Terahertz technology is set to enter the mainstream, enabling better performance in a wide range of applications

7 min read

Developing sources and detectors of electromagnetic radiation has been fundamental to scientific progress. But such devices have been lacking in one part of the spectrum – the so called 'terahertz gap'. This area, in which practical technologies for generating and detecting the radiation do not exist, lies between the microwave and infrared, covering frequencies from 0.1 to 10THz.

The THz gap matters because radiation from this part of the spectrum has special features. Terahertz light has remarkable properties: it is intrinsically safe, non ionising, non destructive. Detecting reflected THz radiation makes it possible to create spectroscopic information and 3D images with unique spectroscopic signatures – terahertz fingerprints – not found at other wavelengths like optical and infrared. THz imaging also produces results more quickly than X-rays.

T-Light from Menlo Systems is a compact turnkey femtosecond laser said to be suited to use in applications ranging from ultra fast spectroscopy and material characterisation to THz physicsTeraSys 4000 from Rainbow Photonics is a spectrometer that operates over frequencies ranging from 300GHz to 4THz. It is said to be ideal for use in spectroscopy, production technology and security

That is why its potential applications range so widely – from medical imaging, biological research, pharmaceutical monitoring and semiconductor testing to security, communications, manufacturing and quality control.

Science has used THz detection for decades to investigate things like dust in our galaxy and in telescopes like those at Mauna Kea and the Herschel Space Observatory. But these are multi million dollar projects, using highly specialised, custom built THz systems, often involving extreme cooling – a long way from the commercial world.

The conventional electronic sources and detectors used for radio and microwaves, which are powerful, compact, easy to use at room temperature and affordable, remain stubbornly difficult to produce for the THz band.

But things are changing and the field of THz photonics is growing in importance. This explains the emergence of several companies in the field, like Teraview, Toptica, Menlo Systems and Rainbow Photonics, who claim it will become a major area of technology over the next decade.

An academic who agrees is Daniel Mittleman, Professor in the Electrical and Computer Engineering department at Rice University in the US. He sees several application areas where THz photonics now has major potential. First is the development of techniques for table top generation of very high intensity THz pulses.

"Peak fields of 1MV/cm are not unusual any more. This opens up an entirely new realm of THz nonlinear optics, with huge impact in condensed matter spectroscopy, for example."

Secondly, there is the future of wireless communications.

"It is inevitable that consumer accessible wireless networks will operate at more than 100GHz," he says. "Although 60GHz is already in production, the technologies required for frequencies in the range from 100 to 400GHz are going to look very different."

A fundamental reason for the growth in THz photonics is semiconductors, notably the improving capabilities of silicon CMOS devices at higher frequencies.

"If you can replace a $100,000 laser with a $1 silicon chip that does the same thing, then the world will beat a path to your door. The technology is not quite there yet. But I envision Moore's law will continue to carry CMOS technology to higher frequencies and higher power, and it will have a growing impact on the THz world in the years to come."

Finally, THz cameras are coming of age – megapixel focal plane arrays with fast read-out capabilities.

"For many years, this was a 'Holy Grail', and now it's here," Mittleman says. "They are still too expensive, of course, but that will change in time. As the price falls, this is going to have a huge impact on many of the proposed applications, especially in security and sensing."

A major source of THz radiation is the quantum cascade laser (QCL), which typically operates between 2 and 5THz and is revolutionising the THz field. Previous THz sources were broadband, time domain based systems using a femtosecond, mode locked laser, which excited a photoconductive switch to generate the THz radiation. But these have drawbacks, like high cost and their relative low power. The QCL was the first semiconductor device that could emit high power, narrowband THz radiation.

The THz QCL emerged from research in Cambridge by Professors Giles Davies and Edmund Linfield, both now at Leeds University, which is a leading centre for THz R&D. Unlike typical semiconductor lasers, that emit through the recombination of electron–hole pairs across the material band gap, the laser emission by QCLs is achieved by exploiting phenomena that emerge from a repeated stack of semiconductor multiple quantum well heterostructures.

"The QCL is a really nice example of theoretical physics working," says Paul Dean, EPSRC Research Fellow at Leeds University. "Their operation involves complex quantum mechanics regarding things like the electron transport. So, in order to design them, you need sophisticated modelling tools.

The THz QCL hasn't reached full commercialisation yet because it still requires cryogenic cooling, restricting use in many industrial applications. If they could work at room temperature, it would make a big difference."

But progress is being made, thanks to improved designs and better control of the molecular beam epitaxy process for producing the semiconductors. This has enabled the Leeds team to develop THz systems capable of working at temperatures of more than 77K. The importance of this is development is that liquid nitrogen can be used for cooling, rather than helium. This reduces the cost drastically – from more than £1 per litre to a fraction of a penny – and makes the whole process easier. Many standard tools, such as MRI scanners, use liquid nitrogen cooling.

