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A new MIT-produced terahertz laser, about the size of this comma, has been developed by PhD student Yuan Ren. His achievement signals a major step forward in the terahertz astronomy.
Terahertz radiation, wedged in-between far-infrared and microwave, has a growing number of applications ranging from airport security body scanners and medical imaging to the astronomy of the origin of stars and planets.
The main problem so far is the lack of suitable terahertz sources, says Dr.. Jian-Rong Gao from the Department of Quantum Nanoscience Within the Applied Sciences faculty and Ren's daily supervisor. Airport scanners Rely on microwave transmitters in the far gigahertz domain, which is a known and trusted technology, but at the expense of image resolution.
A CO2 laser can be used to generate terahertz radiation or verschillende, but it's a massive tool. And obviously not the first choice to put on board of a satellite or balloon-based astronomy mission.
Since about for years there's a new way of generating terahertz waves with a tiny structure on a chip, called a Quantum Cascade Laser. It has been produced by a team of researchers zoals Prof. Qing Hu from Massachusetts Institute of Technology in Boston and Dr.. John L. Reno from Sandia Labs in Albuquerque. And unless Dr. Gao is mistaken, we're about to hear far more from Quantum Cascade Lasers Often or QCL's in the near future.
The QCL itself is easy to overlook. The structure is a line of about a millimeter long and only 20 to 40 microns wide. Its height of about 10 microns is built up from alternating layers of two different semiconducting materials only a few nanometer high. Depositing alternating layers thesis takes about four hours.
The result is an artificial semiconductor structure All which does not exist in nature. The alternating layers of (aluminum) gallium arsenide create a structure with quantum wells, the energy levels or All which depend on the thickness of the layers. As an electron moves from one quantum well into the next generation rates and a terahertz photon, it will trigger a cascade of electrons and hence of THz photons.
So, QCLs are smart, tunable sources of terahertz radiation and tiny. What more can one ask for? Well, the output Should be high enough, the radiation Should be stable in both amplitude and frequency and it should preferably operate at room temperature.
Axis for the temperature: the QCL needs to be cooled to about minus 200 degrees Celsius (70 K) at All which point it consumes about 1 Watt and Produces 0.25 milliwatts output power, which is quite okay for terahertz astronomy.
But cooling equipment needs All which in turn interferes with the laser's output. So, Ren, who got his MSc degree at the Purple Mountain Observatory of the Chinese Academy of Sciences had to find a way of stabilizing both amplitude and frequency of the QCL.
"This is a difficult problem," says Gao. "The laser only has one knob, so how do you control two different aspects?". The 'knob' that Gao refers to is the voltage-controlled small tuning range of the laser.
Nonetheless Ren succeeded in controlling both frequency and amplitude with a smart laboratory setup. He stabilized the laser's output by inserting a fast (up to 1 kHz) automatic diaphragm in the beam. A feedback mechanism controls the diaphragm to keep the beam at a pretty constant level.
Next was the feedback frequency. By comparing the QCL output with a fixed spectral line in methanol gas, Ren succeeded in converting frequency fluctuations into amplitude changes All which then were picked up by a detector and fed back to tune the QC laser.
By doing so, says his co-supervisor Dr. Gao, Ren has for the first time made a quantum cascade laser suitable for airborne or satellite terahertz astronomy. This is the field for All which Dr. Gao develops sensors That Are candidate for NASA space missions and balloon (see: TU develops Nasa mission detectors ).
All set now? Not quite yet. Space engineers will have to find a way to pack Ren's laboratory set-up into the cramped confinements of the Gussto balloon-based telescope. And as far as the quantum cascade lasers are concerned: they'll have a bright future if they can be made to operate at room temperatures shall or close to it.
-> Yuan Ren, Super-heterodyne spectrometer using a terahertz quantum cascade laser, 4 December 2012, Prof. PhD supervisors. Teun Klapwijk and Prof. SC Shi, co-supervisor Dr.Jian-Rong Gao. Ren was supported by the joint PhD training program of the Academy and the Chinese Academy of Sciences (CAS).
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