Self-assembling of liquid crystal droplets on lithium niobate substrates driven by pyroelectric effect

It is known that one of the major attractive feature of liquid crystals (LCs) lies in the fact that their optical properties can be modulated by electric, optical or magnetic fields. Recently, it was discovered that liquid crystal droplets, in particular conditions, can be driven in desired locations following electric field lines. The pyroelectric properties of the periodically poled lithium niobate (PPLN) crystal are exploited and PPLN crystal is used as substrate. In fact, heating and/or cooling this material, it has already been demonstrated that surface charges can appear, by pyroelectric effect, leading to very interesting phenomena. Then, being a polar LC molecule, it undergoes a force due to the existing fields, able to move the material. Results show that fragmented droplets coalesce to form bigger droplets in fixed locations, on a longer time scale. In some cases, one single drop can be observed onto each hexagonal domain. These drops behave as microlenses and the whole sample could be viewed as a dynamical optical micro-element able to switch from a diffuser state (fragmentation state) to a microlens array (coalescence state), without the need of an external voltage. Moreover, the birefringent properties of liquid crystals can make such microlenses also tunable.