The name laser comes from the acronym for light amplification by stimulated emission of radiation. Laser is the descendant of the maser which stands for microwave amplification by stimulated emission of radiation. The difference between them is in the range of output frequencies. Laser and maser are both based on the phenomenon of stimulated emission, which was postulated by Einstein in the 1910s. The laser medium can be gas, liquid, amorphous solid, or semiconductor. The semiconductor laser is also called injection laser, junction laser, or laser diode. Maser action was first realized by Townes and his students, and by Basov and Prokhorov, both in 1954, using ammonia gas. For their pioneering work on optical amplification, Townes, Basov, and Prokhorov received the Nobel Prize in physics in 1964. Laser action was obtained first on solid ruby (non-semiconductor) in 1960, and then on helium-neon gas in 1961. The analysis of laser conditions in semiconductor was made by Bernard and Duraffourg, and Dumke. In 1962, four papers reported the injection laser almost simultaneously. These include the works of Hall et al., Nathan et al. and Quist et al. on GaAs while Holonyak and Bevacqua studied GaAsP material. Improvement from heterostructures was suggested by Kroemer, and by Alferov and Kazarinov in 1963. Eventually Hayashi et al. in 1970 achieved CW operation at room temperature using a double-heterojunction laser. The laser, a very important device in modern optoelectronics among other applications, is the subject of many texts and reference books.
A broad-area laser can be seen as a laser which is basically a vertical p-n junction with special design for optical reflection from vertical walls. It is necessary, however, for both n-type and p-type regions to be doped to degeneracy to achieve population inversion, to be discussed later. It is usually started with an n+-substrate on which an epitaxial layer is grown. Liquid-phase epitaxy, vapor-phase epitaxy, МВБ, and MOCVD have all been used, with LPE being the most common technique in commercial types. The top, heavily doped region can be incorporated by diffusion, ion implantation, or deposited during epitaxy. It is also necessary that the semiconductor have a direct energy gap for efficient light emission to achieve lasing, unlike an LED where isoelectronic impurity can be added to semiconductors of indirect energy gap. Common starting substrates for epitaxial growth are GaAs and InP. Most laser materials, especially common ones in practice, are III-V compounds. This is because most II-VI compound semiconductors are difficult to dope to both n- and p-type.
Another unique structural requirement for a laser is an optical resonator, called Fabry-Perot etalon, in the direction of the laser light output. This means that the two vertical walls that form the resonator should be perfectly perpendicular to the junction, and have mirror-like smoothness with optimum reflectivity. This can be achieved by etching, polishing, or most commonly, by cleaving. A typical length L is between 200-500 ., but a precise dimension is not required since L is much larger than the wavelength of emission. For this reason, practical lasers arc discrete components. One of the Fabry-Perot mirrors can be totally reflecting so light comes out from only one side. The mirror walls parallel to the laser output are roughened to be highly absorbing to prevent lasing in the transverse direction. To confine light output in the lateral direction, stripe geometries are used.
We first consider the optical processes of a two-level system, not necessarily restricted to semiconductors, with energies Ex and E2, and electron concentrations and N2. The three main processes are absorption (A), spontaneous emission (B), and stimulated emission (C). Absorption is characterized by the absorption coefficient (a) and is the principle process in photodetectors and solar cells. In spontaneous emission, light produced is random in space and time. It is the predominant mechanism in an LED. In stimulated emission, a photon input is required to stimulate an electron transition to yield another photon of identical wavelength and phase (coherent). Stimulated emission is the main mechanisms for lasing.To acquire population inversion, pumping from external sources such as light can be used. In the case of an injection laser, forward bias to the р-n junction provides carrier injection. In each side of the junction, minority carriers are injected to recombine with the majority carriers to produce light. In practical devices, the p-region is more effective in light emission due to higher electron injection (higher mobility). For the sake of simplicity, at low temperature, states below the Fermi levels are completely filled and those above are completely empty. The I-V characteristics of a laser are similar to the conventional p-n junction diode. Even though both sides of the junction are degenerate, the transition region is less abrupt compared to a tunnel diode so that there is no negative differential resistance under forward bias.
., but a precise dimension is not required since L is much larger than the wavelength of emission. For this reason, practical lasers arc discrete components. One of the Fabry-Perot mirrors can be totally reflecting so light comes out from only one side. The mirror walls parallel to the laser output are roughened to be highly absorbing to prevent lasing in the transverse direction. To confine light output in the lateral direction, stripe geometries are used.