Graphene tunes THz laser for the first time

Researchers at the University of Manchester have used a graphene layer to reversibly alter the output of a THz quantum cascade laser. This is the first time such a laser has been electronically tuned.

The research, reported in Science, was led by Dr Subhasish Chakraborty and Sir Kostya Novoselov.

“Current terahertz devices do not allow for tuneable properties, a new device would have to be made each time requirements changed, making them unattractive on an industrial scale," said Novoselov.

Control in the proof-of-principal device does not give anything as straight-forward as adjustment of output amplitude or frequency, but alters the number of modes (output wavelengths) simultaneously present in the resulting laser beam.

However, Chakraborty told Electronics Weekly, the techniques developed open the door to sophisticated control of output frequency and amplitude, and the researchers can see a clear path to creating a multi-use semiconductor THz laser that can be electronically tuned for their different applications.

Because of their inflexible fixed outputs, THz quantum cascade lasers have been largely ignored by industry, he added, and electronic tuning could be the key to wider adoption.

A quantum cascade laser is a long thin horizontal device. A cavity is formed by its abruptly cut ends, out of which the beams emerge.

The beam consist not of a single frequency, but of a collection of frequencies (called modes) which can be modified by, in this case, a layer of gold deposited on top of the device. Crosswise slits in the gold decide which modes are promoted and which are suppressed.

This is normally a fixed situation - the pattern of slits set the output modes.

In the Manchester case there is a layer of graphene on top of the gold, separated by a polymer insulating layer to allow the graphene to be biased.

Biasing the graphene modifies the characteristics of the slits and therefore changes the output spectrum.