In this paper, we present the first microfluidic junctions for droplet generation directly engraved on lithium niobate crystals by micromachining techniques, preparatory to a fully integrated opto-microfluidics lab-on-chip system. In particular, laser ablation technique and the mechanical micromachining technique are exploited to realise microfluidic channels in T- and cross junction configurations. The quality of both lateral and bottom surfaces of the channels are therefore compared together with a detailed study of their roughness measured by means of atomic force microscopy in order to evaluate the final performance achievable in an optofluidic device. Finally, the microfluidics performances of these water-in-oil droplets generators are investigated depending on these micromachining techniques, with particular focus on a wide range of droplet generation rates.

Lithium Niobate Micromachining for the Fabrication of Microfluidic Droplet Generators

Formation of relocatable umbilical defects in a liquid crystal with positive dielectric anisotropy induced via photovoltaic fields

Exciting experimental results on the response properties of hybridized photo responsive liquid crystal test cells are reported, where iron doped lithium niobate substrates were used to photo generate electric fields and indium tin oxide coated cover glasses were used to confine these photo generated fields in a liquid crystal layer. Samples were investigated in a modified inverted optical polarizing microscope with white probe light (crossed polarizers) and exposed with a Gaussian laser beam focused to a small spot (14 μm FWHM). Test cells filled with nematic LC showed homeotropic director alignment. Upon exposure, this alignment was maintained at the exposure spot center and the LC director was selectively realigned in a surrounding single ring. This ring had a thickness of a few microns and its diameter increased with increasing exposure intensity (112 μm at 0.7 mW, 204 μm at 1.1 mW). This characteristic director realignment was traced back to the optically generated electric field distributions by simulations. In samples filled with chiral nematic LC, uniformly standing helix alignment was found. Textural transitions were induced at the focus position, which again led to the formation of well-defined circular defects. We could show that these defects can be permanently stored within the chiral nematic LC. Polarized optical microscopy of a rotated sample revealed that a point like defect with +1 topological charge was enclosed in each of these defects. Photovoltaic fields generated in small lithium niobate particles dispersed in a LC were found to cause promising optical responses and particle movement.

Confined photovoltaic fields in a photo-responsive liquid crystal test cell

Abstract Photovoltaic fields induced in x-cut Fe-doped lithium niobate (Fe:LiNbO3)were used to achieve optically induced defect formation and light valving in a vertically aligned nematic liquid crystal. Initially, the optical axis of the LC was vertically aligned (along the surface-normal of the planar, photovoltaic substrates) throughout the whole sample. Samples were exposed with a focused continuous wave laser beam and investigated via microscopic imaging in-between crossed polarizers. The optical axis of the planar, x-cut Fe:LiNbO3 substrates was in the substrate plane and oriented parallel to one of the polarizers, which resulted in an initially dark state. Optically induced surface fields (with high in-plane components) generated within the substrates led to director reorientations and defect formation. Accordingly, the samples were locally switched into a transmissive state. The area affected by exposure was larger (300 μm) than the FWHM of the Gaussian exposure beam (14 μm). Switching from dark to bright states (light valving) could be achieved in the investigated samples much more eficiently than in previously investigated samples with z-cut Fe:LiNbO3-substrates. Realignments of the LC director were induced at lower optical power density (140 mW/cm2) than would be required to excite the intrinsically present nonlinear optical responses in a nematic LC such as the light induced Fredericks transition.

/pdf/photovoltaic-light-valving-induced-in-a-vertically-aligned-4jdhg7h5zz.pdf

Photovoltaic light valving induced in a vertically aligned nematic liquid crystal on a x-cut Fe:LiNbO3substrate

Field-induced defect creation was investigated in chiral nematic liquid crystal test cells, where a Fe-doped lithium niobate thin-cut was applied as field generating substrate. These photo ...

Defect formation in N* LCs via photovoltaic fields: impact of surface treatment

Liquid crystal cells with LiNbO3:Fe crystals as substrates, are described. The photovoltaic field generated by the substrates is able to reorient the liquid crystal director thus giving rise to a phase shift on the light propagating through the cell, as in liquid crystal light valves. The process does not require the application of an external electric field, thus being potentially useful for applications requiring a high degree of compactness. A detailed characterization of several cells based on lithium niobate crystals with different iron concentration has been carried out. The correlation between the LiNbO3:Fe characteristics and the liquid crystal reorientation is also discussed.

All-optical phase shifter with photovoltaic liquid crystal cell

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All-optical phase shifter with photovoltaic liquid crystal cell