9.9 QUANTUM CASCADE LASERS

A quantum cascade laser is a fundamentally different type of semiconductor laser in which energy is extracted step by step from electrons passing through a series of quantum wells in a semiconductor structure. Each of the quantum wells has energy levels designed to trap electrons passing through the structure, and an electric field is applied along the length of the structure. Electrons enter the quantum well and are trapped in an upper energy state, where they can be stimulated to emit light and drop to a lower energy level in the quantum well. From that state they can drop to a lower level and tunnel out of that quantum well and into another, as shown in Figure 9-18. The electrons drop successively lower in energy as they pass along the structure, like marbles rolling down stairs.

The slanted steps in Figure 9-18 show the energy level of the conduction band, which is lower in the quantum wells than in the intermediate zones. The energy levels within the quantum well are sublevels within the conduction band. The slant comes from the variation of the electric field along the length of the laser. The electric potential creates a gradient in energy levels that essentially pulls the electron through the structure, like a marble rolling down stairs. Ideally, the electron could be stimulated to emit energy each time it drops to a new quantum well, so one electron

could emit many photons, but it is not that easy. Figure 9-18 shows a simplified view. In practice, each step shown represents a series of thin layers that combine to function as one level.

The energy levels in the quantum wells are tailored by selecting their composition and thickness. In practice, the separation of the levels produces laser emission at wavelengths from a few micrometers to more than 100 µm. Like other semiconductor lasers, quantum cascade lasers are made of III V materials, but they are not junction lasers or diodes.

Quantum cascade lasers are important because they emit infrared wavelengths longer than are readily available from other semiconductors. Those wavelengths are important for sensing.