A cholesteric liquid crystal (CLC) formed by chiral molecules represents a self-assembled one-dimensionally periodic helical structure with pitch p in the submicrometer and micrometer range. Because of the spatial periodicity of the dielectric permittivity, a CLC doped with a fluorescent dye and pumped optically is capable of mirrorless lasing. An attractive feature of a CLC laser is that the pitch p and thus the wavelength of lasing λ can be tuned, for example, by chemical composition. However, the most desired mode to tune the laser, by an electric field, has so far been elusive. Here we present the realization of an electrically tunable laser with λ spanning an extraordinarily broad range (>100 nm) of the visible spectrum. The effect is achieved by using an electric-fieldinduced oblique helicoidal (OH) state in which the molecules form an acute angle with the helicoidal axis rather than align perpendicularly to it as in a field-free CLC. The principal advantage of the electrically controlled CLC OH laser is that the electric field is applied parallel to the helical axis and thus changes the pitch but preserves the single-harmonic structure. The preserved single-harmonic structure ensures efficiency of lasing in the entire tunable range of emission. The broad tuning range of CLC OH lasers, coupled with their microscopic size and narrow line widths, may enable new applications in areas such as diagnostics, sensing, microscopy, displays, and holography.cholesteric liquid crystals | lasing | electric tunability | heliconical structure C hiral interactions in cholesteric liquid crystals (CLCs) result in supramolecular helical structures (1). The nematic directorn defining molecular orientation is perpendicular to the helix axis and rotates continuously along the axis, generating a regular rightangle helix with pitch p. Because the dielectric tensor is periodic in space, the CLC is a 1D photonic bandgap structure, selectively reflecting circularly polarized light of the same handedness as the helix. The reflection band edges are at wavelengths λ o = pn o and λ e = pn e , where n o and n e are, respectively, the ordinary and extraordinary refractive indices of the local uniaxial structure (1).In optically pumped thin layers of CLCs doped with fluorescent dyes, selective Bragg reflection of light gives rise to mirrorless lasing at the reflection band edges. Early demonstrations of lasing from CLCs (2-6) stimulated considerable interest in these easily produced micrometer-sized laser light sources due to their potential in spectroscopic, communications, sensing, and display applications. One major appeal of CLC lasers is the tunability of the emission wavelength by controlling the pitch. This can be achieved by changing the temperature (6-8), composition or cell thickness (9-11), mechanical strain in cross-linked CLC elastomers (12-14), and using reversible photochemical reactions (15, 16). The most sought-after control is via an applied electric field, which, although possible in principle (17-23), has not yet achiev...

We demonstrate a giant flexoelectro-optic behavior of liquid crystal dimer CB7CB. Flexoelectric properties of CB7CB experimentally characterized by measured angle of an in-plane rotation of helical axis (HA) in polymer stabilized uniform lying helix cholesteric liquid crystal. The 45° rotation of HA providing full intensity modulation of transmitted through a pair of crossed polarizers light, is achieved with 4.5 V/μm with a sub-millisecond electro-optic switching time. Reported properties enable application of CB7CB in applications of the flexoelectric effect in fast switching photonic and electro-optic devices.

We report wide range and reversible tuning of the selective reflection band of a single crystal cholesteric liquid crystal elastomer (CLCE). The tuning is the result of mechanical shortening of the helical pitch achieved by imposing a uniform uniaxial strain along the helical axis. On doping the CLCE sample with a laser dye, we observe lasing from the CLCE in both glassy and rubbery states. By changing the cholesteric pitch, mechanical compression provides tuning of the laser emission from the dye doped CLCE over a significant part of the fluorescence band of the laser dye. In this work we demonstrate for the first time that both the CLCE pitch and the lasing wavelength are linearly dependent on the strain imposed on the CLCE film.

We explore flexoelectro-optic effect in polymer stabilized cholesteric liquid crystals (CLC) with uniform lying helix texture. A method to enlarge the flexoelectric response of the CLC mixture is demonstrated by mixing two nematic liquid crystal host materials with large and small effective flexoelastic coefficients. By a choice of material composition of CLC mixture, a large angle of flexoelectrically driven in-plane rotation of helical axis with a submillisecond response time is obtained. Our findings will be useful for material design of future CLC mixtures for applications in flexoelectro-optic displays.