Between microwaves and visible light on the electromagnetic spectrum lies terahertz radiation. Although it has promise for medical and industrial imaging, chemical detection and more, many of these applications are only viable with small, power-efficient sources of terahertz rays. However, the standard method for producing them involves a bulky tabletop device needing lots of power.
A recent issue of Nature Photonics describes a novel design for terahertz lasers etched onto microchips. Created by researchers working with Qing Hu, a professor of electrical engineering and computer science at MIT, the device has been selected by NASA to provide terahertz emission for its Galactic/Extragalactic ULDB Spectroscopic Terahertz Observatory (GUSTO) mission. The GUSTO mission is aimed at determining the composition of the interstellar medium — the matter that fills the space between stars. It will deploy instrument-laden balloons to the Earth’s upper atmosphere, necessitating a lightweight terahertz emitter.
Designing the device to direct light was a challenge for researchers because it is based on a quantum cascade laser that naturally emits radiation in two opposed directions. For conventional lasers, this can easily be remedied by placing a mirror over one end to reflect energy back in the desired direction. But the researchers’ photonic wire lasers are too small in comparison to the long wavelength of terahertz radiation for the mirror method to be effective.
The solution was to exploit a peculiarity of the laser’s long rectangular ridge design, also known as a waveguide. The application of an electric field induces an electromagnetic wave along the length of the waveguide. Reflections moving back and forth in the waveguide coincide perfectly with those of the waves moving in the opposite direction, essentially creating an inert “standing wave” that will not radiate out. By cutting regularly-spaced slits into the waveguide, however, Hu’s group allowed the radiation to come out – and then used reflectors placed behind each of the holes to reinforce the wave in one direction while canceling it out in the other.
This effectively redirected 80 percent of the energy that would have normally exited out the back of the laser, boosting the power output.
“They’ve come up with a very elegant scheme to essentially force much more of the power to go in the forward direction,” said Ben Williams, an associate professor of electrical and computer engineering at the University of California, Los Angeles. “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.”