The role of the laser crystal extends beyond merely initiating stimulated emission. The crystal structure allows the emitted photons to bounce within it, stimulating further emissions, and causing an avalanche. The laser diode chip is the small black chip at the front; a photodiode at the back is used to control output power. SEM (scanning electron microscope) image of a commercial laser diode with its case and window cut away. Laser diodes offer high power for their size and produce electrical-power-efficient laser radiation. As photonics push into industrial microfabrication, space-based LiDAR, and femtosecond biophotonics, understanding laser crystal functionality becomes essential. The bonding combinations of states become the valence bands (VB) of the crystal, and the anti-bonding combinations of these states become the conduction band (CB).
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