p n Excess holes Excess electrons

Fermi Level n-type p-type Holes

Fermi Level n-type p-type Holes Voltage

Fermi Level n-type p-type Holes Voltage

Laser-Diode-Technology/852182/1033

p n Excess holes Excess electrons

They are basically a diode junction. In order to get LASER action there needs to be a region where BOTH excited electron states and holes (vacant electron states) are present. This is achieved using heavily doped n and p material and applying a forward bias to the junction. Fig. 16 shows the energy levels either side of the diode junction.16

Diode Pumped Solid State (DPSS) laser: Diode lasers make a convenient pump source for various gain media, typically those solid state media utilizing Neodymium as the dopant (Neodymium:Yttrium Aluminum Garnet or Nd:YAG, Nd:Yttrium Vanadate or Nd:YVO4). The 1064 nm output is typically frequency doubled to 532nm. The familiar green laser pointer is a DPSS laser.

Figure 3.2 Indirect and Direct gap semiconductors In equilibrium, the charge carriers occupy their lowest energy states, electrons at the bottom of the conduction band, and holes at the top of the valence band. In silicon these states do not have the same momentum. Therefore if a recombination is to result in the emission of a photon, which has little momentum, a quantum of lattice vibration (a phonon) must also be created to carry away the excess momentum. This is known as an indirect process and such semiconductors are known as INDIRECT GAP semiconductors. The two particle process is not favoured, and recombinations in indirect gap semiconductors usually occur by thermal or collisional processes. The silicon chips in a computer do not glow, they just get warm. GaAs however is a DIRECT GAP semiconductor. The minima of electron and hole energies occur at the same momentum. Thus recombination can result in a photon alone. LED action was first observed in GaAs in The light is emitted in a narrow range of wavelengths which is determined by the size of the band gap of the semiconductor. The red, green and yellow LEDs available today are made using semiconductor compounds with different band gaps, for example GaAs, GaAlAs, AlInGaP, GaAsP. The LED must satisfy two more conditions before it is possible to make a laser diode. Firstly stimulated emission must be able to dominate over absorbtion and spontaneous emission, and secondly there must be an optical cavity. Fortunately both are relatively simple to achieve. The population inversion required for stimulated emission to dominate is realised by increasing the current through the diode. This increases the density of electron-hole pairs in the junction region, and thus creates an inversion. The optical cavity is made by cleaving the crystal along two parallel crystal planes, perpendicular to the junction plane. This creates optically flat and parallel surfaces. They do not need to be coated to act as laser mirrors as the high refractive index of GaAs gives the GaAs-air interface a reflectivity of 35% which is found to give sufficient feedback to sustain laser action.