Michael Wittek

Broad-band microwave characterization of liquid crystals using a temperature-controlled coaxial transmission line

Liquid crystals (LCs) promise to be suitable passive tunable material for microwave devices with excellent features concerning tunability and losses. In order to optimize the synthesis of LCs and the design of tunable microwave devices based on them, LCs have to be characterized at microwaves. For a microwave analysis between 360 MHz-23 GHz, use was made of a broad-band characterization method with a two-port coaxial line, temperature controlled within -7/spl deg/C and 115/spl deg/C. Two LCs are investigated: K15, as a quasi-standard, and MDA-03-2838, as a novel mixture with increased dielectric anisotropy and reduced microwave losses tan/spl delta/.

Tunable passive phase shifter for microwave applications using highly anisotropic liquid crystals

A tunable broadband inverted microstrip line phase shifter filled with Liquid Crystals (LCs) is investigated between 1.125 GHz and 35 GHz at room temperature. The effective dielectric anisotropy is tuned by a DC-voltage of up to 30 V. In addition to standard LCs like K15 (5CB), a novel highly anisotropic LC mixture is characterized by a resonator method at 8.5 GHz, showing a very high dielectric anisotropy /spl Delta/n of 0.32 for the novel mixture compared to 0.13 for K15. These LCs are filled into two inverted microstrip line phase shifter devices with different polyimide films and heights. With a physical length of 50 mm, the insertion losses are about 4 dB for the novel mixture compared to 6 dB for K15 at 24 GHz. A differential phase shift of 360/spl deg/ can be achieved at 30 GHz with the novel mixture. The figure-of-merit of the phase shifter exceeds 110/spl deg//dB for the novel mixture compared to 21/spl deg//dB for K15 at 24 GHz. To our knowledge, this is the best value above 20 GHz at room temperature demonstrated for a tunable phase shifter based on nonlinear dielectrics up to now. This substantial progress opens up totally new low-cost LC applications beyond optics.

Passive Phase Shifter for W-Band Applications using Liquid Crystals

In this paper a ridged waveguide phase shifter at W-band frequencies is presented. A taper matches the ridged waveguide phase shifter to the rectangular waveguide feed. Three different liquid crystals (LC) are used: a quasi-standard LC K15, a highly anisotropic mixture MDA-03-2844, and a dual-frequency switching LC MDA-05-1132, all synthesized by Merck KgaA. The return loss of the phase shifter device is always below -10dB in the whole W-band. The maximum achievable figure-of-merit of these materials is 78deg/dB at 108 GHz

Hysteresis and memory factor of the Kerr effect in blue phases

The performance of a polymer-stabilized blue phase system based on a nematic host with large dielectric anisotropy and a chiral dopant with high helical twisting power is investigated and the influence of the reactive monomer composition on the electro-optic characteristics is studied. Field-induced birefringence with a Kerr coefficient greater than 1u2009nm V−2 can be achieved in a large temperature range from well below 20u2009°C to above 55u2009°C. The disturbing influences of electro-optic hysteresis and memory effects can be reduced by diligent choice of the composition and appropriate electric addressing.

23.1: Invited Paper: New Materials for Polymer-Stabilized Blue Phase

Compared to other liquid crystalline mesophases, the Blue Phase exhibits extremely fast switching via induced birefringence by an external field. However, the pure Blue Phase is characterized by a rather narrow temperature range on the order of a few K. For display applications, one very promising way to broaden the stable temperature range is achieved by polymer-stabilization of the Blue Phase. A crucial prerequisite for efficient stabilization is the optimal material selection/matching of reactive mesogens RMs and the chiral host. New material concepts will be introduced and discussed.

IP3 Measurements of Liquid Crystals at Microwave Frequencies

The nonlinear behaviour of a phase shifter employing two different nematic liquid crystals (NLCs) as a tunable dielectric is investigated in this work. The NLCs used are the quasi-standard NLC 5CB and a highly anisotropic mixture MDA-03-2844. The third order intercept point IP3 was measured at 7.4 GHz. The utilized measurement setup allows the application of 40 dBm total power to the device under test (DUT) with an IP3 of the empty setup of at least 60 dBm. The output IP3 is at least 52 dBm for 5CB and 58 dBm for the mixture. The proposed phase shifter design therefore exhibits high linearity

Recent developments in liquid crystals for microwave applications

Recent developments in Liquid Crystal mixtures for tunable microwave applications have led to two material classes with increased performance. A high tunability material class provides a tunability above 30%, while a low loss material class has shown loss tangents down to 0.08.

Recent advances in fast-switching liquid-crystal mode based on uniform lying helix (ULH) (Conference Presentation)

Uniform lying Helix (ULH) is one of the most promising new LC modes for fast switching applications with alternative driving schemes such as FSC (field sequential colour). ULH operates via flexoelectric switching, unlike current commercially available liquid crystal modes that operate via dielectric switching. The realisation of mixtures with suitable flexoelectric properties requires unique materials composed of specially designed ´bi-mesogens´ and also the addition of a chiral dopant. Recent advances, updated mixture performance and other challenges such as alignment and driving for this new mode are discussed.

Novel materials for polymer-stabilized blue phase

Blue phase shows one the fastest switching via induced birefringence by an external electrical field observed in liquid crystals. However, the pure blue phase typically has a narrow temperature range. Applying polymer-stabilization using reactive mesogens, a blue phase with a largely increased temperature range of up to 60 K can be obtained. Prerequisite for a good stabilization is the adequate matching of RMs and the optimized polymerization conditions. To achieve a low operating voltage of the polymer stabilised Blue phase display device, the material properties of the host - namely the product of the optical birefringence Δn and the electrical anisotropy Δε - has to be optimised. New host developments yielding a largely increased Δn * Δε ~ 40 at room temperature are presented.