Vector beams generated by microlasers based on topological liquid-crystal structures

Structured light with designable intensity, polarization and phase fields is today of high relevance, with application ranging from imaging, metrology, optical trapping, ultracold atoms, classical and quantum communications and memory. Specifically, vortex and vector beams can be generated directly in the laser cavity, however, a controllable, geometrically simple and easy to manufacture laser microcavity that generates structured light on demand, especially tailored polarization, is still an open challenge. Here we show that tunable laser vector beams can be generated from self-assembled liquid-crystal (LC) micro-structures with topological defects inside a thin Fabry-P\'erot microcavity. The LC superstructure provides complex three dimensional birefringent refractive index profiles with order parameter singularities. The topology of the LC structures is transferred into the topology of the light polarization. The oriented fluorescent dye emission dipoles enable the selection of optical modes with a particular polarization, as enabled by the birefringence profile in the laser cavity. The proposed lasers have no principal limitation for realizing structured light with arbitrarily tailored intensity and polarization fields

Fotonski načini v anizotropni topološki mehki snovi

This thesis numerically explores ideas for the realisation of novel photonic modes using anisotropic topological soft matter, with the control over material birefringence being the central underlying concept. The work is focused on analyzing the interplay between light and material, especially by use of topological defects and concepts of photonic topological insulators. The main methodological approach is Finite Difference Frequency Domain (FDFD) method, which is specifically developed to calculate eigenstates in the anisotropic optical resonators by adapting and improving upon existing FDFD approaches and can account for arbitrary spatially varying optical axis of uniaxial birefringent material. First optical system explores eigenmodes of different liquid crystal structures, like nematic droplets, defects and layers, embedded in Fabry-Pérot resonator. Shapes of the intensity profiles of the emerging modes are found to depend on the shape of the cavity, determined by its refractive index profile, while the polarization profiles show strong dependence on the spatially varying nematic director field. Under second research direction, we explore the possibilities for deflection and control of the light beam intensity profiles by using stacks of liquid crystal cells. Combinations of individually controlled building blocks show the ability to control the light beam continuously. The proposed device can operate in a broader wavelength spectrum and is capable of splitting the beam and controlling each part individually. A third system looking at the effects of pixelization on photonic crystal band gaps is studied on the case of cylindrical pillars in a square lattice. We show that already simple approximations of the circular pillar cross sections act similarly to the original structure with only a small mismatch in the photonic band structure. The demonstrated concept is used to construct a perturbed pixelated photonic crystal based on the Kagome lattice, which supports the existence of unidirectional states at the domain boundaries. Further, unidirectional states are also shown in a perturbed pixelated Kagome lattice photonic crystal based on liquid crystals, which offer a possibility of constructing a topological photonic crystal with reconfigurable boundaries. In the fourth line of research, characteristics of negative birefringence nematic liquid crystal reordering in the vicinity of umbilical defects in the presence of external electric field are examined. More generally, the work explores new concepts for using soft materials for real-time shaping and control of the light at the microscopic level.V tej doktorski disertaciji numerično raziščem ideje za realizacijo novih fotonskih načinov z uporabo anizotropnih topoloških mehkih snovi, pri čemer je osrednji osnovni koncept nadzor nad optično osjo materiala. Delo je osredotočeno na analizo medsebojne sklopitve svetlobe in materiala, zlasti z uporabo topoloških defektov in konceptov fotonskih topoloških izolatorjev. Glavna uporabljena metoda je metoda končnih diferenc v frekvenčni domeni (FDFD), ki sem jo razvil posebej za izračun lastnih stanj v anizotropnih optičnih resonatorjih s prilagajanjem in nadgradnjo obstoječih izvedb FDFD metode in lahko upošteva poljubno prostorsko spreminjajočo se optično os enoosnega dvolomnega materiala. V prvem optičnem sistemu raziščem lastne načine različnih struktur tekočih kristalov, kot so nematske kapljice, defekti in plasti tekočih kristalov, vgrajenih v Fabry-Pérotov resonator. Ugotovim, da so oblike intenzitetnih profilov pripadajočih svetlobnih načinov odvisne od oblike votline, ki je določena z njenim profilom lomnega količnika, medtem ko polarizacijski profili kažejo močno odvisnost od prostorsko spreminjajočega se direktorskega polja. Pod drugo raziskovalno smerjo raziskujem možnosti odklona in nadzora intenzitetnih profilov svetlobnega snopa z uporabo skladov tekočekristalnih celic. Kombinacije individualno nadzorovanih gradnikov kažejo sposobnost neprekinjenega nadzora svetlobnega snopa. Predlagana naprava lahko deluje v širšem spektru valovnih dolžin in lahko razdeli žarek ter nadzoruje vsak del posebej. Kot tretji sistem preučim učinke pikselacije na vrzeli v pasovnih strukturah fotonskih kristalov na primeru valjastih stebrov v kvadratni mreži. Pokažem, da že preprosti približki krožnega preseka stebra delujejo podobno kot prvotna struktura z le majhnim neskladjem v pasovnih strukturah. Prikazani koncept uporabim za konstrukcijo pikseliranega fotonskega kristala na osnovi rešetke Kagome s premaknjenimi položaji stebrov, ki podpira obstoj enosmernih svetlobnih stanj na mejah med topološko različnimi fotonskimi kristali. Nadalje enosmerna stanja prikažem tudi v pikseliranem fotonskem kristalu na osnovi rešetke Kagome s premaknjenimi položaji stebrov in tekočih kristalov, ki ponujajo možnost izdelave topološkega fotonskega kristala s premičnimi mejami. Kot četrto vejo raziskav preučim značilnosti preurejanja tekočih kristalov z negativno dvolomnostjo v bližini umbiličnih defektov ob prisotnosti zunanjega električnega polja. Na splošno delo raziskuje nove koncepte uporabe mehkih materialov za oblikovanje in nadzor svetlobe na mikroskopski ravni v realnem času

