Semiconductor lasers are widely used in modern life. In telecommunications they send signals for thousands of kilometres along optical fibres. In consumer electronics, semiconductor lasers are used to read the data on compact disks and CD-ROMs. Other applications include laser printers and laser pointers. Although they are just the size of a grain of salt, typically a few microns in cross-section and a few hundred microns long, semiconductor lasers are an integral part of the modern world.
The quantum cascade laser is based on a fundamentally different principle to normal semiconductor lasers. It was invented and first demonstrated in 1994. It uses only one type of charge carrier, electrons, and is therefore called a unipolar laser.
In essence the quantum cascade laser operates like an electronic waterfall. Electrons cascade down a series of identical energy steps built into the material during crystal growth, emitting a photon at each step. This is unlike diode lasers which emit only one photon over the similar cycle. In practice this means that quantum cascade lasers can outperform diode lasers operating at the same wavelength by factors greater than 1000 in terms of power due to the cascading effect and the ability to carry large currents.
The other revolutionary aspect of the quantum cascade laser is that it can be designed to emit at any wavelength over an extremely wide range using the same combination of materials in the active region.
In the June issue of Physics World, some of the inventors of the quantum cascade laser, Federico Capasso, Deborah L Sivco, Alfred Y Cho, and Claire Gmachl from Bell Laboratories in the US explain the design of quantum cascade lasers.