Resonant X-ray Scattering: A Tool for Structure Elucidation in Liquid Crystals

The use of resonant X-ray scattering to determine structures in liquid crystal systems is surveyed. This powerful experimental technique utilises "forbidden reflections" to determine the subtle differences in interlayer orientation that differentiate several smectic systems. The technique relies on the materials containing an atom to which the X-ray energy can be tuned, usually sulphur or selenium. Experiments are often carried out on free-standing films that provide a highly monodomain structure that allows high-resolution measurements to be made, and, hence, structural details to be determined. Alternatively, resonant scattering has been demonstrated for materials contained in glass devices that permit the application of electric fields to the system, in a manner analogous to that used in liquid crystal devices. The resonant scattering technique provides unequivocal descriptions of the packing in smectic systems, and the way in which the packing is distorted in electric fields. This Minireview describes the principles behind resonant X-ray scattering, its application to liquid crystals and some of the potential for the future.

Nanofabricated media with negative permeability at visible frequencies

A great deal of attention has recently been focused on a new class of smart materials--so-called left-handed media--that exhibit highly unusual electromagnetic properties and promise new device applications. Left-handed materials require negative permeability micro, an extreme condition that has so far been achieved only for frequencies in the microwave to terahertz range. Extension of the approach described in ref. 7 to achieve the necessary high-frequency magnetic response in visible optics presents a formidable challenge, as no material--natural or artificial--is known to exhibit any magnetism at these frequencies. Here we report a nanofabricated medium consisting of electromagnetically coupled pairs of gold dots with geometry carefully designed at a 10-nm level. The medium exhibits a strong magnetic response at visible-light frequencies, including a band with negative micro. The magnetism arises owing to the excitation of an antisymmetric plasmon resonance. The high-frequency permeability qualitatively reveals itself via optical impedance matching. Our results demonstrate the feasibility of engineering magnetism at visible frequencies and pave the way towards magnetic and left-handed components for visible optics.

The absorption of polarized light by vertebrate photoreceptors

A physiologically realistic model has been constructed for a theoretical study of the mechanisms by which the vertebrate visual system absorbs linearly polarized light. Using a 4 x 4 matrix technique, analytic solutions to Maxwell's equations have been deduced for rod and cone photoreceptors, allowing calculation of the absorbance as a function of wavelength for a variety of illumination geometries. With the use of experimentally measured optical parameters, the calculated absorbance spectra show excellent agreement in both magnitude and form with microspectrophotometric data. Moreover, failing to correct for the true nature of reflection or scattering in the sample, results in the elevated absorbance commonly seen at shorter wavelengths in experimental measurements. Finally, calculated dichroic ratios also accurately predict experimental results, mirroring the differences seen between rods and cones.

Differences in the optical properties of vertebrate photoreceptor classes leading to axial polarization sensitivity

Polarization microspectrophotometry recordings were made to investigate possible differences in the way different spectral classes of photoreceptors from coho salmon (Oncorhynchus kisutch) absorb linearly polarized light. The results strongly suggest that rods and cones absorb transversely illuminating polarized light differently. Cones were found to exhibit a tilted optical geometry in which the maximum absorbance occurred when the E-vector was at a small angle to the transverse axis of the outer segment. Solutions to Maxwell's equations were deduced to investigate the effect of this tilt under conditions of axial illumination. Calculations show an approximate 10% difference in the absorbance of orthogonal polarizations, suggesting the possibility of axial dichroism in the cones of this species.

Theory of layer structure in ferroelectric liquid crystal devices in applied electric fields

We propose a model for the free energy of a ferroelectric liquid crystal formed by cooling a sample from the smectic-A phase between parallel substrates. Under these circumstances the smectic layers may deform into V-shaped structures known as chevrons. Application of a strong electric field causes the layers to return to a flat shape, but this can occur in a number of ways. In the model presented here, it is a parameter related to the layer compression modulus that is the principal factor in determining the nature of the field-induced transition from chevrons to flat layers. When this parameter is large, the transition is sudden, but when it is small the chevron first takes on a rounded form before flattening. At intermediate values the tip of the chevron first flattens, and then this flat region gradually grows to encompass the entire layer.