Numerical modeling of optical modes in topological soft matter

Vector and vortex laser beams are desired in many applications and are usually created by manipulating the laser output or by inserting optical components in the laser cavity. Distinctly, inserting liquid crystals into the laser cavity allows for extensive control over the emitted light due to their high susceptibility to external fields and birefringent nature. In this work we demonstrate diverse optical modes for lasing as enabled and stabilised by topological birefringent soft matter structures using numerical modelling. We show diverse structuring of light—with different 3D intensity and polarization profiles—as realised by topological soft matter structures in radial nematic droplet, in 2D nematic cavities of different geometry and including topological defects with different charges and winding numbers, in arbitrary varying birefringence fields with topological defects and in pixelated birefringent profiles. We use custom written FDFD code to calculate emergent electromagnetic eigenmodes. Control over lasing is of a particular interest aiming towards the creation of general intensity, polarization and topologically shaped laser beams

Controllable shifting, steering, and expanding of light beam based on multi-layer liquid-crystal cells

Shaping and steering of light beams is essential in many modern applications, ranging from optical tweezers, camera lenses, vision correction to 3D displays. However, current realisations require increasingly greater tunability and aim for lesser specificity for use in diverse applications. Here, we demonstrate tunable light beam control based on multi-layer liquid-crystal cells and external electric field, capable of extended beam shifting, steering, and expanding, using a combination of theory and full numerical modelling, both for liquid crystal orientations and the transmitted light. Specifically, by exploiting three different function-specific and tunable birefringent nematic layers, we show an effective liquid-crystal beam control device, capable of precise control of outgoing light propagation, with possible application in projectors or automotive headlamps

Spotting plants' microfilament morphologies and nanostructures

The tracheary system of plant leaves is composed of a cellulose skeleton with diverse hierarchical structures. It is built of polygonally bent helical microfilaments of cellulose-based nanostructures coated by different layers, which provide them high compression resistance, elasticity, and roughness. Their function includes the transport of water and nutrients from the roots to the leaves. Unveiling details about local interactions of tracheary elements with surrounding material, which varies between plants due to adaptation to different environments, is crucial for understanding ascending fluid transport and for tracheary mechanical strength relevant to potential applications. Here we show that plant tracheary microfilaments, collected from Agapanthus africanus and Ornithogalum thyrsoides leaves, have different surface morphologies, revealed by nematic liquid crystal droplets. This results in diverse interactions among microfilaments and with the environment; the differences translate to diverse mechanical properties of entangled microfilaments and their potential applications. The presented study also introduces routes for accurate characterization of plants' microfilaments.info:eu-repo/semantics/publishedVersio