Photonic properties of liquid crystalline systems with a helical superstructure

The helical superstructure of SmC* liquid crystalline phases involves a periodic modulation of the refractive index n and, like in a photonic crystal, light of appropriate wavelength l = n p is selectively reflected when travelling along the helix axis (“photonic bandgap”). Distorting the helical structure by even weak external fields also changes the selective reflection and, thereby, gives rise to decent effects like tunable lasing, as it was recently demonstrated by Finkelmann et al. for chiral nematic liquid crystals and in the case of smectic C* liquid crystals by Ozaki et al. On the other hand, light travelling perpendicular to the helix axis is diffracted by the periodic helical structure and the diffraction patterns give detailed information about the helical modulation (i.e. its pitch p) and its response to external forces.
 

The central scope of this project concerns the question how the SmC* helical structure is distorted and, above a certain threshold field, even suppressed by an external electric field. The distortion results from the competition between elastic (favoring the undistorted helical structure) and electric (favoring a uniform polarization field, incompatible with the helix) interactions. The understanding of this process is of central interest for possible non-display applications of SmC* liquid crystals in tunable lasers and diffraction gratings.
 

The picture below show a ferroelectric liquid crystal sample (thickness: 50 µm), whose helical superstructure becomes visible in a polarizing microscope in terms of a bright-dark modulation with a periodicity (pitch) of about 6 µm (left). From an optical standpoint the sample represents a phase grating, leading to diffraction of laser light.

From about U = 10 V on, the helical structure changes to a completely homogeneous texture (right), which does not exhibit any characteristic diffraction spots, apart from the diffuse scattering in vertical direction.