Sep 4, 2017: Terahertz (sub-millimeter) waves has the potential for many innovative applications like subsurface inspection, explosive detection, etc. However, it is a technical challenge to generate and sense them.
Terahertz are of wavelengths of 1 mm down to 30 µm, corresponding to frequencies from 300 GHz to 3 THz (1 THz = 1012 Hz). These wavelengths pose a challenge for practical use because they are in the zone between the standard RF spectrum and optical world.
However, because of their potential in specific applications is immese, efforts are being made to improve their sources. Under a project funded by the National Science Foundation (NASA), and the US Department of Energy, a team of researchers from Sandia National Laboratories, MIT, and the University of Toronto has developed an innovative physical structure that enhances the power output of chip-mounted terahertz lasers by almost 80%, resulting in the best-performing chip-mounted terahertz source.
It has been selected by NASA to provide emissions for the Galactic/Extragalactic ULDB Spectroscopic Terahertz Observatory (GUSTO) mission, that would get launched in 2021 to to determine the composition of the “interstellar medium,” and terahertz rays are well-suited to the spectroscopic measurement of oxygen concentrations.
The new approach, which is one of the best ways to generate a high-quality terahertz output beam, is a variation on a third-order laser with distributed feedback. It naturally emits radiation in two opposite directions, which means 50% loss of directed output power.
By exploiting an inherent aspect of the tiny laser’s design, the new design sends 80% of the light from the back of the laser to the front. A standard quantum laser with a long rectangular ridge serves as a waveguide. The application of an electric field induces a standing electromagnetic wave along the length of the waveguide. The group then cut regularly spaced slits into the waveguide, which allow terahertz rays to radiate out.
These slits are spaced in such a way that the waves can emit each other only along the axis of the waveguide. At more oblique angles from the waveguide, they cancel each other out. They then put reflectors behind each of the slits in the waveguide, a step that can be part of the manufacturing process of the waveguide itself.
The new design is not tied to any particular “gain medium,” the combination of materials used in the body of the laser. If the research team comes up with a better gain medium, then they can double its output power, too. The tean has increased power without designing a new active medium, which is quite difficult.