Semiconductor lasers

Semiconductor lasers or laser diodes play an important part in our everyday lives by providing cheap and compact-size lasers. They consist of complex multi-layer structures requiring nanometer scale accuracy and an elaborate design. Their theoretical description is important not only from a fundamental point of view, but also in order to generate new and improved designs. The description can be done at various levels of accuracy and effort, resulting in different levels of understanding. It is common to all systems that the laser is an inverted carrier density system. The carrier inversion results in anelectromagnetic polarization which drives an electric field E(t). In most cases, the electric field is confined in a resonator, the properties of which are also important factors for laser performance.

How diode lasers make light

In a laser diode, we take things a stage further to make the emerging light more pure and powerful. Instead of using silicon as the semiconductor, we use a different material, notably an alloy of aluminumand gallium arsenide (indium gallium arsenide phosphide is another popular choice). Electrons are injected into the diode, they combine with holes, and some of their excess energy is converted into photons, which interact with more incoming electrons, helping to produce more photons—and so on in a kind of self-perpetuating process called resonance. This repeated conversion of incoming electrons into outgoing photons is analogous to the process of stimulated emission that occurs in a conventional, gas-based laser.

Semiconductor lasers

Artwork: The basic setup of a laser diode. Laser light is produced when electrons and photons interact in a p-n junction arranged in a similar way to a conventional junction diode or LED. One end of the diode is polished so the laser light can emerge from it. The other ends are left roughened to help confine the light.

In a conventional laser, a concentrated light beam is produced by “pumping” the light emitted from atoms repeatedly between two mirrors. In a laser diode, an equivalent process happens when the photons bounce back and forth in the microscopic junction (roughly one micrometer wide) between the slices of p-type and n-type semiconductor, which is technically known as a Fabry-Perot resonant cavity (a kind of interferometer). The amplified laser light eventually emerges from the polished end of the gap in a beam parallel to the junction. From there, it goes on to read music from your CD, scan the price on your cornflakes, print out your college dissertation, or do a thousand other useful things!

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