LCP research on the back cover of Advanced Optical Materials

(13-07-2018) Inge Nys, LCP research group, got once again the attention of the Advanced Optical Materials editors with an article on Liquid Crystal Superstructures: Surface‐Mediated Alignment of Long Pitch Chiral Nematic Liquid Crystal Structures

In the issue of Advanced Optical Materials 13/2018, released on the 4th of July, the backcover picture originated from the work of Inge Nys, Jeroen Beeckman and Kristiaan Neyts from the Liquid Crystals and Photonics.
Liquid Crystal Superstructures: Surface‐Mediated Alignment of Long Pitch Chiral Nematic Liquid Crystal Structures (Advanced Optical Materials 13/2018), Inge Nys  Jeroen Beeckman  Kristiaan Neyts
Liquid Crystal Superstructures: Surface‐Mediated Alignment of Long Pitch Chiral Nematic Liquid Crystal Structures (Advanced Optical Materials 13/2018), Inge Nys Jeroen Beeckman Kristiaan Neyts
In this article, we have applied surface topography to steer the directional growth of long pitch chiral nematic liquid crystal structures. A periodic grating is created by e-beam lithography and gives rise to micrometer-scale periodic variations of the surface anchoring. The surface topography is combined with an applied electric field to steer the growth of cholesteric fingers and the liquid crystal director configuration in the cells is simulated.​

 

 

Layman's abstract
Liquid crystals are soft materials in which long organic molecules prefer to align parallel to their neighbors. This type of material can organize itself into complex structures, which can be responsive to external stimuli such as heat or electric fields. Nowadays liquid crystals are widely used in displays (so-called LCDs or liquid crystal displays) but they can also be used in other applications such as tunable lenses or smart windows. In most applications, the orientation of the liquid crystal molecules is modified by an electric field, and this changes the properties of transmitted light. In this way a pixel in a LCD can be switched from bright to dark by applying a voltage. To develop new functional devices it is essential to control the alignment of the liquid crystal at the solid surfaces.
 
In this study, a new method is developed to define the orientation of the liquid crystal at the interface with a solid surface. On top of a flat glass substrate, a periodic structure with grooves is made by electron-beam illumination. This structure aligns the liquid crystal molecules alternatingly parallel and perpendicular with the substrate. It turns out that these periodic variations match very well with chiral liquid crystals, which tend to spontaneously form a helical structure with a well-defined period. When this material is placed on the surface with linear grooves, the helical units form preferably along the grooves. The growth of the helical units can be controlled by applying an electric field over the liquid crystal layer. This offers interesting possibilities for the realization of diffraction gratings and for the stabilization of uniform lying helix structures that can be used in lasing and display applications. The effect of the patterned  structure on the chiral liquid crystal material has been analyzed by making numerical simulations of the liquid crystal orientation and the optical transmission. 
 
The straight-forward technique to produce surface structures by lithography has the advantage that small-scale structures with complex shapes are easily achievable. In the future, the combined effect of different surface structures and electric fields on the alignment of liquid crystals can be further exploited to manipulate the liquid crystal orientation. This can further stimulate the development of electro-optic devices such as displays, diffraction gratings and lasers based on liquid crystals. ​