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A tiny terahertz laser which could be used in imaging and chemical detection has been selected by NASA to provide terahertz emission for the Galactic/ExtraGalactic ULDB Spectroscopic Terahertz Observatory (GUSTO) mission.
The laser was developed at Massachusetts Institute of Technology (MIT) following over 20 years of research by Distinguished Professor of Electrical Engineering and Computer Science, Qing Hu.
Terahertz radiation is found in the band of the electromagnetic spectrum between microwaves and visible light. It has promising applications in medical and industrial imaging and chemical detection, many of which require small, power-efficient sources of terahertz rays. However, the standard method of producing rays involves a bulky, power-guzzling table top device.
Hu’s group at MIT investigated sources of terahertz radiation that can be etched onto microchips, and together with colleagues at Sandia National Laboratories and the University of Toronto have established a novel method for boosting the power output of chip-mounted terahertz lasers by 80%.
The design – the best-performing chip-mounted terahertz source yet reported – is a new variation 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, MIT
A drawback of previous devices is that it naturally emits radiation in an opposing direction; since most applications of terahertz radiation requires directed light, the device therefore wastes half its energy output. The new design reroutes 80% of the light usually escaping the rear of the laser so it travels in the desired direction.
As Khalatpour explains, the design is not tied to any particular 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. 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.
Ali Khalatpour, a Graduate Student in Electrical Engineering and Computer Science, MIT
Bidirectional emission – emission of light in opposing directions – is not uncommon in laser design and is easily fixed in many conventional lasers by placing a mirror over one end of the laser. However, terahertz radiation has such a long wavelength and the Researchers’ new lasers – photonic wire lasers – are so small, that much of the electromagnetic wave traveling the laser’s length actually lies outside the body. A mirror at one end would reflect back a miniscule fraction of the wave’s total energy.
The Researchers’ solution exploits a peculiarity of the laser’s design. A quantum cascade laser comprises of a long rectangular ridge called a waveguide where materials are arranged so that when an electric field is applied, it induces an electromagnetic wave along the length of the guide. This standing wave is essentially inert as the wave reflects back and forth in the waveguide in such a way that the crests and troughs of the reflections coincide with those of the waves moving in the opposite direction and will not radiate out of the waveguide.
So Hu's group cut regularly spaced slits into the waveguide, which allow 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.
The group put reflectors behind each of the holes in the waveguide, a step that can be 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% of the energy that would have exited in the wrong direction is now redirected the other way.
The work - published in Nature Photonics was funded by the National Science Foundation, the US Department of Energy and NASA, who have selected the device to be part of the GUSTO mission. The mission aims to determine the composition of the interstellar medium - the matter that fills the space between stars - and is employing terahertz rays as they are uniquely well-suited to spectroscopic measurement of oxygen concentrations.
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