According to Edholm’s law, the demand for point-to-point bandwidth in wireless short-range communications has doubled every 18 months over the last 25 years It can be predicted that data rates of around 5–10 Gb/s will be required in ten years In order to achieve 10 Gb/s data rates, the carrier frequencies need to be increased beyond 100 GHz Over the past ten years, several groups have considered the prospects of using sub-terahertz (THz) and THz waves (100–2000 GHz) as a means to transmit data wirelessly Some of the reported advantages of THz communications links are inherently higher bandwidth compared to millimeter wave links, less susceptibility to scintillation effects than infrared wireless links, and the ability to use THz links for secure communications Our goal of this paper is to provide a comprehensive review of wireless sub-THz and THz communications

Review of terahertz and subterahertz wireless communications

We demonstrate the first electronically switchable Bragg structure for THz frequencies. The structure works as stop-band filter and as mirror. It exhibits the 60 GHz broad stop-band around 300 GHz which can be removed by reorienting liquid crystal molecules in an external electric field. Our first proof-of-principle experiments agree very well with transfer matrix calculations.

Liquid crystal based electrically switchable Bragg structure for THz waves

We report on a first experimental study of the molecular properties of nematic liquid crystals in the terahertz range In the beginning, we extract the frequency and temperature dependent refractive index and absorption coefficient of the cyanobiphenyls 5CB, 6CB and 7CB from terahertz time domain spectroscopy measurements and investigate the impact of the alkyl chain length on the macroscopic liquid crystal characteristics, focusing especially on the pronounced odd and even effect Next, we deduce the principle polarizabilities and the order parameter S by applying Vuks’ approximation and Haller’s approach On this basis, we calculate the main polarizabilities along the longitudinal and transverse axis and link the observed terahertz properties to the molecular structure of the liquid crystals

Molecular properties of liquid crystals in the terahertz frequency range

The effects of magnetic birefringence of liquid crystal E7 on the photonic bandgaps and the transmitting properties in two-dimensional photonic crystal waveguides are investigated by using the plane wave expansion and finite-difference time domain methods. Detailed calculations on the shifts and variations in the frequency width of bandgaps reveal that under the control of an external magnetic field the two-dimensional liquid-crystal-filled photonic crystal waveguide can serve as a switch and continuously tunable bandpass filter at the terahertz wave band.

Liquid-crystal-filled photonic crystal for terahertz switch and filter

We systematically study the terahertz (THz) properties of a set of nematic liquid crystals. We use THz time domain spectroscopy to obtain data between 0.2 THz up to 2.5 THz. Such a broadband set of terahertz data has not yet been reported. From the measured data, we extract macroscopic data, such as the birefringence, the refractive indices and the absorption coefficients for ordinary and extraordinary polarization. The impact of different side chains and core structures on the terahertz anisotropy are discussed. This paper will serve as a data base and can be very useful for the design of new terahertz liquid crystal material.

Z. Ghattan

Papers

Sub-terahertz on–off switch based on a two-dimensional photonic crystal infiltrated by liquid crystals