TEM₀₀ Mode Semiconductor Laser

A semiconductor laser is a coherent radiation light source, and three fundamental conditions must be met to enable it to generate laser output:

1. Gain condition: Establish an inverted carrier distribution in the lasing medium (active region). In semiconductors, the energy of electrons is represented by a series of nearly continuous energy levels forming an energy band. Therefore, to achieve population inversion in semiconductors, the number of electrons in the higher-energy conduction band must significantly exceed the number of holes in the lower-energy valence band. This is accomplished by applying a forward bias to homojunctions or heterojunctions and injecting the necessary carriers into the active layer. Electrons are excited from the lower-energy valence band to the higher-energy conduction band. When a large number of electrons in the population-inverted state recombine with holes, stimulated emission occurs.

2. To achieve coherent stimulated emission in practice, it is necessary to enable multiple feedback of the stimulated emission within an optical resonant cavity, thereby forming laser oscillation. The resonant cavity of a laser is formed by the natural cleavage planes of semiconductor crystals acting as mirrors. Typically, the end without light emission is coated with a high-reflectivity multilayer dielectric film, while the emitting surface is coated with an anti-reflective film. For Fabry-Pérot (F-P) cavity semiconductor lasers, the natural cleavage planes perpendicular to the P-N junction plane can conveniently form the F-P cavity.

3. To achieve stable oscillation, the laser medium must provide sufficient gain to compensate for the optical losses caused by the resonant cavity and the losses from laser output through the cavity facets, thereby continuously amplifying the optical field within the cavity. This requires strong current injection, meaning a sufficient population inversion. The higher the degree of population inversion, the greater the gain achieved, necessitating the fulfillment of a certain current threshold condition. When the laser reaches this threshold, light of a specific wavelength can resonate within the cavity and be amplified, ultimately forming a laser output that is continuously emitted.

It is evident that in semiconductor lasers, the dipole transition of electrons and holes is the fundamental process of light emission and amplification. For novel semiconductor lasers, quantum wells are widely recognized as the driving force behind their development. The question of whether quantum wires and quantum dots can fully exploit quantum effects has extended into this century. Scientists have attempted to create quantum dots in various materials using self-organizing structures, and GaInN quantum dots have already been employed in semiconductor lasers. Additionally, researchers have developed another type of stimulated emission process known as quantum cascade lasers, which operate based on transitions from a sub-band level in the semiconductor conduction band to a lower state within the same band. Since only electrons in the conduction band participate in this process, it constitutes a unipolar device.