Paintable band-edge liquid crystal lasers Gardiner, Damian J; Morris, Stephen M; Hands, Philip J W ...
Optics express,
2011-Jan-31, 2011-01-31, 20110131, Letnik:
19, Številka:
3
Journal Article
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In this paper we demonstrate photonic band-edge laser emission from emulsion-based polymer dispersed liquid crystals. The lasing medium consists of dye-doped chiral nematic droplets dispersed within ...a polymer matrix that spontaneously align as the film dries. Such lasers can be easily formed on single substrates with no alignment layers. The system combines the self-organizing periodic structure of chiral nematic liquid crystals with the simplicity of the emulsion procedure so as to produce a material that retains the emission characteristics of band-edge lasers yet can be readily coated. Sequential and stacked layers demonstrate the possibility of achieving simultaneous multi-wavelength laser output from glass, metallic, and flexible substrates.
► Liquid crystalline chromophores have been synthesized for photonic band-edge laser devices. ► These are bimesogenic compounds consisting of the repeating unit from the semiconducting polymer, ...CNPPV. ► Single mode laser emission was observed in chiral nematic samples doped with the chromophores up to a concentration of 10wt.%. ► The excitation threshold is found to increase with the concentration of the chromophore.
We present results on laser action from liquid crystal compounds whereby one sub-unit of the molecular structure consists of the cyano-substituted chromophore, {phenylene-bis (2-cyanopropene)}, similar to the basic unit of the semiconducting polymer structure poly(cyanoterephthalylidene). These compounds were found to exhibit nematic liquid crystal phases. In addition, by virtue of the liquid crystalline properties, the compounds were found to be highly miscible in wide temperature range commercial nematogen mixtures. When optically excited at λ=355nm, laser emission was observed in the blue/green region of the visible spectrum (480–530nm) and at larger concentrations by weight than is achievable using conventional laser dyes. Upon increasing the concentration of dye from 2 to 5wt.% the threshold was found to increase from Eth=0.42±0.02μJ/pulse (≈20mJ/cm2) to Eth=0.66±0.03μJ/pulse (≈34mJ/cm2). Laser emission was also observed at concentrations of 10wt.% but was less stable than that observed for lower concentrations of the chromophore.
We present experimental results on the bulk flexoelectric coefficients e and effective elastic coefficients K of non-symmetric bimesogenic liquid crystals when the number of terminal and lateral ...fluoro substituents is increased. These coefficients are of importance because the flexoelastic ratio e/K governs the magnitude of flexoelectro-optic switching in chiral nematic liquid crystals. The study is carried out for two different types of linkage in the flexible spacer chain that connects the separate mesogenic units: these are either an ether or an ester unit. It is found that increasing the number of fluorine atoms on the mesogenic units typically leads to a small increase in e and a decrease in K, resulting in an enhancement of e/K. The most dramatic increase in e/K, however, is observed when the linking group is changed from ether to ester units, which can largely be attributed to an increase in e. Increasing the number of fluorine atoms does, however, increase the viscoelastic ratio and therefore leads to a concomitant increase in the response time. This is observed for both types of linkage, although the ester-linked compounds exhibit smaller viscoelastic ratios compared with their ether-linked counterparts. Highly fluorinated ester-linked compounds are also found to exhibit lower transition temperatures and dielectric anisotropies. As a result, these compounds are promising materials for use in electro-optic devices.
It is known that bimesogenic liquid crystals exhibit a marked "odd-even" effect in the flexoelastic ratio (the effective flexoelectric coefficient to the average elastic coefficient), with the ratio ...being higher for the "odd-spaced" bimesogens (those with an odd number of alkyl groups in the spacer chain) than their neighboring even-spaced counterparts. To determine the contribution of each property to the flexoelastic ratio, we present experimental results on the flexoelectric and elastic coefficients of two homologous nonsymmetric bimesogens which possess odd and even alkyl spacers. Our results show that, although there are differences in the flexoelectric coefficients, there are substantially larger differences in the effective elastic coefficient. Specifically, the odd bimesogen is found to have both a low splay elastic coefficient and a very low bend elastic coefficient which, when combined, results in a significantly lower effective elastic coefficient and consequently a higher flexoelastic ratio.
•Light-scattering devices based on binary mixtures of smectic A and nematic LC.•Induced clear state requires less applied electric filed for high dielectric mixtures.•Pyrromethene 597 laser dye has ...been used as a gain medium for the random laser.•Dye’s life span and repeatable electro-optic behaviour has been enhanced.•This study demonstrates the random laser emission with an improved stability.
In this paper, we focus on the development of liquid crystal (LC) visible-light scattering devices for random lasers. These light-scattering devices are based upon binary mixtures that consist of an organosiloxane smectic A LC and a wide temperature range nematogen LC. Both the temperature range of the smectic A phase and the dielectric anisotropy of the binary mixture are increased compared with that of the neat organosiloxane compound. In the latter case, the increase in the dielectric anisotropy results in a reduction of the magnitude of the electric field required to induce a clear state. Furthermore, it is found that the electric field threshold continues to decrease with increasing concentration of the nematic compound. For the random laser devices, the Pyrromethene 597 laser dye was added to a mixture that was optimized for scattering and it was found that the absorption properties of the dye becomes unstable in the presence of the electro-hydrodynamic instabilities that are required to generate scattering in the LC cells. This is believed to be due to electro-chemical reactions that occur at the electrodes. To avoid dye degradation and ensure repeatable electro-optic behaviour, a reduction–oxidation (redox) couple is dispersed within the dye-doped binary mixture. It is shown that the addition of redox dopants helps to stabilize the dye in the scattering mixtures, and also increases the long-term repeatability of the scattering behaviour. Finally, we conclude by characterizing the random laser emission of the dye-doped binary mixture and demonstrate improved stability.
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