Terahertz waves penetrate the world of imaging

24 Sep 2002

Foiling terrorists, detecting cancer and designing new drugs are just some of the potential applications for equipment that exploits the unique properties of terahertz waves. Oliver Graydon looks behind the scenes at one of the hottest sectors in photonics research today.

From Opto & Laser Europe October 2002

Detecting weapons concealed underneath clothing, analysing the contents of suspicious-looking envelopes, or even spotting the onset of cancer: these are just some of the exciting prospects that have been turning terahertz wave research into one of the most talked-about topics in photonics.

Terahertz waves are electromagnetic waves that have a frequency of between 100 GHz and 30 THz and lie between the infrared and microwave parts of the spectrum. What makes these waves so fascinating to scientists is their ability to penetrate materials that are usually opaque to both visible and infrared radiation.

For example, terahertz waves can pass through fog, fabrics, plastic, wood, ceramics and even a few centimetres of brick - although they can be blocked by a metal object or a thin layer of water. The way in which terahertz waves interact with living matter has potential for highlighting the early signs of tooth decay and skin or breast cancer, or understanding cell dynamics.

Growing demand The list of potential customers for terahertz wave technology is growing all the time. The military wants high terahertz and millimetre-wave imagers that are able to "see through" bad weather; chemists want spectroscopy equipment for analysing the structure of new drugs; and airports arguably need better security-screening equipment.

As a result, research on terahertz waves is fast evolving beyond being a mere scientific curiosity. There is now a buzz of activity as researchers around the world race to build the first practical terahertz imaging and spectroscopy equipment.

"The field has exploded - it's incredible," said Paul Planken from Delft University of Technology, who is an active researcher in the field. "A number of years ago I tried to make a list of all the papers published on terahertz generation and detection. I started with 1969 and there was just one paper. Now I see so many papers being published that I refuse to believe that 10 years from now there will be no real-life applications for terahertz technology."

The Delft team has built a state-of-the-art system for generating and detecting terahertz waves and has already applied it to various imaging tasks, such as filming the diffusion of a gas through polystyrene foam. The team is now investigating biomedical applications and has recently demonstrated a method of dramatically improving the resolution of a terahertz imaging system.

Powerful potential Although the majority of the initial research on terahertz imaging was carried out by Martin Nuss and colleagues at Bell Labs in the early 1990s, today the leading research group in the field is undoubtedly Xi Cheng Zhang's group at Rensselaer Polytechnic Institute, US. For more than a decade the group has been pushing back the frontiers of terahertz technology and has published a huge number of papers on the topic. The Institute has recently established a dedicated facility for terahertz studies, the Center for Terahertz Research, and has received a $1m (€1.03m) donation from the W M Keck Foundation.

At this year's CLEO conference in May in California, Zhang and co-workers from the University of Adelaide, Australia, and the New York State Department of Health, US, reported initial results on the use of terahertz waves to screen potential biohazards.

The team found that terahertz waves could detect and classify unknown powders that were sealed inside an envelope. Using a transmission terahertz imaging technique operating at 0.3 THz, the team successfully detected and distinguished between samples of flour, salt, baking soda and bacterial spores placed inside a paper envelope.

Outside academia, one of the first companies attempting to cash in on the market potential and commercialize terahertz technology is Teraview, a start-up based in Cambridge, UK. With a headcount of 13, the firm was spun out of Toshiba Europe's Research Laboratory in April 2001 to develop equipment for medical imaging, drug development and security screening.

The start-up has not wasted any time. Having built prototype medical-imaging equipment, the Cambridge-based firm has tested its terahertz technique for skin-cancer detection in field trials at UK hospitals, as well as using it to image semiconductor chips for electronics companies.

"Although there are lots of universities carrying out research in the field, we are the first company dedicated to commercializing terahertz technology," claimed Don Arnone, Teraview's chief executive officer. "We're starting to bring products to market and will have a product launch shortly."

Teraview's prototype medical imager, the TPI Scan, resembles a photocopier on wheels. It squeezes the laser, optics and electronics needed for terahertz imaging into a self-contained 1 m long, 1 m high and 60 cm wide trolley that weighs 150 kg. The TPI Scan can scan a sample of up to 25 x 25 mm in size in less than 1 min, and features an integral camera that simultaneously generates a visible image of the sample. The resolution of the scanning is 200 µm in one axis and 20 µm in the other.

The equipment is likely to retail for around £250,000 (€395,000) and will be used to help diagnose skin lesions and plan surgery. It features an external CD writer, USB port and network connection for data transfer.

