QCLs: mass-market devices or just interesting physics?

An industry exploiting QCLs would not be limited by potential applications. For example, QCLs are the only solid-state lasers that can access the mid-infrared region of the spectrum where toxic gases and vapors have their tell-tale absorption features. This gas-sensing market is worth $7 billion worldwide, and opportunities for QCLs also exist in a variety of other lucrative markets: detection of explosives, oil and gas exploration, medical diagnostics, and perhaps even countermeasure systems to protect aircraft from missile attack.

One company hoping to exploit the opportunities provided by QCLs is UK-based laser-applications business Cascade Technologies, a spin-off from the University of Strathclyde. The company was founded in January 2003, but was busy raising funds until April 2004, and aims to produce a highly sensitive gas sensor by April 2005. Cascade has developed prototypes emitting at around 10.25 µm which have been used for the analysis of car fumes, detecting levels of carbon dioxide and of ethylene, an unburnt hydrocarbon.

"Cascade s first product is a packaged laser system which enables QCLs to be operated in a user-friendly way," said Richard Cooper, the company s director of operations. Its advantages include "ease of use, that it is simple to set up and install, and its small size". Cascade s new system will be demonstrated at the 5th Workshop on Quantum Cascade Lasers, which will take place in Freiberg, Germany, on September 23-24.

Sensing the opportunity The gas sensor is sensitive to the parts-per-trillion level, can operate at room temperature with up to 1 million measurements per second, is portable, and requires no consumables, claims Cooper. "The initial price of the gas sensor will be comparable to high-end gas chromatography mass spectrometry [GCMS], but it offers superior performance and requires only limited training to use," he said. GCMS is the present industry choice and achieves sensitivities in the part-per-billion range, but it takes around 40 minutes to produce a reading, according to Cooper.

Cascade is not the only company seeking to exploit QCLs for gas sensing. Physical Sciences, based in Andover, MA, has developed two separate systems using 5 ns pulses, one for detecting carbon monoxide and the other for detecting nitrogen dioxide. With a sensitivity of 10 parts per billion, these products are suitable for atmospheric monitoring, pollution emission monitoring, and the analysis of exhaled human breath.

Cooper explains that systems from competitors tend to use significantly shorter pulses and vary the laser frequency by superimposing a slow voltage ramp onto the pulse train. One pulse corresponds to one data point, so several seconds are required to build up a picture. "Meanwhile, the gas has moved on," pointed out Cooper. He believes Cascade s other advantage is that its system has fewer complex, expensive elements.

Cascade plans to exploit a discovery by its technical director, Erwan Normand, that will make positive use of a QCL feature that was previously seen as a disadvantage. The technique uses a 300 ns pulse that contains a spread of wavelengths caused by changes in laser temperature. The resulting pulse covers a spectral window broad enough to fingerprint several molecules at once, and gives a "full picture in less than a millionth of a second".

Since the global gas-sensing market is so valuable, Cascade needs to take only a small cut to be successful. Cooper explains that other potential uses for its systems include the oil and gas industry, for both process and environmental monitoring, and in the longer term medical diagnostics, such as determining diseases from breath analysis. The point-of-care medical diagnostic industry alone is valued at $22 billion.

The source of QCL chips for Cascade, along with the US-based Jet Propulsion Lab, the Fraunhofer Institute in Germany, and many others, is Alpes Lasers, a spin-off from the University of Neuchâtel, which currently employs about 10 people. It was founded in 1998 by Matthias Beck, Antoine Muller and Jérôme Faist, co-inventor of the device. Alpes is growing slowly but steadily, and senior R&D engineer Yargo Bonetti believes it is the only company in the world offering commercially grown QCLs.

A solitary manufacturer Bonetti feels that the QCL market is big enough to support two companies of Alpes s size, but demand is insufficient for a large enterprise to enter the business. "Laser demand is not sufficient to produce thousands or tens of thousands of lasers per week," Bonetti explained. He notes that four or five competitors have emerged in the last year only to withdraw, presumably because the venture offers little reward for the efforts involved. US-based Applied Optoelectronics, for example, was manufacturing QCLs until about 12 months ago.

Alpes conducts its MBE growth of QCLs at the University of Neuchâtel. Currently developing CW sources, Alpes offers either pulsed singlemode QCLs in the regions of 5.3-6 µm and 10-10.5 µm, or pulsed multimode QCLs, operating either at 5.0-6.2 µm or at 8.5-10.5 µm. Peak power outputs are 100-500 mW. The company also offers a "starter kit", including the electronics required to drive the device and a temperature controller. Prices for off-of-the-shelf QCLs are €10,000-20,000 ($12,000-24,000), but tailor-made devices sell for around €50,000.

