What makes a material a good lens for CO2 lasers? I read on Wikipedia that zinc selenide and germanium make good lenses for $\mathrm{CO}_2$ lasers. My question is, Why?
 A: CO2 lasers are high power lasers. Low absorption and good thermal conductivity are important. Between the two, Ge has the higher absorption. Ge has is only good for lower power lasers. ZnSe is more common. 
When the task is to focus a laser beam, the only imperfections are spherical aberration and diffraction. There are no off axis or chromatic aberrations. 
Both materials have high indices of refraction at 10.6 um. For ZnSe, n = 2.4. For Ge, n = 4. Typical glass in the visible region is around 1.5. 
A high index of refraction means a lens of a given focal length can be made with a larger radius of curvature. This reduces spherical aberration. 
Given that the wavelength is 20 times longer than visible light, the size of the diffraction limited spot produced by a given aperture is 20 times larger than for visible diffraction light. That means you don't have to reduce spherical aberration very far before you have a diffraction limited lens. Typically, a singlet lens is diffraction limited. 

Other materials are sometimes used. GaAs is used on occasion. NaCl is occasionally used in the IR, but I don't know what wavelength. 
A: Lenses and windows in in any laser should be transparent to the wavelength being used. CO2 lasers typically transmit in infrared. Germanium is transparent in infrared, as is zinc selenide.
