Achromatic terahertz quarter-wave Fresnel rhomb retarder

Achromatic terahertz (THz) quarter-wave retarder is widely desired to manipulate the polarization states of broadband THz beams, which are essential for spectroscopic applications, such as circular dichroism spectroscopy and steering THz vortex beams. A retarder based on Fresnel reflection exhibits the potential for designing an achromatic THz quarter-wave retarder. However, special care should be taken to make a Fresnel retarder capable of manipulating the beam ellipticity by simply rotating its fast axis without affecting its propagation path. Hereby, we design a 4-bounce achromatic quarter-wave Fresnel rhomb retarder free of affecting beam propagation, which can easily change the input beam's ellipticity by simply rotating the retarder's fast axis.