Abstract
A liquid crystal (LC) layer that is too thick exhibits a small terahertz birefringence due to the limited long-range force of the alignment layers that exert on it. An LC layer that is too thin has a small terahertz birefringence due to its invisibility to incident terahertz waves. Therefore, an LC layer may have a large terahertz birefringence at a specific thickness. It is well known that the birefringence of an LC layer dominates the shift of the resonance frequency of the metamaterial imbedded into the LC layer. As a result, this work studies the effect of the thicknesses of LC layers on the shift of the resonance frequencies of metamaterials. LC layers with various thicknesses ranging from 310 µm to 1487 µm are deposited on terahertz metamaterials, and each of the layers is aligned by two polyimide layers that are rubbed in a direction. The terahertz metamaterials have a maximum frequency shifting range of 21 GHz as 710 µm thick LC layers with mutually orthogonal rubbing directions are deposited on them. The maximum frequency shifting range arises from the competition between the long-range force of the polyimide layers and the interaction between the LC layers and their incident terahertz waves.
Highlights
Metamaterials have attracted considerable interest thanks to their unusual electromagnetic resonance [1,2,3,4,5,6,7]
Before the polyimide-coated metamaterials are deposited with the liquid crystal (LC) layers, their terahertz spectra are measured using a commercial terahertz spectrometer (TPS 3000, TeraView, Cambridge, UK) in transmission mode
The polyimide-coated metamaterials can be used to study the effect of the thicknesses of the LC layers on the shift of the resonance frequencies of the metamaterials
Summary
Metamaterials have attracted considerable interest thanks to their unusual electromagnetic resonance [1,2,3,4,5,6,7]. Chen et al used a dual frequency LC layer deposited on a terahertz metamaterial to develop an electrically controllable terahertz filter [6]. Chiang et al fabricated metamaterial-imbedded LC cells, and the alignment layers in the cells are rubbed in various directions with respect to the gaps of the split-ring resonators of the metamaterials [7]. The alignment layers with the various rubbing directions shift the resonance frequencies of the metamaterials. Wang et al used a metamaterial-imbedded LC cell with a graphene electrode to develop an electrically controllable terahertz absorber [8]. The resonance frequency of the metamaterial is tuned from 0.75 THz to 1.0 THz by applying a voltage of 10 V to the graphene electrode, and the absorption of the metamaterial has an amplitude modulation of 80% at the frequencies
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