While generating THz radiation has been one technical challenge, detecting it has been another because THz photons carry about 100 times less energy than those of visible light, making them harder to detect. A lot of thermal processes that happen in the detectors can cause problems, hence the need for powerful cooling.

One detector innovation developed recently at Leeds is to use the QCL as both source and detector. The THz radiation from the laser is reflected by the external target and goes back into the laser.

"Just by measuring the voltage across the laser, you can measure quite accurately the radiation coming off your target. That has simplified things because instead of systems requiring the laser and a cryogenically cooled detector, you only need a laser with some simple focusing optics. We are now looking to explore the commercialisation of this," Dean says.

Another advance towards compact, sensitive and fast THz detectors was announced recently by Italian and French researchers. By exploiting the excitation of plasma waves in FETs, they have been able to create the first detectors based on semiconductor nanowires. The team also developed the first THz detectors made of mono- or bilayer graphene.

"Our work shows that nanowire FET technology is versatile enough to enable 'design' via lithography of the detector's parameters and its main functionalities," explained Miriam Serena Vitiello, leader of the Terahertz Photonics Group in the Nanoscience Institute in Pisa.

The new nanowire detector operates at room temperature, can reach detection frequencies greater than 3THz, has a maximum modulation speed in the MHz range and a noise performance already competitive with the best commercial technologies, Vitiello says.

The nanodetectors can handle large area, fast imaging across both THz and sub THz spectra, making possible a range of spectroscopic and real time imaging applications – and possibly fast megapixel THz cameras.

Vitiello says: "The aim now is to push performance into the ultrafast detection realm, explore the feasibility of single photon detection by using novel architectures and material choices, develop compact focal plane arrays, and to integrate on chip the nanowire detectors with THz quantum cascade microlasers. This will allow us to take THz photonics to a whole new level of compactness and versatility, where it can address many 'killer' applications."

Even though much THz photonics development has been for scientific uses, one company that straddles the scientific and commercial worlds is QMC Instruments. This operates from within Cardiff University's School of Physics and Astronomy, marketing work done by the University's Astronomy Instrumentation Group (AIG). QMCI and AIG develop THz instrumentation principally for scientific organisations. They are used in diverse applications such as atmospheric remote sensing, astronomy, semiconductor materials characterisation, hot plasma fusion diagnostics and electron spin resonance spectroscopy.

"The challenges of detecting astronomical THz radiation are considerable," says Richard Wylde, QMCI's managing director. "Small signals at long wavelengths must be detected in the presence of much larger backgrounds," he explains. "It requires highly sensitive detectors operating at ultra low temperatures – less than 1K – specialised filters and optics to block unwanted radiation and heat, as well as innovative low loss optical designs."

THz instruments have been built for ground based facilities, balloon borne experiments and many satellite projects, helping in pioneering surveys of the remnant light from the Big Bang and resulting in significant progress in our understanding of the early universe.

Some are massive projects, such as the billion dollar ALMA (Atacama Large Millimetre Array) telescope project in Chile, for which QMCI supplied devices such as cooled polarisers; or in fusion reactors, where it has supplied a multi-channel detector for laser interferometry of the plasma in a Chinese superconducting tokamak (the torus which contains the plasma). For structural biology, QMCI has built systems for detecting electron spin resonance.

One recent project involved a THz passive imaging camera developed by NEC, which commissioned QMCI to provide a filter to reject high frequency radiation. This solved the serious issue of ghost image generation and resulted in the launch of the Soltec THz Imager, which has been used successfully by rescue workers in fires.

Technical advances have recently centred on improvements in detector sensitivities, helped by enhancements in cooling technology.

"We have seen improvements of two to three orders of magnitude and are reaching sensitivities of 10-18W," Wylde says. "These could have commercial significance, for example in security scanning."

One novel use of THz imaging has been performed by Reading University, which has used optics made by QMCI to build THz radiometers to analyse artworks to see if they have been painted or plastered over. Using a pulsed THz imaging system sited at the Louvre in Paris, art heritage researchers can see what lies beneath coats of plaster or paint. They are also working on archaeological applications of pulsed THz imaging.

Dr John Bowen, from Reading's School of Systems Engineering, has recently worked on other cultural objects, including an Egyptian bird mummy and Palaeolithic cave art.

"We used a portable, fibre coupled THz time domain spectroscopic imaging system, which allowed us to measure specimens in both transmission and reflection geometry, and present time and frequency based image modes. The results confirm earlier evidence that THz imaging can provide information complementary to that obtainable from X-ray CT scans of mummies, giving better visualisation of low density regions. In addition, THz imaging can distinguish mineralised layers in metal artefacts."

Perhaps one of the most interesting applications for THz technology came recently in the Rosetta mission, where the Philae probe landed on the surface of comet Churyumov-Gerasimenko – or 67P. QMCI provided the project with a radiometer to look at the amount of isotopic oxygen in the water vapour being boiled off as the comet neared the Sun, investigating the theory that all water on the Earth might have come from comets.