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04 Feb 2009
Quantum cascade lasers have opened up a whole host of important applications in the mid-infrared. Marie Freebody speaks to Federico Capasso to find out about the latest progress in the laser's development and the factors that are shaping its commercialization.
Federico Capasso invented the first quantum cascade laser (QCL) in 1994 along with colleagues at Bell Laboratories, US. He has remained active in the field for 15 years and is now the Robert L Wallace Professor of Applied Physics and the Vinton Hayes Senior Research Fellow in Electrical Engineering at Harvard University, US.
Can you explain how a QCL works?
A QCL is radically different from a conventional semiconductor diode laser where the emission wavelength depends on the semiconductor bandgap. A QCL contains a series of quantum wells, and its emission wavelength can be varied simply by changing the thickness of the quantum well layers.
From this point of view, the QCL is an enabling device that allows you to reach all of the mid-infrared spectrum (3–25 µm), a large part of the far-infrared (50–300 µm) and beyond into sub-millimetre wavelengths.
The other key difference is that a QCL uses a cascaded process, which means that once an electron jumps from one upper energy level to a lower level, it is not lost. It can be re-injected into the next stage to emit a second photon. This cascading process allows you to recycle the same electron many times resulting in a very powerful laser.
Why is QCL research important and what are the main applications?
QCLs have opened up mid-infrared pho-tonics, which is very important for sensing applications in fields such as environmental air-quality monitoring, medical diagnostics and homeland security. Before the invention of the QCL, there was no compact mid-infrared semiconductor laser that could operate at room temperature with high power in both pulsed and CW mode.
A QCL can access two important windows (3–5 µm and 8–13 µm) in which the atmosphere is reasonably transparent, allowing detection of pollutants. There are also numerous applications in spectroscopy such as combustion diagnostics, breath analysis for medical purposes and process control in industry.
In terms of defence, chemical sensing can detect explosives or toxic gases. The field of countermeasures is also important in which a powerful QCL can blind the detector of heat-seeking missiles.
How difficult is it to transfer QCLs from the laboratory to the market?
Commercialization of these lasers is in full swing. You can already buy QCL wafers, devices and sensors from a number of companies in Europe and the US. We are currently collaborating with Hamamatsu Photonics in Japan, which is commercializing QCLs. In Europe, Alcatel has a partnership with Thales and has created a company called III-V Laboratories that is manufacturing and selling QCLs.
There is no fundamental difficulty in commercializing these lasers because the underlying technology is the same as found in conventional lasers. A few years ago, my group joined forces with Agilent Technologies to show that a high-performance CW room temperature QCL could be fabricated using the same standard thin-film growth technology (MOCVD) used by semiconductor manufacturers.
Commercialization of QCLs will follow the same curve as traditional lasers – initially the production volume is low and the laser costs are high. But as production volume increases, the price decreases.
What have been the most important technological advances in the field?
The Razeghi group at Northwestern University, US, as well as my group in collaboration with the US-based gas sensor manufacturer Pranalytica, have developed QCLs with higher power levels and higher power efficiencies. Both groups have demonstrated watt-level power output at room temperature with a power efficiency of over 10%. Until recently QCL power efficiency was only around a few percent.
Another important advance made by the Jerome Faist group has been creating QCL sources that span a large wavelength range. Our goal is to make a compact laser spectrometer to compete with a Fourier transform infrared spectrometer.
Currently a terahertz QCL can only operate when cooled to around 200K but the ultimate goal is to achieve higher operating temperatures. Last year, we demonstrated the first room-temperature terahertz QCL-based source by creating a dual-frequency QCL. We employed difference frequency generation to obtain a beam in the terahertz regime. The power level was still small but hopefully we will develop a milliwatt level source in the future.
What will the next big breakthrough be and what key challenges remain?
Remaining challenges include developing a modelocked QCL that emits picosecond pulses, obtaining greater broadband capability and creating a high-performance QCL that emits shorter wavelengths (around 2 3 µm). Another difficult task is to push QCLs to emit at telecoms wavelengths, which is likely to take 5–10 years.
Theory suggests that a power efficiency of 30–35% in the mid-infrared is possible. In the terahertz regime, a thermoelectrically cooled QCL could be developed in the next couple of years that operates above 200K and a room-temperature terahertz nonlinear optical QCL source that emits at milliwatt level power could be possible in the next five years.
• This article originally appeared in the February 2009 issue of Optics & Laser Europe magazine.
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