TPX THz Windows

Overview

TPX windows let a terahertz beam pass into or out of an enclosure with minimal loss and essentially no dispersion. They are the standard window for nitrogen purge boxes, sample chambers and detector housings on a terahertz beamline, finished to 80/50 scratch-dig with diameter held to 0.2 mm.

Why a window matters more than it looks

Water vapour absorbs strongly and structurally across the terahertz band, so any measurement made in ordinary room air carries deep absorption lines that have nothing to do with the sample. The standard remedy is to enclose the whole terahertz path and purge it with dry nitrogen, and that enclosure needs windows that do not themselves distort the pulse. A dispersive or lossy window would defeat the purpose.

TPX compared with silicon

TPX is visible-transparent, light, cheap and easily machined into large sizes, and its index is low enough that surface reflections are modest. High-resistivity silicon has a much higher index and therefore stronger surface reflections, but it is far more rigid and holds a pressure differential, which is why silicon is the usual choice for a vacuum window and TPX for an atmospheric purge enclosure.

Design notes

Fresnel reflection at each face produces a delayed replica of the pulse in the time domain. With a thin window that replica lands close to the main pulse and can be hard to separate, so specify thickness with the intended scan range in mind, or tilt the window slightly so the replica walks out of the beam path. Allow enough clear aperture for the beam at the window position, which in a collimated section can be much larger than at the focus.