the energy gap in LT-GaAs, are ideal as they can generate optical pulses as short as 10 fs. These lasers are available as commercial, turnkey systems. Steering mirrors Silver-coated mirrors are optimum for use as steering mirrors for infrared pulses around 800 nm. Their reflectivity is higher than gold and much higher than aluminum at that wavelength. Beamsplitters A beamsplitter is used to divide a single ultrashort optical pulse into two separate beams. A 50/50 beamsplitter is often used, supplying equal optical power to the terahertz generator and detector, though it is common to provide the terahertz generation path with more power given the inefficiency of the terahertz generation process compared to the detection efficiency of infrared (typically 800 nm wavelength) light. Delay stage An optical delay-line is implemented using a movable stage to vary the path length of one of the two beam paths. A delay stage uses a moving retroreflector to redirect the beam along a well-defined output path but following a delay. Movement of the stage holding the retroreflector corresponds to an adjustment of path length and consequently the time at which the terahertz detector is gated relative to the source terahertz pulse. Purge box A purge box is typically used so that absorption of THz radiation by gaseous water molecules is minimized. A dry air source is often used for this purpose, however, a nitrogen gas source may also be used. Water is known to have many discrete absorptions in the THz region that are rotational modes of water molecules. Alternatively, nitrogen, as a diatomic molecule, has no electric dipole moment, and does not (for the purposes of typical THz-TDS) absorb THz radiation. Thus, a purge box may be filled with nitrogen gas so no unintended discrete absorptions in the THz frequency range occur. Parabolic mirrors 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. Uses of THz