The authors of this Optics Letters article propose new designs for terahertz achromatic quarter- and half-wave plates based on phase retardation by total internal reflection. Exploiting well-known Fresnel phase difference between s- and p-polarized light due to total internal reflection, the authors use rhomb prisms to achieve the required polarization control. The phase difference between the components depends only on the angle of incidence and the refractive index of the prism, so as long as the index does not vary significantly with frequency the effect is achromatic. Thus, by adjusting the number of internal reflections in the prism, the authors can control the plate to be either a half-wave plate or a quarter-wave plate. As the prism does not rely on birefringence to achieve quarter/half wave retardation, there is no time delay, hence leading to identical waves for ±λ/4 polarization except for the rotation direction.
In terahertz optics, large aperture beams are often required for propagation or imaging, the consequence being the need of using very large prisms. The authors resolve this issue by using Multi Stacked Prisms (MSP) and adjusting them to produce the desired plate. Finally, the authors apply their setup to a toy experiment by imaging two bundles of hair overlaid in the shape of a cross, proving the efficiency of a MSP to image selectively the two directions of the hair bundles.
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