Abstract
Tunable liquid crystal devices that can change terahertz wave polarization continuously have many potential applications in terahertz optical systems. We present a reflective liquid crystal terahertz waveplate with sub-wavelength metal grating and metal ground plane electrodes. The thickness of the liquid crystal layer can be reduced to ~10% of that needed for the same phase shift at a given frequency in a transmissive waveplate. We experimentally demonstrate the same tunability as in the transmissive type just using half the thickness. We discuss the dependence on the angle of incidence for phase shift tunability, which can achieve beam steering and polarization conversion simultaneously. The proposed design can be applied in terahertz imaging, sensing, and communications.
Highlights
Waveplates are essential and versatile tools for transforming, controlling, or analyzing the polarization state of light
We are limited by the experimental system, there is a potential for tuning the phase difference by varying the incident angle of the THz beam according to Eq (1)
Before 35°, the phase difference at 0 V is larger than that at 22 V, decreasing the operating voltage can compensate for the decrease of the phase difference induced by the incident angle increase
Summary
Waveplates are essential and versatile tools for transforming, controlling, or analyzing the polarization state of light. Magnetic fields were used to tune the birefringence of LCs [6, 7] These devices achieved good tunability, but were bulky, heavy, and highly power-consuming. We have realized broadband tunable LC THz waveplates driven with porous graphene electrodes [12] All of these devices operate in the transmission mode. We demonstrate a large phase shift tunable range and the polarization conversion dependence on the angle of incidence is discussed. These properties suggest a compact method to effectively realize electric-driven tunable THz components
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