As for customers, Teraview has already signed a distribution agreement with Bruckner, which makes analysis equipment for the pharmaceuticals industry. "We're looking to align ourselves with established players, such as Bruckner, in each of our key markets," said Arnone. "We'll produce a terahertz engine [a module for emitting and detecting terahertz waves] and then supply it to commercial collaborators for them to market."

Teraview is also working closely with the semiconductor group at Cambridge University to develop a semiconductor laser that operates in the terahertz region. To date, terahertz waves are usually generated by illuminating a piece of semiconductor, such as gallium arsenide, with femtosecond pulses from a solid-state laser such as a Ti:sapphire laser. Although this approach works well and femtosecond lasers are getting smaller and cheaper all the time, it is still a relatively bulky and expensive solution. Ultimately, the preferred source for many commercial applications would be a compact semiconductor laser.

Quantum leap The development of such emitters has recently taken a leap forward, thanks to the invention at Bell Labs, US, of the quantum-cascade laser, which emits in the mid-infrared at around 4 µm. Semiconductor scientists are now adapting the technology to design lasers that are operational in the far-infrared and terahertz regions.

Earlier this year, researchers from Teraview, the University of Cambridge, and the National Institute for the Physics of Matter (INFM) in Italy made a series of quantum cascade lasers that operate in pulsed and continuous-wave mode at 4.4 THz (wavelength 68 µm). Although the lasers can only currently work at low temperatures of up to 50 K, they emit up to 2 mW of singlemode terahertz radiation. The challenge now is to raise the operational temperature, which, although it may take several years, is definitely feasible.

"I think the development of a quantum cascade laser is a major milestone, certainly - if they are able to make one that works at room temperature," said Planken. "Up until recently most people generated terahertz radiation by illuminating GaAs with femtosecond laser pulses or by optical rectification with telluride electro-optic crystals. Now they have a third candidate."

Whether it will be its applications in medicine, security or pharmaceuticals that take off first is hard to predict, but once terahertz technology has a toehold in one market it is likely to quickly spread to others.

"Some people say that terahertz radiation will replace X-rays, but that's not true," said Planken. "It will not happen because the human body is made up mostly of water, so you do not get much penetration. Water is a very good terahertz application killer."

T-rays explainedThe powerful nature of terahertz analysis stems from the fact that it is a coherent technique that can make both amplitude and phase measurements. Unlike common optical spectroscopic techniques that only measure the intensity of light at specific frequencies, terahertz experiments often measure the temporal electric field of terahertz pulses that have interacted with (i.e. reflected off or passed through) a sample.

A Fourier transformation of this time-domain data discloses the amplitude and phase of the pulse and reveals a wealth of information about the sample. For example, it allows precise measurements of the refractive index and absorption coefficient of a sample. Molecules also have unique rotation and vibration resonance lines in the terahertz spectrum that can be used as terahertz fingerprints.

The most popular way to generate terahertz waves is to illuminate a carefully engineered semiconductor crystal, such as GaAs, with femtosecond pulses of visible light. This bombardment creates ultrashort pulses of terahertz radiation (typically of the order 100 fs) that can be used for imaging and spectroscopy.

As the pulses reflect from different depths within an object, an "image slice" at a desired depth can be built up by carefully controlling the timing of the pulse detection. A 3D image can be constructed by putting together a number of these sliced images.

Coherent detection of the pulses is achieved by illuminating a second crystal with both the terahertz pulses and a portion of the visible femtosecond pulses that is split off from the original beam and has undergone a suitable time delay.

Breakthrough in resolutionPaul Planken and Nick van der Valk at Delft University of Technology in the Netherlands have improved the resolution of a terahertz imaging system by a factor of almost 1000. The development is significant because it could open the door to terahertz studies of tiny structures, such as living cells.

Imaging systems conventionally have a diffraction-limited resolution (spot size) that is equal to about half of the optical wavelength. In the case of terahertz waves, which have a wavelength of about 1 mm, this corresponds to a resolution of about 0.5 mm - far too big for cell imaging. However, the Delft team has now demonstrated that it can create and measure terahertz spot sizes as small as 8 µm, and that 1 µm may ultimately be possible.

The Dutch researchers achieve such small focal spots by using a sharp metal tip to locally bend and concentrate the electric field of the terahertz beam near the surface of a GaP crystal. The mechanism is analogous to how a lightning rod acts to bend electrical field lines.

In the future the method could be used with a terahertz pump to probe experiments by placing thin samples on the crystal underneath the tip. The sample is illuminated by the pump beam from above and a tightly focused probe beam from below. The crystal is then raster scanned under the tip to generate a 2D terahertz image.