A new MIT-produced terahertz laser, about the size of a comma, has been stabilised by PhD-student Yuan Ren. His achievement signals a major step forward in terahertz astronomy.
Terahertz radiation, wedged in-between far-infrared and microwave, has a growing number of applications ranging from airport security body scanners 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 radiation of several terahertz. However it’s a massive instrument and thus not the first choice to put on board on a satellite or balloon-based astronomy mission.
A new way of generating terahertz waves with a tiny structure on a chip, called a Quantum Cascade Lase, has existed for about ten years. It was been produced by a team including researchers 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 will be hearing far more from Quantum Cascade Lasers or QCL’s in the near future.
TinyTiny
Tiny
The QCL itself is easy to overlook. The structure is a line about a millimetre long and only 20 to 40 micron wide. Its height of about 10 micron is built up from alternating layers of two different semiconducting materials only a few nanometres high. Depositing these alternating layers takes about ten hours.
The result is an artificial semiconductor structure that doesn’t exist in nature. The alternating layers of (aluminium) gallium arsenide create a structure with quantum wells, the energy levels of which depend on the thickness of the layers. As an electron moves from one quantum well into the next and generates a terahertz photon, it will trigger a cascade of electrons and hence, THz photons.
So, QCLs are smart, tunable and tiny sources of terahertz radiation. 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.
As for the temperature: the QCL needs to be cooled to about minus 200 degrees Celsius (70 K) at which point it consumes about 1 Watt and produces 0,25 milliwatt of power, which is actually okay for terahertz astronomy.
Cool QCL’sCool QCL’s
Cool QCL’s
But cooling needs equipment, 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.
Ren succeeded nonetheless in controlling both frequency and amplitude with a smart laboratory set-up. He stabilised 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 constant level.
Next was the frequency feedback. By comparing the QCL output with a fixed spectral line in methanol gas, Ren succeeded in converting frequency fluctuations into amplitude changes 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 which Dr. Gao develops sensors that are candidate for NASA space and balloon missions (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 function at room temperatures or close to it.
→ Yuan Ren, Super-terahertz heterodyne spectrometer using a quantum cascade laser, 4 December 2012, PhD supervisors Prof. Teun Klapwijk and Prof. S.C. Shi, co-supervisor Dr. Jian-Rong Gao. Ren was supported by the joint PhD Training programme of the KNAW and the Chinese Academy of Sciences (CAS).
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