A Practical T-Ray Amplifier
Terahertz sensors can see through clothing and sniff for bombs, but they've had to do so with very weak signals
Illustration: Jerome Tignon
2 December 2009—Terahertz time-domain spectroscopy is a tantalizing technique for scientists wanting to characterize materials and for those charged with securing airports and other sensitive locations. Unlike X-rays, terahertz radiation—which lies above microwave wavelengths and below the infrared band, in the 30 micrometer to 1 millimeter range—doesn’t damage biological tissue and provides good resolution. Many molecules—including those in explosives and narcotics—have spectral signatures that lie in the terahertz range. And terahertz radiation is absorbed by water, so it can be used to determine a material’s water content.
Yet "terahertz technology is extremely underdeveloped compared with other portions of the electromagnetic energy spectrum," says Nathan Jukam of the Pierre Aigrain Laboratory at the École Normale Supérieure, in Paris. Terahertz radiation, which is generated by ultrashort-duration lasers, is often weak, and it is difficult to amplify the terahertz signal without using large, complex, and costly laser systems.
Now Jukam, along with colleagues at the École Normale Supérieure, the Denis Diderot University, also in Paris, and the University of Leeds, in England, have found a way to make a compact and practical terahertz amplifier, which they describe in the latest issue of the journal Nature Photonics . Experts say that use of the amplifier could lead to applications in pharmaceutical research, in biological imaging, and for detecting explosives or drugs.
"This is a potentially important development that…could provide a useful source of radiation for applications to terahertz time-domain spectroscopy," says David Auston, associate director of the Center for Energy Efficient Materials at the University of California, Santa Barbara (UCSB).
Jukam and his colleagues realized that when a laser is switched on, it takes a while to build up the lasing action to full power. During this initial period, if you send in another input pulse, the laser amplifies the input. This technique is called gain switching and has been applied to lasers in the near-infrared and visible bands. Once the laser is in steady-state mode (at full power), however, the amplification effect is lost. At that point, the gain is always equal to the loss, known to laser physicists as gain clamping.
At the heart of Jukam’s technique lies a terahertz pulse-producing quantum cascade (QC) laser. QC lasers are remarkably versatile semiconductor lasers that were invented at AT&T Bell Laboratories in 1994. Over the years, QC lasers have been used for remote sensing of gases in Earth’s atmosphere and other environmental applications. They typically lase in the mid- to far-infrared range, but that has recently been extended to the terahertz range. Previously, no one had figured out how to do gain switching in the terahertz range because the QC laser reaches the clamping stage quickly.
The trickiest part was figuring out how to switch on the amplification in the QC laser before the onset of laser action. This was finally done by integrating with the QC laser a photoconductive switch called an Auston switch (invented by UCSB’s David Auston). This switch ensures that the QC laser is switched on to produce amplification and then switched off before the gain clamping sets in—all in a matter of a few tens of picoseconds. This way the QC laser is kept in its amplification mode.
The researchers produced an amplification by a factor of 400—that’s enough to greatly boost the performance of terahertz devices, says Jukam. The next step, he says, is to see if they can extend the duration of the amplification phase. He has high hopes for QC laser–based terahertz amplification: "In some sense, it is the ultimate amplifier."
About the Author
Sawato R. Das is a New York City-based science reporter. In October 2009, he reported on the use of metamaterials in smartphones.