Interlayer structures of the chiral smectic liquid crystal phases revealed by resonant X-ray scattering

The structures of the liquid crystalline chiral subphases exhibited by several materials containing either a selenium or sulphur atom have been investigated using a resonant x-ray scattering technique. This technique provides a unique structural probe for the ferroelectric, ferrielectric, antiferroelectric, and SmC(*)(alpha) phases. An analysis of the scattering features allows the structural models of the different subphases to be distinguished, in addition to providing a measurement of the helical pitch. This paper reports resonant scattering features in the antiferroelectric hexatic phase, the three- and four-layer intermediate phases, the antiferroelectric and ferroelectric phases and the SmC(*)(alpha) phase. The helicoidal pitch has been measured from the scattering peaks in the four-layer intermediate phase as well as in the antiferroelectric and ferroelectric phases. In the SmC(*)(alpha) phase, an investigation into the helical structure has revealed a pitch ranging from 5 to 54 layers in different materials. Further, a strong resonant scattering signal has been observed in mixtures of a selenium containing material with as much as 90% nonresonant material.

Influence of electric fields on the smectic layer structure of ferroelectric and antiferroelectric liquid crystal devices

The electric-field-induced structural rearrangement of smectic layers in the antiferroelectric and ferroelectric phases of three different materials is reported. The materials all have high optical tilt angles (around 30 degrees ), compared with the steric tilt angles deduced from layer spacing measurements (around 18 degrees ). The chevron angles observed in devices agree well with values found for the steric tilt angle across the tilted mesophase range. Electric fields were applied to liquid crystal devices while the smectic layer structures, in both the depth and in the plane of the device, were probed using small angle x-ray scattering. Two separate aspects of the influence of the field on the layer structure were studied. First, the organization of the smectic layers in the antiferroelectric phase is described before, during, and after the application of an electric field of sufficient magnitude to induce a chevron to bookshelf transition. Second, the evolution of the field-induced layer structure change has been investigated as the field was incrementally increased in both the antiferroelectric and ferroelectric phases. It was found that the chevron to bookshelf transition has a distinct threshold in the antiferroelectric phase, but shows low or zero threshold behavior in the ferroelectric phase for all the materials studied.

Orientational ordering in the chiral smectic-C*FI2 liquid crystal phase determined by resonant polarized x-ray diffraction

High-resolution resonant polarized x-ray diffraction experiments near the sulfur K edge have been performed on free-standing liquid crystal films exhibiting the chiral smectic-C*FI2 phase. It is widely accepted that this phase has a four-layer repeat unit, but the internal structure of the repeat unit remains controversial. We report different resolved features of the resonant x-ray diffraction peaks associated with the smectic-C*FI2 phase that unambiguously demonstrate that the four-layer repeat unit is locally biaxial about the layer normal and that the measured angle, describing the biaxiality, is in good agreement with optical measurements.

Resonant x-ray scattering study of the antiferroelectric and ferrielectric phases in liquid crystal devices

Resonant x-ray scattering has been used to investigate the interlayer ordering of the antiferroelectric and ferrielectric smectic C* subphases in a device geometry. The liquid crystalline materials studied contain a selenium atom and the experiments were carried out at the selenium K edge allowing x-ray transmission through glass. The resonant scattering peaks associated with the antiferroelectric phase were observed in two devices containing different materials. It was observed that the electric-field-induced antiferroelectric to ferroelectric transition coincides with the chevron to bookshelf transition in one of the devices. Observation of the splitting of the antiferroelectric resonant peaks as a function of applied field also confirmed that no helical unwinding occurs at fields lower than the chevron to bookshelf threshold. Resonant features associated with the four-layer ferrielectric liquid crystal phase were observed in a device geometry. Monitoring the electric field dependence of these ferrielectric resonant peaks showed that the chevron to bookshelf transition occurs at a lower applied field than the ferrielectric to ferroelectric switching transition.