| 1. |
- Hegde, Gurumurthy, 1979, et al.
(författare)
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Dual Frequency Addressable Optical Device
- 2005
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Ingår i: Journal of Applied Physics. - 1089-7550. ; 97:9
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Tidskriftsartikel (refereegranskat)abstract
- We propose a photonic switch employing a liquid-crystalline material. The material exhibits a change in the sign of the dielectric anisotropy switching from a positive to a negative value at a certain crossover frequency. By application of an electric field this phenomenon can be used to alter the orientation of the sample between two orthogonal directions leading to a large change in the optical transmission characteristics of the medium. Here we demonstrate that this feature can be realized by an unpolarized ultraviolet UV beam, owing to the photoisomerization of the constituent azobenzene molecules. Possible usage of this for optically driven display devices and image-storage applications are suggested.
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| 2. |
- Hegde, Gurumurthy, 1979, et al.
(författare)
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Dynamic Self-Assembly of the Liquid Crystalline Smectic-A Phase
- 2005
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Ingår i: Advanced Materials. - 1521-4095. ; 17
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Tidskriftsartikel (refereegranskat)abstract
- A photoinduced transition in an azobenzene-containing liquid crystal leads to a phase that does not exist in the thermal cycle. The smectic A phase is induced and stabilized only in the presence of UV light, producing fan-shaped focal conic textures typical of the SmA phase, as observed by polarized-light microscopy (see Figure). This dynamic self-assembly is explained by photoinduced nanophase segregation and a frustrated spin-gas model.
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| 3. |
- Hegde, Gurumurthy, 1979, et al.
(författare)
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Evidence of Worm-like micellar behavior in Chromonic liquid Crystals: Rheological, X-ray
- 2007
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Ingår i: J.Phys.Chem.B. ; 111:33
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Tidskriftsartikel (refereegranskat)abstract
- We report rheological, X-ray, and dielectric investigations on a chromonic liquid-crystalline system formed by aqueous solutions of a food coloring agent, Sunset Yellow, in the absence and upon addition of salt. The salt-concentration dependence of the steady-state viscosity at low shear rates has a non-monotonic variation and is qualitatively similar to the behavior seen in wormlike micellar systems, a surprising result since chromonic systems are expected to be non-micellar in character. More interestingly, for a particular low concentration of the salt (20 mM), the viscosity increases by 3 orders of magnitude in comparison with that of the pure chromonic material. The dynamic (oscillatory) rheological data bring out features which can be described in terms of a microstructure formation. X-ray and dielectric studies show that certain characters of the aggregates formed by the Sunset Yellow molecules are not altered by the addition of salt.
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| 4. |
- Hegde, Gurumurthy, 1979, et al.
(författare)
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Influence of a long-chain alkane on the photoinduced nematic-isotropic transition
- 2004
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Ingår i: Physical Review E. - 1550-7998. ; 69:2
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the phenomenon of photoinduced nematic-isotropic (N-I) transition for different concentrations of a long-chain alkane, viz., octadecane ~OD!, and a host liquid crystalline material having a small quantity ~4.5 mol%! of a photoactive azo compound. As expected, the N-I transition temperature diminishes with increasing concentration of OD. Data of time-resolved measurements of the dielectric constant through both the UV-activated trans-cis transformation and the thermal back relaxation process are presented. With increasing concentration of OD, although the response time for the former process increases slightly by less than a factor of 2, the thermal back relaxation shows a greater increase by more than an order of magnitude. As a possible explanation for this surprising result, we discuss an argument based on the structural incompatibility of the constituents of the system investigated.
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| 5. |
- Hegde, Gurumurthy, 1979, et al.
(författare)
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Nonequilibrium Liquid Crystalline Layered Phase Stabilized by Light
- 2007
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Ingår i: J.Phys.Chem.B. ; 111:2
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Tidskriftsartikel (refereegranskat)abstract
- The ability of light to alter/stabilize a particular thermodynamic phase is a power tool to investigate condensed matter from a new dimension. This field of photoinduced phase transitions is currently an important area of research. Being elastically soft and having subtle changes between its many phases a liquid crystal material is an attractive medium to investigate such light-driven phase transitions. The attraction is partly due to the large birefringence changes accompanying these transitions that are useful in developing photonic devices. In all of the cases reported to date, the photoinduced transition always leads to a phase that in any case exists in the thermal cycle. Recently we reported the first exception to such an established phenomenon (AdV. Mater. 2005, 17, 2086). The guest-host ternary mixture consisting of the photoactive azobenzene guest molecules does not exhibit smectic A phase in the absence of UV radiation. However, the smectic A phase is induced and stabilized only in the presence of UV light. In this paper, we map out hitherto unexplored temperature versus UV intensity phase diagrams for various mixtures, which illustrate that light mimics, in a limited sense, the role of a thermodynamic parameter such as, for example, pressure. The threshold UV intensity required to photodrive the appearance of the smectic A phase is seen to have a strong concentration dependence. Our studies also suggest the possibility of observing a double critical point by employing the UV intensity as a fine-tuning parameter.
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| 6. |
- Hegde, Gurumurthy, 1979, et al.
(författare)
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Photo Induced Effects in Nematic Liquid Crystals
- 2005
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Ingår i: Phase Transitions. - 1029-0338. ; 78:6
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Forskningsöversikt (refereegranskat)abstract
- Temperature, concentration of the solvent and pressure are the parameters that are well known to bring about phase transitions in liquid-crystalline systems. In recent years a new parameter has been added to this list: light. The principle behind these photoinduced transitions is the light-driven shape transformation of certain photoactive materials like, e.g., azobenzene. In this article, we present results of various aspects of our recent investigations on such photoinduced transitions in the nematic phase and highlight the feature that light is a new tool to study phase transitions and the associated critical phenomena
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| 7. |
- Hegde, Gurumurthy, 1979, et al.
(författare)
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Photoinduced Effects in the Vicinity of the Smectic-A-Smectic-CA* Transition; Polarization
- 2006
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Ingår i: Physical Review E. - 1550-7998. ; 73:1
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Tidskriftsartikel (refereegranskat)abstract
- We report detailed measurements of the photoinduced effects on the electric polarization, tilt angle, response time, and rotational viscosity in the vicinity of the smectic-A–antiferroelectric-smectic-C Sm-CA * transition of a guest-host system consisting of photoactive azobenzene-based guest molecules and nonphotoactive host molecules. In the Sm-CA * phase all the parameters, except the tilt angle, exhibit both the primary and secondary photoferroelectric effects. The tilt angle dependence of the polarization in the absence of light and in light-on conditions have been analyzed in terms of the predictions of the generalized mean-field and microscopic models.
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| 8. |
- Hegde, Gurumurthy, 1979, et al.
(författare)
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Polymer network as a template for control of photoconductivity
- 2004
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Ingår i: Liquid Crystals. - 1366-5855. ; 31:9
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Tidskriftsartikel (refereegranskat)abstract
- We report a novel method of confining a photoconducting liquid crystalline material using a polymer templating approach. The attractive feature of this approach is that the magnitude of the photocurrent of the photoconducting material does not diminish, i.e. it is unaltered by the polymer matrix. The results are compared with another method of encapsulation that was recently reported and wherein the photoconductivity decreases upon having the photoconducting material in the polymer matrix. The difference in the behaviour between the two methods is explained using a nanophase segregation model. The method described is particularly suitable for creating patterned photoconductors.
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