Re: This may be a very silly question.
A lot of materials have well defined atomic energy level transitions that can be used in a laser. For a laser they need to ahve a few specific properties. Gases can be used (for example HeNe or argon lasers) but also dyes in a solution. Rare earth metals can make good lasers, but they need to be incorporated into a host crystal in order to work properly. This makes an Nd;YAG laser for example, neodymium put in a YAG crystal.
Diode lasers use a semiconductor as medium, they differ that they don't use the atomic transition of a material but it's bandgap.
All lasers need to be pumped, thsi can be done with an electrical discharge through the gas or by pumping them optically. Diodes are pumped by the current that runs through them.
A quite new type of lasers is the OPSL, optically pumped semiconductor. Here a semiconductor material is pumped optically. This type of lasers has some very good advantages, and is one of the laser types that is available in a wavelength range instead of only at a specific wavelength.
