crests coincide — only along the axis of the waveguide. At more oblique angles from the waveguide, they cancel each other out. So the researchers simply put reflectors behind each of the slits, 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,” said Ben Williams, an associate professor of electrical and computer engineering at the University of California at Los Angeles. “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.”