Characterization of Nematic Liquid Crystals at Microwave Frequencies

Abstract

The use of nematic liquid crystal (LC) mixtures for microwave frequency applications presents a fundamental drawback: many of these mixtures have not been properly characterized at these frequencies, and researchers do not have an a priori clear idea of which behavior they can expect. This work is focused on developing a new procedure for the extraction of the main parameters of a nematic liquid crystal: dielectric permittivity and loss tangent at 11 GHz under different polarization voltages; splay elastic constant K11, which allows calculation of the threshold voltage (Vth); and rotational viscosity γ11, which allows calculating the response time of any arbitrary device. These properties will be calculated by using a resonator-based method, which is implemented with a new topology of substrate integrated transmission line. The LC molecules should be rotated (polarized) by applying an electric field in order to extract the characteristic parameters; thus, the transmission line needs to have two conductors and low electric losses in order to preserve the integrity of the measurements. This method was applied to a well-known liquid crystal mixture (GT3-23002 from MERCK) obtaining the permittivity and loss tangent versus bias voltage curves, the splay elastic constant, and the rotational viscosity of the mixture. The results validate the viability of the proposed method.

Rethinking Figure-of-Merits of Liquid Crystals Shielded Coplanar Waveguide Phase Shifters at 60 GHz

“…The constraint on existing FoM metrics could partially be remediated by incorporating the mmWave phase-tuning speed, which is remarkably limited by the fall time (FT) [12,30] of the bulk LC layer where energy is stored. An updated definition of FoM can accordingly be adapted in Equation ( 5), catering to speed-sensitive applications, such as the emerging LC-based flat-panel antenna array for automotive radars.…”

“…Mobile broadband terminals [1][2][3] are becoming a prime need and rely increasingly on a continuously tunable liquid crystals (LC)-based [4][5][6][7] millimetre-wave (mmWave) beam steering flat-panel [8] antenna array, in lieu of rotating parabolic dishes [9]. The recent upsurge of high-bandwidth low-loss use cases places new demands on the selection of highly anisotropic LCs, which is as important for meeting the phase-tuning requirement as it is vital for low power dissipations in gigahertz (GHz) [10][11][12][13] and towards terahertz (THz) [14][15][16][17] regimes. Figure-of-merit (FoM) [18,19] is hence introduced to balance the above two key performance characteristics.…”

Liquid Crystal-Filled 60 GHz Coaxially Structured Phase Shifter Design and Simulation with Enhanced Figure of Merit by Novel Permittivity-Dependent Impedance Matching

“…However, for the proof-of-concept standalone device physics characterization stage of LC-based low-power-consuming (e.g., bias voltage up to 20 V [11,24,41]) transmission lines (instead of the power-consuming waveguides [43][44][45][46][47]), the LF-mmW decoupling network is not necessarily needed for the modern generation of vector network analyzers (VNA), wherein a port bias is normally embedded in the VNA for the input of an amplified LF bias signal limited up to 30 V. As such, the complicated decoupling network is not encompassed within our computational modeling frameworks in this work. Nevertheless, for follow-up developments of a real-world phased array feeding system, suitable connector mounting and LF-mmW decoupling network implementation (e.g., by embedding bias tees [48][49][50], introducing additional biasing networks [51][52][53][54], or relying on novel structural mechanisms [55,56]) should be conducted as per the application specifications to address the limitations of the current work's analytical and numerical evaluations. More specifically, the perturbations in the achievable differential phase shift, return loss, and insertion loss due to the addition of the decoupling network (e.g., bias tees as depicted in Figure 2) will be experimentally quantified as ongoing optimization endeavors.…”

Time-Domain Characterization of Nematic Liquid Crystals Using Additive Manufacturing Microstrip Lines

“…Resonant characterization methods have been presented in the literature, using multilayer structures based on microstrip transmission lines [11] or planar patch resonators [12] among others, but they are inherently narrowband. Another multilayer structure for the extraction of permittivity as a function of the bias voltage is presented in [13]. In [14], a broadband characterization method was proposed, which needs thru and line standards to carry out the calibration and thus requires fabricating and measuring additional structures.…”