Since most terahertz radiation applications require directed light, researchers have found a way to redirect most of the light that exits the back of the laser so that it travels in the desired direction.
A terahertz design that can be etched onto microchips has managed to improve the power output of chip-mounted terahertz lasers by 80%.
The design, developed by researchers at MIT, Sandia National Laboratories and the University of Toronto, is a new variation on a device called a quantum cascade laser with distributed feedback.
“We started with this because it was the best out there,” said 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. 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% of the light that usually exits the back of the laser, so that it travels in the desired direction.
The design, according to Khalatpour, is not tied to any particular “gain medium,” or combination of materials in the body of the laser.
“If we come up with a better gain medium, we can double its output power, too,” Khalatpour said. “We increased power without designing a new active medium, which is pretty hard. Usually, even a 10% increase requires a lot of work in every aspect of the design.”
Bidirectional emission, or emission of light in opposed directions, is a common feature of many laser designs. With conventional lasers, it’s easily remedied by putting a mirror over one end of the laser. However, 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 the 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 an electromagnetic wave along the length of the waveguide.
This wave, however, is what’s called a “standing wave.” If an electromagnetic wave can be thought of as a regular up-and-down squiggle, then the 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.
The researchers 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.
The 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’s using terahertz rays because they’re uniquely well-suited to spectroscopic measurement of oxygen concentrations.