Flexible control of terahertz chirality in twisted nematic liquid crystal cell with submillimeter thickness

Liquid crystal (LC), an anisotropic and tunable material, is widely used in phase shifters, polarization converters, and chiral devices. However, it is difficult for simple LC devices to achieve strong chiral response in the terahertz regime, even for cholesteric LCs. In this paper, we observe a strong optical rotation effect in a twisted nematic (TN) LC cell with submillimeter thickness; moreover, the magnetic field disturbance breaks the mirror symmetry of the whole structure and a discontinuous TN LC molecular alignment is formed, thus realizing the strong circular dichroism effect. The above two effects can also be actively tuned by electrically controlling the arrangement of LC molecules. First, without applying an external magnetic field, the dynamic modulation of the optical rotation angle in the above TN LC device can be as high as about ${75}^{\ensuremath{\circ}}$. Then, when a magnetic field in the $x$-axis direction is applied, the dynamic manipulation range of circular dichroism in the TN LC device can be up to 35 dB. Meanwhile, the dynamic polarization conversion function (from LP to CP or LP to LP) can also be achieved. The optical rotation and circular dichroism effect of this TN LC with submillimeter thickness offer a clear physical picture for the development of efficient and active terahertz polarization conversion and chiral devices.