A laser gain medium is a material that amplifies light through a process called stimulated emission. This medium can be a solid, liquid, or gas, and it contains atoms or molecules that can be excited to higher energy levels. When these excited particles return to their lower energy state, they release photons, which are the basic units of light. The gain medium is essential for producing the coherent light characteristic of lasers. Common examples of gain media include ruby, neodymium-doped glass, and carbon dioxide. The choice of gain medium affects the laser's wavelength, efficiency, and application in various fields like telecommunications and medicine.