The complex refractive indices of the liquid crystal mixture E7 in the terahertz frequency range

Abstract

We have used terahertz time-domain spectroscopy to investigate the complex optical constants and birefringence of a widely used liquid crystal mixture E7 in both nematic and isotropic phases ͑26°C-70°C͒. The extinction coefficient of E7 at room temperature is less than 0.035 and without sharp absorption features in the frequency range of 0.2-2.0 THz. The extraordinary ͑n e ͒ and ordinary ͑n o ͒ indices of refraction at 26°C are 1.690-1.704 and 1.557-1.581, respectively, giving rise to a birefringence of 0.130-0.148 in this frequency range. The temperature-dependent ͑26°C-70°C͒ order parameter extracted from the birefringence data agrees with that in the visible region quite well. Further, the temperature gradients of the terahertz optical constants of E7 are also determined. The optical constants of E7 in the terahertz or sub-millimeter wave range are found to deviate significantly from values predicated by the usual extended Cauchy equations used in the visible and near-infrared.

Tunable Metamaterials Made of Graphene-Liquid Crystal Multilayers

“…At λ = 10.6 µm, the ordinary (n o ) and extraordinary (n e ) refractive indices of E7 are determined around 1.5 and 1.7, respectively. As mentioned earlier the n o and n e of the LC are temperature dependent and can be shown as [27,28]:…”

“…The permittivity components of the LC molecules are temperature dependent and the anisotropy of the material can be easily controlled by the electric fields [26][27][28]. Consequently, the optical response of the whole MM structure is modified.…”

Recent Advances in Birefringence Studies at THz Frequencies

“…For LiNbO 3 , very high values of Δn up to 1.78 at 1 THz were found [34], and even higher values of 3.3 at 1 THz were observed for rutile [2]. For liquid crystals (LC), birefringence due to the alignment of the molecular chains was reported for pure materials [43][44][45][46][47] as well as for mixtures [45,[48][49][50][51][52], and also for a fiber-reinforced LC polymer component [6]. In this case, birefringence was also assigned to the orientation of the LC molecules and not to the alignment of the embedded fibers.…”

MULTIPLE TIME SCALES OPTICAL NONLINEARITIES OF LIQUID CRYSTALS FOR OPTICAL-TERAHERTZ-MICROWAVE APPLICATIONS (Invited Review)

“…Nematic liquid crystals are uni-axial, characterized by refractive indices n and the n ⊥ , for light polarization parallel and orthogonal to the director axis, respectively. The magnitude of nematic birefringence Δn = n − n ⊥ is among the largest of all known materials, on the order of 0.2 that holds from the visible through far infrared and well into the microwave regime [1][2][3][41][42][43][44][45][46][47][48], c.f. Figure 3 and Table 1.…”

“…Figure 3 and Table 1. NLC with birefringence approaching unity in the visible regime are now routinely being produced [47,48], and there are increasing efforts to synthesize NLC with large birefringence and low loss in the Terahertz and longer wavelength regime [41][42][43][44][45][46] paralleling the growth in interests in developing tunable electromagnetic devices.…”