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Diode emits practical terahertz pulses

19 Jun 2003

Scientists in the US fabricate a liquid-nitrogen-cooled semiconductor laser that produces 3.4 THz radiation.

Researchers at the Massachusetts Institute of Technology and Sandia National Laboratories have made a convenient source of terahertz radiation.

Their quantum cascade laser (QCL) emits at 3.4 THz, equivalent to 88 µm, and is the first semiconductor-based terahertz source to operate under liquid-nitrogen cooling (Electronics Letters 39 915).

The terahertz band – which sits between the infrared and microwave regions of the spectrum – is creating lots of interest currently as it has imaging applications in security. This is because terahertz waves penetrate clothes and plastic, but are blocked by metal objects.

However, scientists have yet to find a convenient, compact source of terahertz rays. A diode source based on a quantum cascade structure was developed by a UK-Italian collaboration last year (see related story), but it only worked up to 50 K – a temperature too low for liquid nitrogen operation.

The new device made by Benjamin Williams and colleagues emits 3.4 THz pulses at a maximum temperature of 87 K. Although the maximum laser output of 3.4 mW is seen at just 5 K, the device still emits about 1 mW at the liquid nitrogen temperature, 77 K.

Based on gallium arsenide/aluminium gallium arsenide, the laser consists of 175 cascaded four-well modules. This structure was grown on a wafer using molecular beam epitaxy.

The team recorded the emission spectra of 200 ns pulses from the laser. At injection currents below 4.8 A, the emission was singlemode, but as the operating current was increased, the gain shifted to higher frequencies and became increasingly multimode in character.

The team attributes the improved temperature performance of the QCL to an increase in both the cavity length and width. The 2.56 mm cavity length more than halved mirror losses, while the wider waveguide provided a more uniform current distribution.

Michael Hatcher is technology editor of Opto & Laser Europe magazine.

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