Liquid Crystal Sample
LC cells assembly and microscope characterization
A cell consists of two parallel glass plates held at a constant distance by spacers and sealed after filling. The LC occupies the space between both glasses. In order to minimize defects and to achieve a proper functionalization, the glasses undergo a thorough cleaning procedure. As we want the LC to align in a determined way on the glass substrate, the inner surfaces of the cell are treated with a specific polymer imposing either homeotropic or planar alignment. The functionalization is performed either via spin coating or simply dipping the substrates in the polymer solution. Additionally, the polymeric layers inducing a planar alignment have to be rubbed on a velvet to obtain a defined azimuthal alignment along the rubbing direction. Afterwards the cell can be assembled with a UV curable glue, choosing a specific spacer from few to hundreds of micrometers in thickness. Now LC can be introduced and subsequently sealed. A polarizing microscope is used to observe the obtained result and the LC alignment. If no polymer is used for alignment, contacts can be introduced in the design of the cell and AC voltages up to 1KV are applied.
LC free standing films
Because of the high surface tension, smectic liquid crystal can form free standing films by moving a razor blade upon a hole in a copper block. The uniqueness of such films lies in the fact that they serve as simple models of two-dimensional fluids. Free standing films has been mainly studied in the contest of topological and geometrical properties, such as liquid inclusions and dislocations.
In a freely suspended film, the air-surface interface has a similar but weaker effect on the surface director orientation as compared to that of a treated glass substrate imposing homeotropic alignment (perpendicular to the surface). The layer depth distance over which this orientation persists is known as the correlation length, which is analogue to the skin-depth effect (director orientation decreases exponentially with the correlation length as scale factor).
The liquid crystal holder project is originally inspired by the linear slide target inserter from the Ohio State University Scarlet laser facility [1,2]. This enabled in-vacuum film formation at high repetition rate (10 Hz). Our liquid crystal holder consists of a wiper that glide vertically over a circular aperture within a copper frame. The wiper is guided by a bridge that can vary the force pressing the wiper against the frame. Aperture diameters of few mm are used for various applications and enable different thickness control. These apertures are beveled with a sharp edge: that means LC forms inside the bevel with a fixed position. Temperature regulation is applied for precise thickness control and is achieved by heating the frame with resistors, cooling it down with Peltier elements. Moreover, an automatic LC release has been added to be able to operate the system remotely.
[1] Poole, P.L., et al., Liquid crystal films as on-demand, variable thickness (50-5000 nm) targets for intense lasers. Physics of Plasmas, 2014. 21(6).
[2] Poole, P.L., et al., Moderate repetition rate ultra-intense laser targets and optics using variable thickness liquid crystal films. Applied Physics Letters, 2016. 109(15).