Marcus J. Coles

FLEXOELECTRIC LIQUID CRYSTAL BIMESOGENS

Recently we have shown that short pitch chiral nematogenic bimesogens may be synthesized that couple weakly via the bulk dielectric anisotropy, but extremely strongly via the flexoelectric effect to give exceptional flexoelectric, room temperature,fast electro-optic switching characteristics. Typical switching times are 100 μs with bipolar switching angles of 90° at fields of 5 V μm−1. In the present work we show how changes to the molecular structure, namely replacing the ether linkages with ester groups, control the bulk flexoelectric switching properties of these bimesogenic nematic materials. Due to thestrong monotropicity of the ester bimesogens, mixtures with the more enantiotropic ether linked bimesogens were made, and the effect on the flexoelectric coefficient behaviour investigated.

Electro-optic Telecommunication Devices at 1550 nm Employing Electroclinic and Ferroelectric Switching of an Organosiloxane Liquid Crystal

ABSTRACT We demonstrate three electro-optic telecommunication devices operating at a wavelength λ = 1550 nm: an optical switch or modulator, a variable attenuator, and a rotatable waveplate. These devices make use of the electroclinic and ferroelectric properties of a chiral smectic organosiloxane liquid crystal. Under moderate electric fields of ≤23 V/μm we observed an optical power modulation of up to 38 dB and a switching time of ∼100 μs. The waveplate could be continuously rotated over 38°. We also present birefringence data of this liquid crystal at λ = 1550 nm as a function of temperature and the implications on the development of liquid crystal telecommunication devices.

Novel electroclinic organosiloxane materials for optoelectronic devices

The paper describes an enhancement of the electroclinic characteristics of low molar mass dimeric organosiloxane liquid crystals. The degree of polymerisation of the siloxane core unit was varied in order to study the effect on phase transitions and electro-optic properties. It was found that the SmA*-SmC* phase transition temperature could be moved to any position in the range from 50°C to 10°C if we varied the number of SiMe2 groups in the flexible linkage of the dimeric molecule. More importantly, because the organosiloxane liquid crystal material had some distribution of the number of SiMe2 groups, it showed a rather broad SmA*-SmC* phase transition in contrast to the sharp phase transition of conventional electroclinic materials. The electroclinic coefficient reached the maximum value of 8 degrees/V/μm and was at least 1 degree/V/μm over a temperature range as broad as 10°C. The induced electroclinic tilt angle was as high as 22-23 degrees with good linearity and moderate applied electric fields.

Fast electro-optic liquid crystal switch and attenuator with large extinction ratio at 1550 nm

We demonstrate an electro-optic switch and a variable attenuator for telecommunication applications at λ=1550nm by employing the ferroelectric and electroclinic properties of an organosiloxane liquid crystal. In the ferroelectric SmC* phase an optical switch has been realised with an extinction ratio of 36dB between crossed polarisers. The switching time was ~200microseconds. In the SmA* phase the analogue nature of the electroclinic effect was employed to obtain a variable attenuator. The maximum attenuation range between crossed polarisers was 35dB for an applied electric field of +-9V/micron. The response time of the device was about 100microseconds, independent of the applied electric field. Both devices where demonstrated in the same 21.5micron thick cell which provided a retardance of λ/2 at λ=1550nm.

Comparison of relative flexoelectric coefficients measured by different techniques

While it is well known that it is possible to determine the effective flexoelectric coefficient of nematic liquid crystals using hybrid cells, this technique can be difficult due to the necessity of using a D.C. field. We have used a second method, requiring an A.C. field, to determine this parameter and here we compare the two techniques. The A.C. method employs the linear flexoelectrically induced linear electro-optic switching mechanism observed in chiral nematics. In order to use this second technique a chiral nematic phase is induced in an achiral nematic by the addition of a small amount of chiral additive (~3% concentration w/w) to give helix pitch lengths of typically 0.5-1.0 μm. We note that the two methods can be used interchangeably, since they produce similar results, and we conclude with a discussion of their relative merits.