Which of the following laser types is recognized as a gaseous medium?

The CO2 laser is recognized as a gaseous medium because its laser action is based on a mixture of gases, primarily carbon dioxide, with other gases like nitrogen and helium often added to enhance efficiency and output. The laser generates light at a wavelength of 10.6 micrometers through the excitation of the carbon dioxide molecules when they are stimulated by an electrical discharge or another form of energy. This characteristic of being a gaseous medium is distinct, as it fundamentally relies on gas-phase processes for the generation of coherent laser light.

In contrast, the gallium arsenide diode is a semiconductor laser, meaning it operates based on solid-state physics rather than a gaseous phase. Rhodamine 6G, on the other hand, is a dye laser that uses a liquid dye as its medium, not a gas. Similarly, Nd:YAG, or Neodymium-doped Yttrium Aluminum Garnet, is a solid-state laser that utilizes a crystal as its lasing medium. Each of these other options embodies a different material state, which is why they do not classify as gaseous mediums.