Search

The tunability of individual coupling amplitudes in photonic lattices is highly desirable for photonic Hamiltonian engineering and for studying topological transitions in situ. In this work, we demonstrate the tunneling control between individual lattice sites patterned inside an optical microcavity. The tuning is achieved by applying a voltage to a liquid crystal microcavity possessing photonic Rashba-Dresselhaus spin-orbit coupling. This type of spin-orbit coupling emerges due to the high birefringence of the liquid crystal material and constitutes an artificial gauge field for photons. The proposed technique can be combined with strong-light matter coupling and non-Hermitian physics already established in liquid crystal microcavities.

We create a one-dimensional photonic crystal with strong polarization dependence and tunable by an applied electric field. We accomplish this in a planar microcavity by embedding a cholesteric liquid crystal (LC), which spontaneously forms a uniform lying helix (ULH). The applied voltage controls the orientation of the LC molecules and, consequently, the strength of a polarization-dependent periodic potential. It leads to opening or closing of photonic band gaps in the dispersion of the massive photons in the microcavity. In addition, when the ULH structure possesses a molecular tilt, it induces a spin-orbit coupling between the lattice bands of different parity. This interband spin-orbit coupling (ISOC) is analogous to optical activity and can be treated as a synthetic non-Abelian gauge potential. Finally, we show that doping the LC with dyes allows us to achieve lasing that inherits all the above-mentioned tunable properties of LC microcavity, including dual and circularly-polarized lasing.

In Bose-Einstein condensates, spin-orbit coupling produces supersolidity. It is a peculiar state of matter, which, in addition to the superfluid behaviour of weakly interacting Bose condensates, shows a periodic modulation of its density typical for crystals and called stripe phase. Here, we report the fabrication of a new type of samples allowing to achieve room-temperature supersolidity for a quantum fluid of light. The structure is an optical microcavity filled with a nematic liquid crystal (LC) sandwiched between two layers of the organic polymer MeLPPP. We demonstrate the formation of cavity exciton-polaritons in the presence of Rashba-Dresselhaus spin-orbit coupling (RDSOC), which is tuned by external voltage controlling the LC birefringence. In the RDSOC regime, we demonstrate exciton-polariton condensation in the two distinct degenerate minima of the dispersion. The condensate real space distribution shows both polarization and density stripes, which stem from the interference between phase-coherent condensate components characterized by different wavevectors and polarizations. The possibilities offered by this platform to tune the particle dispersion and to perform full state tomography, including time-resolved, open wide perspectives for detailed future studies of the static and dynamical behaviour of supersolids and of quantum fluids in presence of SOC and topologically non-trivial bands.

Twist-bend (Ntb) and ferroelectric (NF) nematic liquid crystals exhibit several novel effects and new physical properties. The question of what happens in binary mixtures is interesting as a matter of curiosity and pure science. Here, we report experimental studies on the phase diagram and some physical properties of binary mixtures of the above-mentioned nematic liquid crystals. Both N-Ntb and N-NF phase transition temperatures and the corresponding enthalpies decrease significantly and eventually, these transitions disappear in some intermediate compositions. Temperature dependent birefringence above the N-Ntb phase transition temperature shows critical behaviour, and the critical range of the tilt fluctuations becomes wider in the mixtures. The magnitudes and the temperature dependence of the splay elastic constant of the mixtures' high-temperature nematic (N) phase strikingly differ from that of the pristine twist-bend and ferroelectric nematic liquid crystals. The study shows that Ntb and NF liquid crystals are incompatible.

The recent discovery of nematic liquid crystals with polar order (ferroelectric nematic) has created immense interest. Despite intensive research, several physical properties of this liquid crystal are yet to be investigated and understood. Here, we report experimental studies on the birefringence and exoelectric coefficient of a polar nematic (NF) liquid crystal. Our experiments directly reveal that the splay fluctuations influence the birefringence several degrees above the nonpolar (N) to polar-nematic (NF) phase transition temperature and the heat capacity exponent obtained from the tilt angle fluctuations is close to the value reported in adiabatic scanning calorimetry measurements. The flexoelectric coefficient of the nonpolar nematic phase shows a power-law dependence on temperature. Our results demonstrate a strong coupling of splay fluctuations with electric polarization that reduces the splay elastic constant and consequently enhances the flexoelectric coefficient. These results are important for forthcoming applications as well as for understanding all pretransitional effects in ferroelectric nematic liquid crystals., Comment: 5 pages and 4 figures

