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Semiconductor Lasers

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Definition: lasers based on semiconductor gain media

German: Halbleiterlaser

Category: lasers

How to cite the article; suggest additional literature

Semiconductor lasers are lasers based on semiconductor gain media, where optical gain is usually achieved by stimulated emission at an interband transition under conditions of a high carrier density in the conduction band.

The physical origin of gain in a semiconductor (for the usual case of an interband transition) is illustrated in Figure 1. Without pumping, most of the electrons are in the valence band. A pump beam with a photon energy slightly above the bandgap energy can excite electrons into a higher state in the conduction band, from where they quickly decay to states near the bottom of the conduction band. At the same time, the holes generated in the valence band move to the top of the valence band. Electrons in the conduction band can then recombine with these holes, emitting photons with an energy near the bandgap energy. This process can also be stimulated by incoming photons with suitable energy. A quantitative description can be based on the Fermi–Dirac distributions for electrons in both bands.

Most semiconductor lasers are laser diodes, which are pumped with an electrical current in a region where an n-doped and a p-doped semiconductor material meet. However, there are also optically pumped semiconductor lasers, where carriers are generated by absorbed pump light, and quantum cascade lasers, where intraband transitions are utilized.

gain in a semiconductor

Figure 1: Physical origin of gain in a semiconductor.

Common materials for semiconductor lasers (and for other optoelectronic devices) are

These are all direct bandgap semiconductors; indirect bandgap semiconductors such as silicon do not exhibit strong and efficient light emission. As the photon energy of a laser diode is close to the bandgap energy, compositions with different bandgap energies allow for different emission wavelengths. For the ternary and quaternary semiconductor compounds, the bandgap energy can be continuously varied in some substantial range. In AlGaAs = AlxGa1−xAs, for example, an increased aluminum content (increased x) causes an increase in the bandgap energy.

While the most common semiconductor lasers are operating in the near-infrared spectral region, some others generate red light (e.g. in GaInP-based laser pointers) or blue or violet light (with gallium nitrides). For mid-infrared emission, there are e.g. lead selenide (PbSe) lasers (lead salt lasers) and quantum cascade lasers.

Apart from the above-mentioned inorganic semiconductors, organic semiconductor compounds might also be used for semiconductor lasers. The corresponding technology is by far not mature, but its development is pursued because of the attractive prospect of finding a way for cheap mass production of such lasers. So far, only optically pumped organic semiconductor lasers have been demonstrated, whereas for various reasons it is difficult to achieve a high efficiency with electrical pumping.

Types of Semiconductor Lasers

There is a great variety of different semiconductor lasers, spanning wide parameter regions and many different application areas:

Typical Characteristics and Applications

Some typical aspects of semiconductor lasers are:

Such characteristics have made semiconductor lasers the technologically most important type of lasers. Their applications are extremely widespread, including areas as diverse as optical data transmission, optical data storage, metrology, spectroscopy, material processing, pumping solid-state lasers (→ diode-pumped lasers), and various kinds of medical treatments.

Pulsed Output

Most semiconductor lasers generate a continuous output. Due to their very limited energy storage capability (low upper-state lifetime), they are not very suitable for pulse generation with Q switching, but quasi-continuous-wave operation often allows for significantly enhanced powers. Also, semiconductor lasers can be used for the generation of ultrashort pulses with mode locking or gain switching. The average output powers in short pulses are usually limited to at most a few milliwatts, except for optically pumped surface-emitting external-cavity semiconductor lasers (VECSELs), which can generate multi-watt average output powers in picosecond pulses with multi-gigahertz repetition rates.

Modulation and Stabilization

A particular advantage of the short upper-state lifetime is the capability of semiconductor lasers to be modulated with very high frequencies, which can be tens of gigahertz for VCSELs. This is exploited mainly in optical data transmission, but also in spectroscopy, for the stabilization of lasers to reference cavities, etc.

Bibliography

[1]J. V. Moloney et al., “Quantum design of semiconductor active materials: laser and amplifier applications”, Laser & Photon. Rev. 1 (1), 24 (2007)
[2]W. W. Chow and S. W. Koch, Semiconductor-Laser Fundamentals, Springer, Berlin (1999)
[3]B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics, John Wiley & Sons, Inc., New York (1991)

(Suggest additional literature!)

See also: lasers, laser diodes, external-cavity diode lasers, broad area laser diodes, diode bars, diode stacks, edge-emitting semiconductor lasers, surface-emitting semiconductor lasers, quantum cascade lasers, mode-locked diode lasers, semiconductor optical amplifiers

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