These mirrors are generally used as last mirror prior to focusing lens.They create a 90° Phase Shift with the aid of specially constructed thin film coating layers applied to the surface, when hit with a linear polarized beam at 45° to the plane of incidence. Commonly called Phase Shifters, Polarizers, or Circular Polarizing Mirrors, they transform the incoming beam into a circular polarized state. They typically are only used external to the laser cavity as a beam delivery mirror. These operate only with a true 10.6µm laser wavelength and will not work at other wavelengths such as 9.3µm  or 9.6µm without special modifications to the thin film coating design. We offer non-standard wavelength Phase Retardation Reflectors at these other wavelengths as custom parts.