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In collaboration with professor Frederico Capasso’s group at Harvard University, the UK’s University of Leeds has raised the operating temperature for a terahertz quantum cascade laser (THz QCL) by nearly 10°C, from the previous record of just -104°C (169K) to -95°C (178K) – see Belkin et al, Optics Express, Vol. 16, Issue 5, p3242.

The QCL is an emergent compact source for narrowband radiation at frequencies of 1–5THz (wavelengths of 60–300 microns). Capasso’s group demonstrated the first QCL (at mid-infrared frequencies) in 1994. Meanwhile, supported by the UK’s Engineering and Physical Sciences Research Council (EPSRC), the Leeds team is led by Edmund Linfield, professor of terahertz electronics, and Giles Davies, professor of electronic and photonic engineering, in the School of Electronic and Electrical Engineering, who in 2002 (while at Cambridge University) led the team that first demonstrated a QCL operating at terahertz frequencies. The latest laser operates in pulsed mode at an emission frequency of 3THz.

THz rays are useful for imaging defects within materials without destroying the objects or even removing them from their setting. They are invisible to the naked eye and can penetrate many dry, non-metallic materials (such as paper, clothing and plastics) with better resolution than microwave radiation. Also, unlike ultrasound, terahertz waves can provide images without contacting an object, and they don’t pose the same health risks as X-rays.

“The potential uses for terahertz technology are huge,” says Linfield. “But at the moment they are limited to niche applications in, for example, the pharmaceutical industry and astronomy [as well as detecting explosive], as the current systems on the market are expensive and physically quite large,” he adds. Since they operate only at cryogenic temperatures well below 0°C, they require liquid-helium-cooled detectors and bulky optical benches that make field work unfeasible.

“The availability of cheap, compact systems would open up a wide range of opportunities in fields including industrial process monitoring, atmospheric science, and medicine,” says Linfield. The challenge is therefore to create a THz QCL laser that will work at room temperature (about 300K).

The latest THz QCLs comprise layers of a GaAs/AlGaAs structure grown on an undoped GaAs substrate using a molecular beam epitaxy (MBE) system purchased by Leeds through the Science Research Infrastructure Fund (SRIF). The structures were processed in Harvard University’s Center for Nanoscale Science (CNS), a member of the National Nanotechnology Infrastructure Network.

The improvement in operating temperature was achieved by using an active region with a three-quantum-well (rather than four-quantum-well) resonant-phonon depopulation design and by using copper (instead of gold) for the cladding material in the metal-metal waveguides (in order to reduce optical losses from the waveguides).

The groups from Leeds and Harvard are still a long way from creating a QCL operating at room temperature. However, the researchers believe that they can raise the laser’s operating temperature further. “We have some radically new design ideas, and also believe that we can make significant improvements in the way we fabricate the lasers,” says Linfield. Such advances could bring practical handheld terahertz devices a further step closer to realization.

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