Abstract
We demonstrate the all optical control of the molecular orientation of nematic liquid crystals confined in microfluidic channels engraved in lithium niobate. Microchannels are obtained by a novel approach based on femtosecond pulse laser micromachining carried on in controlled atmosphere. The combined effect of photovoltaic and pyroelectric fields generated by light in lithium niobate crystals on the liquid crystal orientation, is reported for the first time. The total space charge field and its dependence on the incident light intensity can be controlled by changing the direction of pump light propagation through the microfluidic chip. The results reported in this manuscript demonstrate that liquid crystals and lithium niobate can efficiently be combined in microfluidic configuration, in order to push forward a novel class of optofluidic devices.
Highlights
For many years lithium niobate (LiNbO3) has been among the most widely used materials in photonics[1]
The fields coming into play are two, since, due to the high light intensity resulting from beam focusing, the pyroelectric field arising from laser heating cannot be neglected and its effect combines with that of the photovoltaic field creating an additional degree of freedom for controlling the liquid crystals (LC) response
The off times shows a slight increase for higher intensity, which may be the indication of the onset of some memory effect in the light-induced modification of lithium niobate charge distribution, to what happen in thermal fixing mechanisms
Summary
Silvio Bonfadini[2,3], Fabrizio Ciciulla 1, Luigino Criante[2], Annamaria Zaltron[4], Francesco Simoni[1], Victor Reshetnyak 5 & Liana Lucchetti[1]. The light-induced control of the optical phase shift by a liquid crystal cell having LiNbO3 substrates[8,9] and the generation and manipulation of defects in LC films deposited on LiNbO3 crystals[10,11], have been reported In all these papers, the key role is played by the bulk photovoltaic effect which arises in LiNbO3 under illumination[12]. The key point of these studies is the possibility of designing a new road map in the field of materials available for optofluidics, where the optically generated electric fields of LiNbO3:Fe can be conveniently used and configured to drive liquid crystal molecular orientation, allowing the fabrication of novel all optical microfluidic devices with a high degree of compactness. The system requires further optimization, the obtained results show several peculiar features such as the fast response time and the possibility of combining the photovoltaic and the pyroelectric fields in order to get a further control on the light induced LC reorientation when it is confined to micrometric geometries
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