text: Off-axis parabolic mirrors are commonly used to collimate and focus THz radiation. Radiation from an effective point source, such as from a low-temperature gallium arsenide (LT-GaAs) antenna (active region ~5 μm) incident on an off-axis parabolic mirror becomes collimated, while collimated radiation incident on a parabolic mirror is focused to a point (see diagram). Terahertz radiation can thus be manipulated spatially using optical components such as metal-coated mirrors as well as lenses made from materials that are transparent at THz wavelengths. Samples for spectroscopy are commonly placed at a focus where the terahertz beam is most concentrated.