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Summary Article: semiconductor laser from The Penguin Dictionary of Physics

Syn. diode laser; injection laser. A small robust cheap and flexible type of LASER fabricated from SEMICONDUCTOR material. It consists essentially of a P-N JUNCTION diode, being a refinement of the LIGHT-EMITTING DIODE.

Laser action is nearly impossible to produce in silicon, and semiconductor materials such as gallium arsenide must be used. Under a forward bias, electrons flow across the junction from the n-side to the p-side, where they form an excess minority-carrier concentration; the process is called electron INJECTION. These electrons can recombine with HOLES in the p-side, emitting a photon by spontaneous emission; the photon energy is approximately equal to the energy gap between conduction and valence bands, Eg. At sufficiently large values of applied voltage, great numbers of electrons can cross the junction. Above a threshold current, stimulated emission (see LASER) can occur: an electron excited into the conduction band is induced to emit a photon by a photon from a previous recombination event. As the injection current increases, the stimulated emission increases. The photon produced by stimulated emission matches the incident photon in both energy and phase; a narrow range of wavelengths is produced by the device as a whole.

The diode is usually constructed to have two flat parallel ends of the crystal, which are perpendicular to a flat p-n junction. These ends act as partially reflecting mirrors and the light can be reflected back into the p-n region, causing further amplification. The diode acts as a resonant cavity, the light and its reflection being in phase. An intense laser beam emerges from the mirror ends of the crystal. A very high current density is required, and to prevent overheating at room temperature the beam has to be pulsed.

A continuous laser beam can be achieved by using a modified crystal. A region of pure GaAs is made adjacent to a region of aluminium gallium arsenide in which some of the gallium atoms in the GaAs crystals have been replaced by aluminium atoms. The junction between these two regions of similar crystal structure (a heterojunction) can be used to reduce the threshold current required to achieve laser action, and a continuous laser beam is possible. The output power is tens of milliwatts at wavelengths between 900-700 nm and the efficiency can be as high as 10%.

A range of different materials and modifications to the basic structure have now been produced to provide lasers of different wavelengths and to optimize the operation. Semiconductor lasers have an extensive range of applications.

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