Ultraviolet lasing in cholesteric liquid crystals

We report the observation of stimulated emission and mirrorless lasing in pure cholesteric liquid crystals. The lasing action is attributed to the combination of the fluorescence and the distributed feedback that are due to the inherent periodic structure of the liquid crystal. If the reflection band matches the intrinsic emission of the cholesteric liquid crystal, the crystal becomes a natural laser material, which will self-lase, without any optical elements or the addition of dyes, under picosecond excitation at 355 nm. Samples have been made to lase at different wavelengths in the near UV by shifting of the edge of the reflection band in the range of 385-405 nm. Typical linewidths observed are of the order of 0.5 nm.     

Dependence of lasing threshold power on excitation wavelength in dye-doped cholesteric liquid crystals

Relationship between threshold power and excitation wavelength has been investigated in distributed feedback (DFB) lasers using dye-doped cholesteric liquid crystals (CLCs). We found that the threshold shows strong dependence on the excitation wavelength. The excitation wavelength was varied over the whole absorption band of a commercial DCM dye, resulting in almost one-order of magnitude lower threshold power by the excitation at the higher energy tail than at the lower energy tail of the absorption band. We confirmed the existence of an optimum wavelength giving the lowest threshold power due to the competition between this effect and decreasing absorption at the higer-energy tail of the absorption band using a different dye, pyromethene 580.     

Dye-concentration-dependent lasing behaviors and spectral characteristics of cholesteric liquid crystals

The laser behavior and spectral changes occurring in cholesteric liquid crystals with varying dye-doped concentrations were investigated when pumped at 532 nm. It was found that the long-wavelength band edge and the laser line exhibit a blue shift over 21 nm with increasing dye concentration. The circularly polarized fluorescence spectra were examined, and the location of the sense reversion of circular polarization was determined to coincide well with the discrete lasing lines. The blue shift can be ascribed to the decrease in average refractive index and pitch of the dye-doped cholesteric liquid crystals. The dependence of the slope efficiency and threshold energy on the dye concentration can be attributed to the shift in photonic stopband and the change in penetration depth of excitation. The temperature and incident angle of pumping beam also have a significant impact on the lasing properties. The optimal dye concentration is found to be 0.5 wt% at 30.5 °C with an incident angle of 10°. The laser emission located at 601.4 nm with slope efficiency of 4 % was achieved above the threshold energy of 14.3 μJ.     

Low-threshold lasing at the edge of a photonic stop band in cholesteric liquid crystals

Low-threshold lasing is observed at the edge of the stop band of a one-dimensional structure-a dye-doped cholesteric liquid-crystal film. The mode closest to the edge has the lowest lasing threshold. The rates of spontaneous and stimulated emission are suppressed within the stop band and enhanced at the band edge. The ratio of right to left circularly polarized spontaneous emission is in good agreement with calculated density of photon states.     

Permeation in cholesteric liquid crystals

It is shown that permeation in cholesteric liquid crystals can be derived quite generally from Leslie's linearized equations for the conservation of linear and angular momentum.     

Continuous wave mirrorless lasing in cholesteric liquid crystals with a pitch gradient across the cell gap

Despite numerous efforts, continuous wave (CW) lasing in dye doped, one-dimensional (1D) photonic bandgap cholesteric liquid crystal (CLC) structures has not been previously reported, to our knowledge. Here we report on the observation of lasing in such structures under both coherent (laser) and incoherent (LED) CW light excitation. To achieve this effect, we used a 1D-photonic bandgap structure made of a polymer stabilized CLC with a pitch gradient across the cell thickness. A spectral reflectivity profile of such a CLC structure reveals local minima in the area within a photonic stopband and close to it. The realization of lasing pumped by low power CW light sources opens the possibility of all-organic, compact, tunable CW lasers for display and medical applications.     

Local-field anisotropy in cholesteric liquid crystals

The first experimental values of the Lorentz tensor components L _j for the cholesteric and smectic A phases of derivatives of cholesterol have been obtained using the dispersion of the refractive indices in the visible range. The temperature dependence of the components L _j has been determined; it is invariant with respect to the cholesteric-smectic A phase transition. The effect of the isotropization of the Lorentz and local field tensors with decreasing anisotropy of the molecular polarizability has been revealed.     

