Magnetically induced birefringence of randomly aligned liquid crystals in the terahertz regime under a weak magnetic field

Abstract

Tunable terahertz (THz) wave cells have been demonstrated by using magnetic-controlled birefringence in a randomly aligned liquid crystal (LC) cell embedded with three kinds of thermotropic LCs (5CB, E7 and BNHR). By using the THz time domain spectroscopy, these three LCs have been investigated under different low magnetic fields. Experimental results show that the randomly aligned LCs in 3mm thickness cells still have high birefringence controlled by a low magnetic field in the THz regime. The phase shift of π for BNHR cell is achieved over the entire testing range at 30mT, and the dynamic response process of BNHR under a weak magnetic field has also been investigated. When the initial magnetic field of 5mT is applied, unlike the continuous tunability of the cells filled with 5CB and E7, the BNHR cell has a great phase shift of 1.5π at 0.35THz before reaching the steady state. During that process the refractive index and absorption of BNHR vary with time due to its high viscosity, and a larger magnetic field can significantly shorten the response time. These indicate that the randomly aligned mm-thick LC layer of THz devices can be used as a tunable THz wave retarder with low a driving magnetic field and high phase modulation depth in a broad THz band. Therefore, a simple manufacturing technique and low magnetic control of these randomly aligned LC layers may explore some novel LC based THz devices for spatial light modulation, filtering and tuning tasks.