We show realisation of images and identification codes based on show-on-demand microstructures. Experimentally, these micron-sized structures are written directly into the liquid crystal using lasers, which fixes the local molecular order at the moment of the structure formation. When reading these structures with the same voltage as when they were created, the structures can be made effectively invisible using the surrounding birefringence. More generally, these results use the concepts of transformation optics and metamaterials and relate them to birefringence and phase matching. The potential applications of this work are for authentication or identification. Experiments were performed at the University of Oxford, whereas theory and modelling at the Faculty of Mathematics and Physics University Ljubljana and Josef Stefan Institute. The work was also highlighted by editors in Nature Photonics (Nat. Photon. 12, 504 (2018)), and EU patent application is filed.
COBISS.SI-ID: 3223396
Generation of fractal topological states in nematic fluids was demonstrated, which were realised by coupling fractal colloidal particles and nematic liquid crystals. The research was a combinations of numerical modelling, which was performed at the Faculty of Mathematics and Physics at the University of Ljubljana with support from the group at Sharif University of Technology, and experiments, which were performed at the Department of Condensed Matter Physics at the Jozef Stefan Institute. In numerical modelling, mesoscopic phenomenological free energy minimisation with finite elements was used, whereas experimentally, two-photon laser polimerisation was used to produces colloidal particles in the shape of fractal Koch snowflakes, which were then observed with optical microscopy techniques. The work was just published in Nature Communications and is a contribution towards the development of responsive complex soft matter.
COBISS.SI-ID: 30202663
We show novel refraction phenomena at the boundary between isotropic medium and anisotropic (hyperbolic) metamaterial with positive/negative eigenindices. Specifically, different regimes of negative and positive refraction are explored for the refraction and reflection of light on the boundary between isotropic material and anisotropic (hyperbolic) metamaterial with the central goal to determine the generalized Snell’s law. The refraction is simulated with custom developed FDTD approach, with an addition of auxiliary differential equation method (ADE), to simulate frequency-dispersive permittivity function, which is necessary to take into account when describing systems with negative ordinary and/or extraordinary index of refraction.
COBISS.SI-ID: 3100004
We demonstrate polarization-selective microlensing and waveguiding of laser beams by birefringent profiles in bulk nematic fluids using numerical modelling. Specifically, we show that radial escaped nematic director profiles with negative birefringence focus and guide light with radial polarization, whereas the opposite – azimuthal – polarization passes through unaffected. A converging lens is realized in a nematic with negative birefringence, and a diverging lens in a positive birefringence material. Tuning of such single-liquid lenses by an external low-frequency electric field and by adjusting the profile and intensity of the beam itself is demonstrated, combining external control with intrinsic self-adaptive focusing. Escaped radial profiles of birefringence are shown to act as single-liquid waveguides with a single distinct eigenmode and low attenuation. Finally, this work is an approach towards creating liquid photonic elements for all-soft matter photonics.
COBISS.SI-ID: 2994020
We demonstrate nematic and cholesteric liquid crystal (LC) gyroids and show their photonic properties as photonic crystals by using numerical modelling. The LC gyroids are designed as composite optical materials, where we take one labyrinth of passages to be a solid dielectric, whereas the other (complementing) labyrinth of passages is taken to be filled by chiral or achiral nematic LC, with the intermediate gyroid surface imposing homeotropic (perpendicular) surface anchoring. The nematic inside the gyroid matrix is shown to exhibit a variety of possible orientational profiles which are characterised by complex networks of topological defects. The diversity of possible nematic states is shown to lead to a rich structure of photonic bands, which can be tuned by the LC volume fraction and the cholesteric pitch, including control over full – direct and indirect – band gaps. The work is published as the invited contribution in 30th anniversary Special Issue featuring 30 invited articles, each "written by an early career researcher whose work will help to shape the future of liquid crystals research".
COBISS.SI-ID: 2977892