Ruby is an aluminum oxide crystal in which some of the aluminum atoms have been replaced with chromium atoms. Chromium gives ruby its characteristic red color and is responsible for the lasing behavior of the crystal. Chromium atoms Absorb green and blue Light and emit or reflect only red light.
For a ruby laser, a crystal of ruby is formed into a cylinder. A fully reflecting mirror is placed on one end and a partially reflecting mirror on the other. A high-intensity lamp is spiraled around the ruby cylinder to provide a Flash of white light that triggers the Laser action. The green and blue wavelengths in the flash excite electrons in the chromium atoms to a higher Energy level. Upon returning to their normal state, the electrons emit their characteristic ruby-red light. The mirrors reflect some of this light back and forth inside the ruby crystal, stimulating other excited chromium atoms to produce more red light, until the light pulse builds up to high power and drains the energy stored in the crystal.
The optically pumped, solid-state laser uses sapphire as the Host lattice and chromium as the active ion. The emission takes place in the red portion of the spectrum.