5-12 March 2023

Prof. Dr. Ridvan Karapinar

Home Institution
Burdur Mehmet Akif Ersoy University Faculty of Sciences and Arts Department of Nanoscience 15030 Burdur, Turkiye

Host Contact
Prof. Irena Drevensek Olenik

Host Institution
University of Ljubljana, Faculty of Mathematics and Physics, Ljubljana, Slovenia

Aim of the mission
The visit took place within the COST Action CA17139 (EUTOPIA). During the STSM visit, we studied the effect of a photovoltaic field generated on the surface of lithium niobate crystals on droplets of a commercial nematic liquid crystal material (E7) deposited on the crystal surface. Temperature dependence of the transport dynamics on the Z-cut and X-cut LiNbO 3 plates was also studied. Fabrication and characterization of such systems is of great interest in recent studies. These optical systems can be helpful in designing novel electro-optical devices such as droplet routers, mixers and/or mergers.

Summary of the Results
Lithium niobate (LiNbO 3, LN) crystals have interesting electro-optic and piezoelectric properties, which can be used in wide range applications. Iron-doped lithium niobate crystals also show strong bulk photovoltaic effect. By illumination with light in the green spectral region, a spatial redistribution of charge carriers occurs in the illuminated area. This redistribution takes place predominantly along the polar axis (c-axis) of the crystal. As a result, a static space-charge electric field is generated inside the crystal surface. In this work, we investigated the effect of a photovoltaic field generated on the surface of lithium niobate crystals on droplets of a commercial nematic liquid crystal material (E7) deposited on the crystal surface. When the crystal surface was illuminated with a laser light, a wide range of dynamic phenomena were initiated. On the X-cut LN plate, upon the light exposure, the droplets located inside the illuminated region tended to bridge each other and re-arranged into mobile cylindrical structures, while the droplets located outside the laser spot were dragged in the direction of the illuminated area. On the Z-cut LN plate the droplets formed streams that run out in radial direction from the illuminated area. In the experiments, a few LC droplets of E7 were deposited on the crystal surface which was placed onto microscope glass slide. The sample surface was irradiated by a single laser beam (λ= 532 nm) with a Gaussian intensity profile. The beam was directed onto the sample surface via the arm for the episcopic illumination in the polarizing optical microscope (POM). The experiments were performed at different temperatures corresponding to the nematic (N) and the isotropic (I) phases of the material. The corresponding light-induced modifications of the nematic droplet structure were monitored with a camera system connected to the POM and analyzed with a software program. When the laser beam was focused onto the LC droplet, the rearrangement of the inner orientation of the droplet was observed. Fig.1a shows snapshots of the dynamic processes taking place at the nematic droplet deposited on the X-cut plate at room temperature. After starting laser illumination, several tiny filaments were formed. They were oriented along the electric field and exhibited time dependent oscillations as a result of the charge separations on the illuminated region of crystal surface They were transient in nature and exhibited branching into narrower filaments and some filaments reached lengths of several millimeters.  Elongation of the droplets in the direction of the c-axis was observed at all temperatures. This indicates that nematic droplets tend to flow along the electric field on the crystal surface. Then the droplets merged together into the running streams by which the material was transported along the c-axis. Similar experiment was carried out on the droplet of the nematic material placed on the Z-cut Fe-doped LiNbO 3 plate. In this case electric field is perpendicular to the crystal surface and filaments are showing a radial texture (Fig.1b).

During the STSM visit, photo-induced alignment of liquid crystal droplets on crystal surfaces have been investigated. Fabrication and characterization of such systems is the topic of a wide research experience from the Faculty of Mathematics and Physics at University of Ljubljana. During the STSM proposal of a possible future collaborative project has been discussed. This will be a solid basis for the
future collabration. It is hoped that results of this study of STSM visit will be presented in a scientific conference or published in a journal.