Optical Characterization of a Nanoscale Incommensurate Pitch in a New Liquid-Crystal Phase

As a remarkable orientationally ordered soft-condensed matter system, the smectic-$C_{\alpha }^{*}$ phase exhibits an incommensurate nanoscale helical pitch of more than four smectic layers. Recent theoretical advances predict the existence of a new phase showing a helical pitch of less than four smectic layers ($<16\mathrm{n}\mathrm{m}$). By applying a novel optical technique to the study of freestanding films with increments in thickness of one smectic layer, we have measured the size of this pitch using 633 nm HeNe laser light. Our results confirm the existence of the newly predicted phase in one unique compound.

Figure 1

The DOR signal versus ($T\mathrm{\text{−}}{T}_c$) for a series of films of MHPOCBC. Each temperature ramp was performed under the same direction of $E$ in the plane of the film. Long upward arrows illustrate periodicities of the signal with $N$ in the Sm-$C_{\alpha 2}^{*}$ phase. The shorter arrows alternating in direction indicate the 2-layer periodicity of the data in the Sm-$C_A^{*}$ phase. At the top is the chemical structure of MHPOCBC.

Figure 2

DOR data (circles) acquired from $N=62$, $N=163$, and $N=221$ MHPOCBC films while cooling at $80\mathrm{m}\mathrm{K}/\min $ with $E=3\mathrm{V}/\mathrm{c}\mathrm{m}$ in the plane of the film. Simulated results are shown as solid gray lines.

Figure 3

The plot shows the pitch evolutions used in simulating data from the $N=62$ (light gray line), $N=163$ (dark gray line), and $N=221$ (black line) films. Stars represent NTE measurements of the pitch in the Sm-$C_A^{*}$ phase. On the right is a cartoon of the INHP structure with ellipsoids representing molecules. $N_s\approx 2$ is the number of anticlinic surface layers.