A quantum well laser is a laser diode in which the active region of the device is so narrow that quantum confinement occurs. Laser diodes are formed in compound semiconductor materials that (quite unlike silicon) are able to emit light efficiently. The wavelength of the light emitted by a quantum well laser is determined by the width of the active region rather than just the bandgap of the material from which it is constructed.This means that much shorter wavelengths can be obtained from quantum well lasers than from conventional laser diodes using a particular semiconductor material. The efficiency of a quantum well laser is also greater than a conventional laser diode due to the stepwise form of its density of states function.
Quantum wells
In 1972, Charles H. Henry, a physicist and newly appointed Head of the Semiconductor Electronics Research Department at Bell Laboratories, had a keen interest in the subject of integrated optics, the fabrication of optical circuits in which the light travels in waveguides.
Later that year while pondering the physics of waveguides, Henry had a profound insight. He realized that a double heterostructure is not only a waveguide for light waves, but simultaneously for electron waves. Henry was drawing upon the principles of quantum mechanics, according to which electrons behave both as particles and as waves. He perceived a complete analogy between the confinement of light by a waveguide and the confinement of electrons by the potential well that is formed from the difference in bandgaps in a double heterostructure.
C.H. Henry realized that, just as there are discrete modes in which light travels within a waveguide, there should be discrete electron wavefunction modes in the potential well, each having a unique energy level. His estimate showed that if the active layer of the heterostructure is as thin as several tens of nanometers, the electron energy levels would be split apart by tens of milli-electron volts. This amount of energy level splitting is observable. The structure Henry analyzed is today called a "quantum well."
Henry proceeded to calculate how this "quantization" (i.e., the existence of discrete electron wavefunctions and discrete electron energy levels) would alter the optical absorption properties (the absorption "edge") of these semiconductors. He realized that, instead of the optical absorption increasing smoothly as it does in ordinary semiconductors, the absorption of a thin heterostructure (when plotted versus photon energy) would appear as a series of steps.
In addition to Henry's contributions, the quantum well (which is a type of double-heterostructure laser) was actually first proposed in 1963 by Herbert Kroemer in Proceedings of the IEEE and simultaneously (in 1963) in the U.S.S.R by Zh. I. Alferov and R.F. Kazarinov.Alferov and Kroemer shared a Nobel Prize in 2000 for their work in semiconductor heterostructures.