The electroviscous effect deals with the change in the viscosity of fluids due to an external electric field. Here, we report experimental studies on the electroviscous effects in a ferroelectric nematic liquid crystal. It was synthesised accomplishing a new synthetic route which provides higher yield than conventional one. We measure electric field-dependent viscosity under a steady shear at different temperatures. In the low field range, the increase in viscosity ($\Delta\eta=\eta(E)-\eta_0$) is proportional to $E^2$ and the corresponding viscoelectric coefficient ($f\approx10^{-9}$m\textsuperscript{2}/V\textsuperscript{2}) of the ferroelectric nematic is 2 orders of magnitude larger than the apolar nematic liquid crystals and largest ever measured for a fluid. The apparent viscosity measured under a high electric field shows a power-law divergence $\eta\sim(T-T_c)^{-0.7\pm0.05}$, followed by nearly an order of magnitude drop below the N-N\textsubscript{F} phase transition. Experimental results within the dynamical scaling approximation demonstrate rapid growth of polar domains under a strong electric field as the N-N\textsubscript{F} phase transition is approached. The gigantic electroviscous effects demonstrated here are important for emerging applications and understanding striking electrohydromechanical effects in ferroelectric nematic liquid crystals., Comment: 5 figures

In the presence of Rashba-Dresselhaus coupling, strong spin-orbit interactions in liquid crystal optical cavities result in a distinctive spin-split entangled dispersion. Spin coherence between such modes give rise to an optical persistent-spin-helix. In this letter, we introduce optical gain in such a system, by dispersing a molecular dye in a liquid-crystal microcavity. We demonstrate both lasing in the Rashba-Dresselhaus regime and the emergence of an optical persistent spin helix.

We study the flow behaviour of a twist-bend nematic $(N_{TB})$ liquid crystal. It shows three distinct shear stress ($\sigma$) responses in a certain range of temperatures and shear rates ($\dot{\gamma}$). In Region-I, $\sigma\sim\sqrt{\dot{\gamma}}$, in region-II, the stress shows a plateau, characterised by a power law $\sigma\sim{\dot{\gamma}}^{\alpha}$, where $\alpha\sim0.1-0.4$ and in region-III, $\sigma\sim\dot{\gamma}$. With increasing shear rate, $\sigma$ changes continuously from region-I to II, whereas it changes discontinuously with a hysteresis from region-II to III. In the plateau (region-II), we observe a dynamic stress fluctuations, exhibiting regular, periodic and quasiperiodic oscillations under the application of steady shear. The observed spatiotemporal dynamics in our experiments are close to those were predicted theoretically in sheared nematogenic fluids., Comment: 8 pages, 9 figures

Spin-orbit interactions which couple spin of a particle with its momentum degrees of freedom lie at the center of spintronic applications. Of special interest in semiconductor physics are Rashba and Dresselhaus spin-orbit coupling (SOC). When equal in strength, the Rashba and Dresselhaus fields result in SU(2) spin rotation symmetry and emergence of the persistent spin helix (PSH) only investigated for charge carriers in semiconductor quantum wells. Recently, a synthetic Rashba-Dresselhaus Hamiltonian was shown to describe cavity photons confined in a microcavity filled with optically anisotropic liquid crystal. In this work, we present a purely optical realisation of two types of spin patterns corresponding to PSH and the Stern-Gerlach experiment in such a cavity. We show how the symmetry of the Hamiltonian results in spatial oscillations of the spin orientation of photons travelling in the plane of the cavity.

We demonstrate an application of retardation-modulation polarimetry in studies of nanocomposite materials. Molecular ordering is explored on both nonchiral and chiral liquid crystals (LCs) in the bulk state and embedded into parallel-arrays of cylindrical channels of alumina or silica membranes of different channel sizes (12-42 nm). Two arms polarimetry serves for simultaneous measurements of the birefringence retardation and optical activity characterizing, respectively, orientational molecular ordering and chiral structuring inside nanochannels., Comment: Conference article, 5 pages, 5 figures

The orientational and translational order of a thermotropic ferroelectric liquid crystal (2MBOCBC) imbibed in self-organized, parallel, cylindrical pores with radii of 10, 15, or 20 nm in anodic aluminium oxide monoliths (AAO) are explored by high-resolution linear and circular optical birefringence as well as neutron diffraction texture analysis. The results are compared to experiments on the bulk system. The native oxidic pore walls do not provide a stable smectogen wall anchoring. By contrast, a polymeric wall grafting enforcing planar molecular anchoring results in a thermal-history independent formation of smectic C* helices and a reversible chevron-like layer buckling. An enhancement of the optical rotatory power by up to one order of magnitude of the confined compared to the bulk liquid crystal is traced to the pretransitional formation of helical structures at the smectic-A*-to-smectic-C* transformation. A linear electro-optical birefringence effect evidences collective fluctuations in the molecular tilt vector direction along the confined helical superstructures, i.e. the Goldstone phason excitations typical of the para-to-ferroelectric transition. Their relaxation frequencies increase with the square of the inverse pore radii as characteristic of plane-wave excitations and are two orders of magnitude larger than in the bulk, evidencing an exceptionally fast electro-optical functionality of the liquid-crystalline-AAO nanohybrids., Comment: 14 pages, 13 figures, 2 ancillary files, as submitted, Nanoscale (in press)