M6 Light Emission (LED/Laser)

Topic

Radiative recombination—a process also modeled within this framework—results in the spontaneous emission of light within this LED: each electron and hole that recombine via this radiative process (as opposed to the SRH or Auger recombination mechanisms also described by the model) emits a single photon at an unpredictable moment and in a random direction, with an energy corresponding to the semiconductor material's bandgap. Because radiative recombination dominates over SRH and Auger recombination in this LED, a larger fraction of these spontaneous photons is emitted for every electron-hole pair injected into the device.

Conversely, if the semiconductor material achieves population inversion—where more electrons occupy higher energy states than lower energy states available for radiative recombination—and is confined within a resonant cavity (a pair of mirrors reflecting the emitted light back and forth through the material), the result is coherent stimulated emission rather than spontaneous emission; this is the process characterizing a semiconductor laser. A photon traversing the resonant cavity stimulates the emission of a second, identical photon—sharing the same energy, phase, and direction—upon passing near an as-yet-unrecombined electron-hole pair within the population inversion, rather than waiting for that pair to recombine spontaneously and unpredictably. A greater degree of population inversion within the semiconductor material means that, each time a photon traverses the resonant cavity, this coherent stimulated emission generates a larger number of these additional identical photons. Thus, this model—characterized by the LED's spontaneous emission and the laser's coherent stimulated emission—defines the semiconductor laser's threshold condition based on the optical gain provided by that coherent stimulated emission during each pass through the resonant cavity. As long as this optical gain remains below the cavity's optical losses—specifically, transmission through the mirrors and absorption within the semiconductor material—spontaneous emission dominates over coherent stimulated emission, and no coherent light beam builds up within the cavity. However, once the optical gain equals these losses—reaching the threshold condition—coherent stimulated emission becomes dominant, and the semiconductor laser begins to emit its characteristic coherent light beam. Consequently, when the population inversion within the semiconductor material exceeds the level required by the threshold condition, the laser's optical gain surpasses the optical losses, causing the semiconductor laser to emit coherent light.

ID:4506