90 deg Off-Axis Parabolic Mirror, Through Hole Parallel to the Collimated Beam, 2", RFL 152.4 mm, Protected Gold

Overview

This is the through-hole off-axis parabolic mirror in the configuration where the hole runs parallel to the collimated beam, that is along the parent optical axis. Optically it matches the plain 2-inch 90 degree protected-gold parabola: 50.8 mm diameter, 152.4 mm reflected focal length, lambda/8 RMS figure and better than 95% average reflectance from 700 to 2000 nm.

When to choose this orientation

This is the orientation normally used for collinear electro-optic sampling in a terahertz time-domain spectrometer. The optical probe beam is passed through the hole along the collimated section and recombined with the terahertz beam on axis, so probe and terahertz arrive at the sampling crystal perfectly overlapped without a separate beam combiner such as an ITO plate or a pellicle. That removes one dispersive element from the probe arm and simplifies the geometry considerably.

Why a reflective relay in the THz band

Off-axis parabolic mirrors are the standard way to collimate and focus terahertz radiation. Refractive optics are avoided because most materials introduce strong absorption and dispersion across the band, whereas a metal-coated reflector is achromatic from 0.1 to beyond 10 THz and adds no chirp to a single-cycle pulse. The protected gold coating stays above 95% average reflectance from 700 to 2000 nm and above 93% from 2 to 30 um, and its reflectance remains high throughout the terahertz band, so the same mirror also works for visible or near-infrared alignment.

Alignment and handling

A typical terahertz time-domain setup uses four off-axis parabolic mirrors: one to collimate the diverging beam from the emitter, one to focus onto the sample, one to recollimate after it, and a final one - usually a through-hole part - to focus onto the detection crystal. Set the beam height once and hold it across the whole table. The gold coating is soft and only lightly protected: never touch or wipe the optical face, and remove dust with dry nitrogen only.