Graphene tunes THz laser

Researchers at the University of Manchester have used a graphene layer to reversibly alter the output of a THz quantum cascade laser.
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 this case is not a giving clean adjustment of output amplitude or frequency, but altering the number of modes (output wavelengths) simultaneously present in the resulting laser beam.
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 beams 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 polymer coated deposited on top of the device. Crosswise slits in the polymer decide which modes are promoted and which are suppressed.
This is normally a permanently fixed situation, but in the Manchester case there is a layer of graphene on top of the polymer and biasing the graphene modifies the characteristics of the slits and therefore changes the output spectrum.
Hysteresis is present in the system, so control voltage has to be pulled beyond an earlier setting to achieve an earlier spectrum.