Miniature Terahertz Laser Holds Promise for Use in Imaging and Chemical Detection

Terahertz radiation has probable applications in industrial and medical imaging and chemical detection, among other applications. Basically, it can be defined as the band of the electromagnetic spectrum between visible light and microwaves.

However, for Terahertz radiation to be used in many of those applications will rely on small, power-efficient sources of terahertz rays, and the standard technique for generating them requires a bulky, power-hungry, tabletop platform.

For over two decades, Qing Hu, an distingushed Professor of Electrical Engineering and Computer Science at MIT, and his group have been exploring sources of terahertz radiation that can be etched onto microchips. In the recent issue of Nature Photonics, members of Hu’s group and colleagues at Sandia National Laboratories and the University of Toronto describe an innovative design that promotes the power output of chip-mounted terahertz lasers by 80%.

As the top-performing chip-mounted terahertz source so far reported, the Researchers’ device has been chosen by NASA to provide terahertz emission for its Galactic/Extragalactic ULDB Spectroscopic Terahertz Observatory (GUSTO) mission. The mission is aimed at determining the composition of the interstellar medium, or the matter that occupies the space between stars, and it is using terahertz rays because they are uniquely well-matched to spectroscopic measurement of oxygen concentrations. As the mission will deploy instrument-loaded balloons to the Earth’s upper atmosphere, the terahertz emitter has to be lightweight.

The Researchers’ design is a new twist on a device known as a quantum cascade laser with distributed feedback.

We started with this because it was the best out there. It has the optimum performance for terahertz.

Ali Khalatpour, a Graduate Student in Electrical Engineering and Computer Science and First Author on the paper

So far, however, the device has had a major shortcoming, which is that it naturally discharges radiation in two divergent directions. Since a majority of applications of terahertz radiation requires directed light that means that the device consumes half of its energy output. Khalatpour and his colleagues discovered a way to redirect 80% of the light that generally exits the back of the laser, so that it travels in the preferred direction.

As Khalatpour explains, the Researchers’ design is not linked to any specific “gain medium,” or combination of materials in the laser’s body.

“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.”

Big waves

Actually, bidirectional emission, or emission of light in different directions, is a typical characteristic of many laser designs. With conventional lasers, however, it is easily fixed by placing a mirror over one end of the laser.

But the wavelength of terahertz radiation is very long, and the Researchers’ new lasers — referred to as photonic wire lasers — are very small, that much of the electromagnetic wave traveling the laser’s length really lies outside the laser’s body. A mirror positioned at one end of the laser would reflect back a minute fraction of the wave’s total energy.

By exploiting the distinctiveness of the miniature laser’s design, Khalatpour and his colleagues tried to solve this problem. A quantum cascade laser comprises of a long rectangular ridge called a waveguide. In the waveguide, materials are set in such a manner that the application of an electric field triggers an electromagnetic wave along the length of the waveguide.

This wave, however, is what is known as a “standing wave.” If an electromagnetic wave can be thought of as a typical 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 seamlessly coincide with those of the waves traveling in the opposite direction. A standing wave is fundamentally inert and will not discharge the waveguide.

Thus Hu’s team cuts proportionately spaced slits into the waveguide, which permit 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 produce reinforce each other — their crests overlap — only along the axis of the waveguide. At further oblique angles from the waveguide, they annul each other.

Breaking symmetry

In the new study, Khalatpour and his Co-authors — Hu, John Reno of Sandia, and Nazir Kherani, a Professor of Materials Science at the University of Toronto — simply place reflectors behind each of the holes in the waveguide, a step that can be effortlessly integrated into the manufacturing process that creates the waveguide itself.

The reflectors are broader than the waveguide, and they are spaced so that the radiation they reflect will strengthen the terahertz wave in one direction but annul it in the other. Some of the terahertz wave that lies outside the waveguide still makes it around the reflectors, but 80% of the energy that would have escaped the waveguide in the wrong direction is at present 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. 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. 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.

Ben Williams, Associate Professor of Electrical and Computer Engineering, the University of California at Berkeley

The new research was funded by NASA, the U.S. Department of Energy, and the National Science Foundation.