THz Laser Temp. Tweaked

In a step toward moving terahertz devices from niche applications to everyday use, researchers in England and the US have increased the operating temperature of a terahertz (THz) quantum cascade laser by nearly 10 degrees.

Terahertz waves are invisible to the naked eye and can penetrate materials like paper, clothing and plastics while remaining harmless to humans. Terahertz spectra can indicate explosives or analyze complex pharmaceutical substances in ways current technologies, such as x-rays, cannot. However, terahertz systems are impractical because they require expensive lasers that function only at temperatures well below zero, liquid-helium-cooled detectors and bulky optical benches that make field work unfeasible.

A collaboration between the Universities of Leeds and Harvard recorded the highest operating temperature to date for a THz quantum cascade laser (QCL), which at -95 °C (-139 °F) is still a long way from the room temperature needed to make use of the devices more practical, but which presents a step toward the goal of creating a small, portable THz device.

The Leeds team, led by professors Edmund Linfield and Giles Davies from the Faculty of Engineering, achieved the record temperature by using copper instead of gold for the cladding material in the metal-metal THz QCL waveguides. The previous highest operating temperature recorded for the laser was -104 °C.

“The potential uses for terahertz technology are huge, but at the moment they are limited to niche applications in, for example, the pharmaceutical industry and astronomy, as the current systems on the market are expensive and physically quite large. The availability of cheap, compact systems would open up a wide range of opportunities in fields including industrial process monitoring, atmospheric science, and medicine,” said Linfield, professor of terahertz electronics in the School of Electronic and Electrical Engineering at Leeds.

The researchers believe they can raise the laser's operating temperature even more, inching handheld terahertz technology even closer to practical use.

“We hope to obtain further advances by optimizing the methods we used to create the device,” Linfield said. “We have some radically new design ideas, and also believe that we can make significant improvements in the way we fabricate the lasers.”

Terahertz QCLs are created by building layers of compounds of aluminium, gallium and arsenic one atomic monolayer at a time, through a process known as molecular beam epitaxy. Leeds’ Faculty of Engineering is one of a small number of laboratories in the world actively "growing" THz QCLs using a molecular beam epitaxy system purchased through the Science Research Infrastructure Fund.

In molecular beam epitaxy, the chemicals evaporate from heated cells, and land on a heated, rotating, substrate. Minute changes in temperature, combined with a set of shutters that block the chemical beams, enable the team to adjust the amount of each chemical which is deposited on the substrate, gradually building up the layers they need. To ensure the device works perfectly, there must be no pollutants, so the process is carried out under ultrahigh vacuum conditions, approaching the vacuum levels found in outer space.

The research was carried out in collaboration with the group of professor Frederico Capasso at Harvard University, who demonstrated the first quantum cascade laser at mid-infrared frequencies in 1994. Linfield and Davies led the team that first demonstrated a quantum cascade laser at terahertz frequencies, in 2002 when they were at Cambridge University; Davies is now professor of electronic and photonic engineering at Leeds.

The research is supported by the UK's Engineering and Physical Sciences Research Council (EPSRC) was published in a March issue of Optics Express.

For more information, visit: www.leeds.ac.uk