29 Apr 2008
optics.org speaks to researchers who believe that there is no fundamental physical limit to achieving a room temperature terahertz quantum cascade laser source.
In a bid to develop a handheld semiconductor laser that emits terahertz radiation, researchers from the universities of Leeds, UK, and Harvard, US, have developed a pulsed quantum cascade laser (QCL) that operates at temperatures of up to 178 K. The group is now aiming to develop a terahertz QCL that operates at around 240 K (Optics Express 16 3242).
"We have improved the performance of terahertz QCLs to reach a new record maximum operating temperature of 178 K," Mikhail Belkin, a researcher at Harvard University, told optics.org. "The operating temperatures of terahertz QCLs have not reached their limits and we believe that the laser can reach a technologically significant operating temperature of around 240 K."
Terahertz sources are ideal for security imaging, screening and remote sensing applications, however current systems are complex and expensive. The availability of cheap, compact systems could open up a wide range of opportunities in fields such as industrial process monitoring, atmospheric science and medicine.
"Our aim is to produce a terahertz QCL emitting in the frequency range of 1-5 THz that can be operated on a simple thermoelectric cooler at room temperature," commented Belkin. "The device would measure just a few millimetres or less in size."
The group's current device emits at 3 THz and measures 1.5 mm x 125 µm x 10 µm. According to Belkin, the device marks the first significant improvement in the maximum operating temperature of terahertz QCLs since early 2005. "Until now, the highest operating temperature of a QCL was approximately 165 K, which was demonstrated by an MIT group led by Qing Hu," he said.High-temperature challenges
One of the key difficulties in developing room-temperature QCLs are the electron scattering processes that are activated as temperature increases. "The upper laser state lifetime of terahertz QCLs decreases with temperature due to optical phonon scattering of thermally excited electrons and acoustic phonon scattering," explained Belkin. "There is also the effect of electron backfilling of the lower laser state, which becomes more pronounced with increasing temperature."
The active region in the device contains 226 QCL stages, each consisting of only three quantum wells and three barriers. Because it has so few wells and barriers per stage, the structure can support a much higher current density for a given doping compared with other terahertz QCL designs. "High current density allows us to produce sufficient population inversion between the upper and lower laser states even at higher temperatures," explained Belkin.
In addition, the group substituted gold for copper in the metal–metal waveguides resulting in a reduction of waveguide losses, which helped to increase the maximum operating temperature of the device.
According to Belkin, the simple active region design (which was first reported by the group of Dr. H.C. Liu), combined with the innovative waveguide design and superb quality molecular beam epitaxy structure growth, results in a device that operates closer to thermoelectric cooler temperatures than ever before.
"The key problem that limits the temperature performance of current terahertz QCLs is the fast decrease of the upper laser state lifetime with temperature," commented Belkin. "Fundamentally, this could be overcome by using quantum dots or novel materials with higher optical phonon energy (such as GaN/AlGaN). Alternatively, terahertz QCL sources that do not require population inversion could be designed such as those based on intracavity difference frequency generation or Raman lasing for example."