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Federal Research Center 
"Krasnoyarsk Science Center of the Siberian
Branch of the Russian Academy of Sciences"

 Федеральный исследовательский центр «Красноярский научный центр Сибирского отделения Российской академии наук»

Federal Research Center 
"Krasnoyarsk Science Center of the Siberian
Branch of the Russian Academy of Sciences"

Twisted into rolls

5 March 2020 г.

Закрученные в роллы
Krasnoyarsk physicists are studying the parameters of a twisted structure, the so-called rolls, in a defect layer of a liquid crystal as part of a multilayer photonic structure. The study of such models is promising for aplication in various optoelectronic devices. The scientists told the Internet portal "Scientific Russia" about these structures.

As functional elements of nanophotonics, optoelectronics and optical instrumentation, experts consider multilayer photonic structures. The attention of scientists is directed primarily to the photonic band gap (or opacity band), which is a unique property of the structure and which can be modified. This allows expanding the boundaries of the application of photonic crystals. Violations of the periodicity lead to the appearance of defect modes (spectral transparency windows) in the band gap.

The alternation of layers of the photonic structure results in different optical characteristics. Therefore, physicists use a combination of different materials. For example, as a defect layer, Krasnoyarsk scientists took a liquid crystal (nematic) as it forms photonic structures with controlled spectral characteristics. In the nematic, the specialists studied the formation of twisted periodic structures, or abnormal rolls, and in particular, how this transformation occurs and how polarization spectra are transformed following this formation.

The approach proposed by Krasnoyarsk physicists to the study of the photon structure can be used to study the features of spatially periodic structures in liquid crystal systems, and further, in tunable spectral filters, polarization sensors, and optical devices.

“Photonic structures are structures with periodically varying dielectric permittivity (varying refractive index). As a result, such materials acquire unique optical properties. For example, they almost completely reflect light in a certain wavelength range, which can often be found in nature in bright colored objects (opals, wings of some butterflies or beetles, feathers of peacocks, chameleons), ” says Mikhail Krakhalev, Candidate of Physical and Mathematical Sciences, associate professor , senior researcher at the L.V. Kirensko Institute of Physics SB RAS, - The multilayer photonic structure under study represents periodically arranged layers (one-dimensional case) with a “defect” layer in the center. The defect layer differs from other layers forming the periodic structure by thickness or refractive index. What does the presence of a defect layer imply? In this case, within the range of wavelengths that are reflected from periodic structures, there appear certain frequencies at which light travels almost completely forward (defect modes of the photonic structure). The number of these modes and their frequency (wavelength) turn out to directly depend on the parameters of the defect layer: the thickness and refractive index of the substance it is made of. Accordingly, by changing the parameters of the defect layer, one can control the spectral position of optical modes (controlled spectral filters, tunable lasers), or vice versa, by the shift of the defect modes, one can infer the changes which occur in the defect layer (sensors)."

In their study the Krasnoyarsk physicists focused on the experimental study of the defect layer of the photonic structure and analysis of its indicators.

How to change the parameters of a defect layer?

“One of the most effective and widely applied methods is to use as a defect a thin (several micrometers) layer of a liquid crystal, for example, a nematic one. Such a liquid crystal has an orientational order and is an optically uniaxial medium, which implies that the refractive index depends on the direction of the preferred orientation of nematic molecules (by the way, this effect is used in liquid crystal monitors, and TV sets). Accordingly, the position of defect modes of the photonic structure depends on the orientation of the liquid crystal molecules.

Moreover, if a twisted nematic structure rather than a uniformly oriented one is created in the layer, then an additional factor appears which is associated with the twisting degree (the so-called geometric phase). Accordingly, having determined the contribution of the geometric phase to the total change in the spectral position of optical modes, the nematic twist angle can also be measured, ” as explained by the researchers.
So, in nematic liquid crystals complex twisted structures are formed, which are characterized by different conditions and methods of shape formation.
How to obtain such a twisted structure?

“A twisted structure can be formed in a nematic layer by orienting the molecules at one and another boundaries of the layer in different directions. As a result, the orientation of the liquid crystal in the volume will rotate smoothly when moving from one substrate to another (such a structure is implemented, for example, in TN-cells of monitors).

But in our case, we considered a system in which a twist appeared as a result of the circling motion of a liquid crystal under the action of an alternating electric field. It should be noted that the behavior of the resulting fluid flows can be different. The simplest case is a circular motion, leading to the formation of periodically arranged rolls. The twist of the structure occurs during a spiral flow of fluid, which is observed in the case of abnormal rolls. It is worth mentioning that in this case, the origin of twisting is significantly different from the case when twisting occurs due to the surface treatment. In abnormal rolls, the orientation of the molecules is the same at the boundaries of the layer, and in the center of the layer the molecules are twisted at a maximum angle, ”explains Vladimir Gunyakov, Candidate of Physical and Mathematical Sciences, associate professor, senior researcher at the L.V. Kirensko Institute of Physics SB RAS, - The given direction of the convective flow in rolls provides an optimal connection between the flow rate and configuration of the nematic director. However, this situation is accompanied by maximum viscous energy dissipation. Twisting the director in rolls reduces dissipation, contributing to the stabilization of the roll structure.

From the viewpoint of optics, rolls are a phase diffraction grating, whose parameters can be controlled by an electric field. Accordingly, such a system of rolls is an interesting model object of complex scattering media placed in a defect layer of a photonic structure. We were interested in a question: how do the appearance of a scattering medium and the type of forming electroconvective flows (rolls) being formed  affect defect modes? ”

Observation of the model object with a complex geometric shape and internal structure shows that scientists have yet to study this peculiar optical system in order to obtain more detailed information about its behavior as part of complex systems.

“We have shown that a photonic multilayer structure with a defect layer can be used to analyze complex periodic structures formed in liquid crystal systems as a result of electro-convective processes. So, one can easily determine the transition between normal and abnormal rolls, as well as investigate the structure of abnormal rolls by measuring the position of optical modes of the photonic structure. In the future, this approach can be further expanded to other systems with a similar structure, ” Mikhail Krakhalev notes the novelty of the approach in the framework of the study of the photonic nematic structure.

Source: Scientific Russia