Electromechanical and light tunable cholesteric liquid crystals

A light-responsive cholesteric liquid crystal (CLC) mixture is tuned with light and applied DC electric field. Interestingly, the reflection of the CLC is red shifted with UV light and blue shifted with applied DC electric fields. UV light exposure induces a red shift in the reflection of the CLC bandgap by as much as 800 nm from the original spectral position. Spatial variations in pitch (and thus reflection color) are shown to blue shift with applied DC field regardless of the photohistory and restore the notch position or image upon removal of the field. The ability to tune the reflection of the CLC bandgap on demand to the red or blue with multiple stimuli is a never before demonstrated effect that could have potential utility in lasing, optical filtering, or data communication applications.     

Permeative flows in cholesteric liquid crystals

We use lattice Boltzmann simulations to solve the Beris-Edwards equations of motion for a cholesteric liquid crystal subjected to Poiseuille flow along the direction of the helical axis (permeative flow). The results allow us to clarify and extend the approximate analytic treatments currently available. We find that if the cholesteric helix is pinned at the boundaries there is an enormous viscosity increase. If, instead, the helix is free the velocity profile is flattened, but the viscosity is essentially unchanged. We highlight the importance of secondary flows, and, for higher flow velocities, we identify a flow-induced double twist structure in the director field--reminiscent of the texture characteristic of blue phases.     

Comments on thermomechanical coupling in cholesteric liquid crystals

The Onsager reciprocal relations give two more equalities between Leslie's coefficients. It suggests a simple experiment which could check Leslie's predictions.     

Constraints,dissipation functions and cholesteric liquid crystals

Our recently introduced dissipation function theory of molecular liquids and solids is extended to include constrained motions. Lagrangian multipliers are used in the reversible parts of the field equations of motion. Conservation laws of mass, momentum, energy and angular momentum are derived (in both the material and spatial frames) and shown to hold under the same general conditions as in the unconstrained case. Our theory is then applied to liquid crystals. Within our formalism, constitutive relations characterizing linear and nonlinear liquid crystals are given. In the linear regime, for cholesterics, depending on whether the heat flux or the temperature gradient is used in the dissipation function we show that different reciprocal relations follow. Our results for incompressible nematics and cholesterics are compared with the Ericksen-Leslie theory and others.     

Structures and thermoelectric convection in cholesteric liquid crystals

The possibilities for excitation of electromagnetic field structures and convection cells, i.e., temperature and velocity structures, in a thermotropic cholesteric liquid crystal in the presence of flow are studied. Estimates are made and possible experiments for observing such structures are discussed. A special thermoelectric effect is investigated as the cause of these excitations — the influence of a heating-induced change in the pitch of the cholesteric helix of the molecules on the permittivity and the electric conductivity of the material. Fiz. Tverd. Tela (St. Petersburg) 41, 165–170 (January 1999)     

Mapping ultrasonic fields with cholesteric liquid crystals

The termperature-sensitive light-scattering properties of cholesteric liquid crystals have been applied to the mapping of the intensity distribution in the near-field of ultrasonic transducers.     

Reflection properties of distorted cholesteric liquid crystals

The optical reflection properties of distorted cholesteric liquid crystals are theoretically investigated. It is shown that higher-order reflection bands occur with distorted helical structures even for light propagation parallel to the twist axis. For aperiodic structures, the positions of the main reflection bands differ from that of periodic ones. Ferroelectric helical structures could be detected by reflection spectroscopy when the liquid crystal is deformed by an electric field.     

A spin model for cholesteric liquid crystals

A spin model for cholestric liquid crystals is presented. The free energy is obtained in the mean field and from that the director correlation functions. It is found that one of the correlation functions diverges as (k₃² + bk_⊥⁴)^?1 where k₃ is the component of the wave-number parallel to the pitch axis, x_⊥ the component perpendicular to the pitch axis and b is a constant. This divergence persists to all orders in the small parameter q₀a where a is a molecular length and q₀ is 2π over the pitch. This divergence implies that the cholesteric state is unstable with respect to fluctuations.