A compact broadband terahertz range quarter-wave plate

TL;DR

This paper presents a novel, compact broadband terahertz quarter-wave plate using sapphire discs, optimized via basin-hopping Monte Carlo, achieving high phase accuracy and reduced optical path length compared to previous designs.

Contribution

Introduction of a sapphire-based achromatic quarter-wave plate with optimized thicknesses and rotations, offering improved phase accuracy and a significantly smaller form factor.

Findings

Predicted phase error of only 0.5% from ideal π/2

Optical path length reduced by approximately a factor of 10

Constructed design performs similarly to larger configurations

Abstract

We detail the design and characterization of a terahertz range achromatic quarter-wave plate based on a stack of aligned variable thickness birefringent sapphire discs. The disc thicknesses and relative rotations of the discs are determined through a basin-hopping Monte Carlo thermal annealing routine. The basin-hopping scheme allows an improved refinement of the required thicknesses and rotations to give a predicted phase error from the ideal $\pi/2$ of only $0.5 \%$, which is a factor of approximately 6 better than previous efforts. Moreover, the large contrast between extraordinary and ordinary axes of sapphire allow us to greatly decrease the overall optical path length of our wave plate design by approximately a factor of 10 over similar designs based on quartz discs. However, this very same contrast requires very precise tolerances in the milled thicknesses of the discs and their assembly. We detail a method to compensate for differences in the thickness from their calculated ideal values. We have constructed one of our designs and found it similar in performance to other configurations, but with our much more compact geometry.