Molecular Orientation Change Nearby Topological Defects Observed by Photo-Induced Polarization/Phase Microscopy.

Haruka Sakanoue,Kenji Katayama,Woon Yong Sohn

doi:10.1021/acsomega.9b01611

Haruka Sakanoue, Kenji Katayama + Show 1 more

Open Access

https://doi.org/10.1021/acsomega.9b01611

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Journal: ACS omega	Publication Date: Aug 14, 2019
Citations: 3	License type: cc-by

Affiliation: Chuo University, Japan Science and Technology Agency

Abstract

Topological defects in liquid crystals (LCs) have been intensively studied and intentionally generated in an organized way recently because they could control the alignment and motion of LCs. We studied how the topological defects could change the molecular orientation/alignment from the observation of photo-induced orientation change of a photo-responsive LC. The photo-induced dynamics was observed by an LED-induced time-resolved polarization/phase microscopy with white light illumination. From the color image sequence, we found that the molecular orientation change started from the topological defects and the orientation change propagated as a pair of defects and was connected, and further disordering was induced as a next step after the initial orientation change finished.

Highlights

Topological defects for liquid crystals (LCs) are the positions where the molecular orientation cannot be defined and are found when LCs are placed in a cell without an alignment layer
Arbitrary change of the refractive index change and the rotation angle were assumed (Δn = 0.005, Δα = 10 degrees), the transmittance before and after the change was calculated, and the trasmittance difference after the change was calculated to see the spectral shape change due to each process
When the LC orientation angle is larger or smaller than 45 degrees, the transmittance becomes smaller. This is the origin of the pattern formation of topological defects, where the orientation of LC molecules rotates 360 degrees around the topological defects for +1 or −1 charge, and the bright and dark regions are repeated as the orientation angle changes

Summary

■ INTRODUCTION

Topological defects for liquid crystals (LCs) are the positions where the molecular orientation cannot be defined and are found when LCs are placed in a cell without an alignment layer. There are various types of topological defects, and they are categorized by the topological charge, which is determined by the rotational angle of molecules around each defect. At these positions, elastic energy is higher than those in neat alignment, which could be understood from Frank’s free energy.[1]. We intentionally generated topological defects inside a planer cell without an alignment layer, and the photoinduced molecular orientation change was observed under the polarization/phase microscope with a millisecond time resolution. By the observation of color image sequences and the analysis of spectral response, we investigated how the molecular orientation change proceeded around the topological defects

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ REFERENCES