Design and simulation of an achromatic phase controller for THz polarization imaging system

Biological applications predominantly rely upon terahertz (THz) frequency range for segregation of abnormal tissues from normal ones using polarization imaging. Achromatic THz phase retarders are an essential part of THz polarization imaging system. For the first time, design and simulation of a terahertz achromatic quarter-wave plate using two different birefringent materials, i.e. crystalline quartz and lithium niobate (LiNbO₃), are demonstrated in the frequency band ranging from 0.8 to 1.3 THz. Phase deviation of the proposed system is within 90° ± 2.2°, which is achieved with only two crystals having different thicknesses without varying their orientation angles. The proposed system exhibits promising behaviour in terms of thickness variation and transmittance. Distinguishable retardance stability in terms of phase deviation and remarkable reduction in overall system thickness has been achieved over the said THz frequency range as compared to other reported systems. This designed achromatic phase retarder finds potential application in THz imaging system used for early detection of colon and gastric cancers in this particular frequency band.