TPX Plano-Convex THz Lens TLA25050, 25.4 mm Diameter, 50 mm Focal Length

Overview

TPX, or polymethylpentene, is one of the few polymers that combines low absorption with an almost flat refractive index across the entire terahertz band. That near-zero dispersion is what makes it usable for single-cycle terahertz pulses, where a dispersive material would stretch the pulse and destroy the time-domain waveform. The TLA25050 is a 25.4 mm diameter plano-convex lens with a 22.4 mm clear aperture and a 50 mm focal length.

Refractive versus reflective

Off-axis parabolic mirrors remain the first choice for a terahertz beamline because they are perfectly achromatic and introduce no material absorption. However, a lens is far more compact and far easier to align, since it needs only two degrees of freedom instead of the four or five of an off-axis parabola. For a compact instrument, for a sample-chamber window that also has to focus, or for a stage where the terahertz path is short and dispersion is not critical, a TPX lens is the pragmatic choice.

A useful alignment property

TPX is transparent in the visible, so a HeNe or diode alignment laser passes straight through the lens and can be used to set the optical axis, beam height and focus position before any terahertz radiation is generated. High-resistivity float-zone silicon, the other common terahertz lens material, is opaque in the visible and does not allow this. TPX is also light and easily machined, which keeps mount loading low.

Design notes

Orient the convex face toward the collimated side to minimise spherical aberration. Because the refractive index of TPX is around 1.46 in the terahertz band, Fresnel reflection at each surface is a few percent and can produce a delayed etalon replica in the time-domain trace; if that replica falls inside the scan window, either increase the scan range so it can be windowed out, or slightly tilt the lens. Keep the lens inside the purged volume along with the rest of the terahertz path.