QCL lifetimes are long and difficult to evaluate. Tests at elevated temperatures suggest it takes 10-50 years (or longer) for a QCL s output power to fall by 20%. Bonetti reveals that some early devices failed after only one or two years because of soldering or bonding issues, although these problems are now largely resolved.

Production times for QCLs at Alpes vary considerably, from just three weeks for established structures to about nine months for custom designs at new wavelengths. Bonetti believes the high level of skill needed to fabricate QCLs discourages potential competitors. "All the processes involved in producing these lasers are quite well-known, and are also published. However, to know how to basically make these lasers does not solve the problem of [actually] making them. There s a lot of know-how that s not documented." Bonetti acknowledges that the small quantities of lasers made today hinder yield improvements.

This view is not shared by Carlo Sirtori, another co-inventor of the QCL, who holds joint roles as a consultant at Thales s research and technologies division in Orsay, France, and as a professor at Université Paris 7 Denis-Diderot. He believes QCL growth is comparable to that of vertical-cavity surface-emitting lasers (VCSELs), a well-established technology. Although QCLs have more interfaces than VCSELs, abrupt interfaces are not critical, and QCLs unipolar nature circumvents complications associated with dopants altering the material growth rate. Sirtori believes MBE holds the upper hand over MOCVD for QCL growth, thanks to its greater control of very thin layers, but he added: "If you care less about wavelength, and more about time, then MOCVD has an advantage."

Sirtori says that Thales is trying to implement antimonide materials on InP substrates, to produce high-power QCLs emitting at around 4 µm for applications such as detecting trace gases and optical free-space communication - in the 3-5 µm region the atmosphere is relatively transparent. To achieve these short wavelengths requires a relatively large separation of the electron energy levels in the quantum well of the active region. Material systems such as AlAsSb barriers and InGaAs wells, lattice-matched to InP, are suitable - the large difference between their bandgaps allows formation of deep quantum wells providing emission at around 4 µm.

Of great interest for the future, in Sirtori s opinion, are QCLs operating in the terahertz region of the spectrum. "There is a gap in the region between 100 µm to below the terahertz [1.0 THz = 300 µm], with no [commercial] semiconductor lasers or transistors operating at these frequencies. The transistor dies at about 500 GHz. That is why this device [the terahertz laser] is important." As figure 1 shows there have been only a handful of reports of lasing above 100 µm, and all with devices operating at around liquid-nitrogen temperature.

Converging dimensions "The size of a quantum well in the terahertz laser, the unit of our structure, is about 50 nm," Sirtori continued. "Silicon technology today has a 100 nm gate size, but the industry is studying transistors with a gate length of 50 nm. You can clearly see the merging of electronics and optoelectronics - the size of a gate is comparable to the quantum well where the photon is generated. My feeling is that the two worlds are getting together."

Returning to one of today s issues, the problem of having only one commercial QCL manufacturer, Sirtori commented: "If I would want to produce a system, all my investment towards this new idea would rely on a single small company. I would be really worried."

The factors affecting the growth of an industry for QCL devices are intriguing - the potential markets for QCLs are massive, but initial market penetration will require a reliable supply of devices. Clearly, competition between even only a handful of QCL manufacturers would drive performance up and prices down. However, current demand for QCLs is sufficient for probably only one other small operation, and the decision by Applied Optoelectronics to cease QCL production can only be detrimental to the development of the emerging sector.

Further information Operating principles of a QCL The diagram shows a section from the conduction band of a typical QCL. The valence band has been omitted because QCLs are unipolar devices, exclusively n-doped, with holes playing no part in the device.

The formation of typically 25-75 alternating injector and active regions is achieved through the precise control of several hundred layers of material, each only a few nanometers thick. The active region contains three quantum wells with three discrete energy levels; the injector consists of layers too thin to confine electrons, and instead a miniband and a minigap are formed.

QCLs are essentially electronic waterfalls, with one electron capable of generating tens of photons. Electrons, present in the miniband owing to high n-type doping, are driven (left to right) towards the active region by an applied electric field. Once trapped in the quantum well of the active region, relaxation from the third to the second energy state releases a photon - and its wavelength is solely determined by the difference in energy between these two states.

The device design optimizes light output, with the position of the minigap suppressing electrons exiting the active region by tunneling into the lower injector region without radiating.

Through lattice vibrations the electron then rapidly relaxes into the first state, before entering the next injector region. Again the electron, under the influence of the electric field, traverses the injector region, and emits a photon in the active region. It is this cascading process that enables just one electron to generate many photons.

Although electron relaxation determines the lasing wavelength, the laser cavity supports up to 20 closely spaced wavelengths. A Bragg grating can be used to select a single wavelength, which is essential for many applications. The QCL can then be tuned over a wide range of wavelengths by changes in temperature which alter the cavity s effective length through changes in refractive indices.