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| Accueil>La recherche>nMat>cristaux liquides>cholestériques | ||||||||||||||
PHOTONIC BAND STRUCTURES
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Research
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Our current research activities deal with cholesteric and chiral nematic liquid crystals (lcs): low molar mass lcs, glassy mesomorphic oligomers, crosslinked mesomorphic polymers with a peculiar attention devoted to Polymer-Stabilized LCs – also called anisotropic gels, for which a polymer network is created in the lc volume by UV light induced polymerization. In relation with the experimental conditions, the relevant scale of the structure and properties of the final material is the nanometer or micrometer scale. Polymer-Stabilized LCs are also called LC nanocomposites or microcomposites. |
We study the relation between the conditions of the material design – formulation of mixtures, UV light curing, thermal history, etc. –, the optical and electro-optical properties – Bragg light reflections – and the structure as investigated by transmission electron microscopy, scanning electron microscopy or near-field microscopies. |
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What is well known by the Liquid Crystal Physicist: the selective light reflection in cholesteric liquid crystals |
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| The cholesteric lc structure. Cholesteric liquid-crystalline states of matter are widely present in nature: atherosclerosis, arthropods cuticles, condensed phases of DNA, plant cell walls, human compact bone osteon, chiral biopolymers, etc. The self organized helical structure produces unique optical properties. The elongated chiral molecules of a cholesteric lc exhibit a long-range orientational order which presents a twisted arrangement in a direction perpendicular to their long axis; in a parallel direction, the structure is similar to the nematic lc one, i.e.: the molecules are on average parallel together without any positional order. Therefore the director of the molecules rotates around a helical axis and the distance over which it rotates 360° is called the pitch. |
 Light reflection from the cholesteric helical structure (by permission from Nature 391, 745-746, copyright 1998 Macmillan Publishers Ltd). |
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| Property of selective light reflection by a cholesteric lc. Cholesteric lcs exhibit many remarkable properties due to the existence of the macroscopic helical structure. The most striking features are the exceptionnally large optical rotatory power and the iridescent colours due to the selective reflection displayed by a uniformly oriented Grandjean planar texture, when the helix axis is perpendicular to the observation plane. At normal incidence, the mean reflection wavelength λ0 is related to the pitch p and the mean refraction index n by: λ0 = np n is the average of the ordinary (n0) and the extraordinary of (ne) refractive indices of the locally uniaxial structure : n = (n0 + ne)/2. The spectral width of the reflection band Δλ is defined as: Δλ = pΔn where Δn = ne - n0 is the birefringence. Within Δλ, an incident unpolarized or linearly polarized light beam parallel to the helix axis is split into two opposite circularly polarized components, one of which is transmitted whereas the other is reflected. The sense of rotation of the latter one agrees with the helix screw sense. A wavelength out of Δλ is simply transmitted.  |
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Compared to conventional pigmented color filters, a clc slab has the special feature to combine several optical properties in one layer: it is not only a filter but also a reflector and a polarizer. Since Δn is limited to 0.3 – 0.4 for colourless organic compounds: the bandwidth Δλ is limited to a few tens of nm in the visible spectrum (see below the related contribution n°I from CEMES on this matter). Therefore the reflected light is circularly polarized with the same handedness as the |
incident light, which is the exact opposite of a normal dielectric mirror that changes the handedness.For example, right-handed circularly polarized incident light is reflected by a right-handed helix and, in contrast, if left-handed circularly polarized, the incident light is transmitted without significant loss through the medium. Consequently, as only circularly polarized light with one handedness is reflected, the reflected intensity of ambient unpolarized light from a CLC is never greater than 50% (see below the related contribution n°II from CEMES on this matter). |
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| I.
Why and how to broaden the wavelength bandwidth in clcs ? |
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| However huge the choice is for pitch values, Δn is typically limited to 0.3-0.4 for colourless organic materials. As a consequence, Δλ is commonly less than 100 nm in the visible spectrum and more often around 50 nm. Whereas the selectivity is desirable for several families of applications (narrow-band optical filters, thermal imaging, laser or paint technologies), a drawback lies in the fact that known cholesteric filters have a too limited reflection bandwidth for specific purposes like full-colour or white-or-black polariser-free reflective displays, broadband circular polarisers or smart windows in buildings for which the solar control would be required over a broad wavelength band. From fundamental considerations, it is thus challenging to increase the reflection bandwidth by finding novel helical structures, with a non-monotonous spatial distribution of the periodicity, and to investigate how the optical response of the material has to be revised. Below is a selected number of results from CEMES on the subject
of broadband reflective cholesteric structures:
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In addition, the bandgap filters can be switched between broadband
reflective, scattering and transparent states by subjecting them
to an electric field. |
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| II.
How to go beyond the 50% limit of reflectance of cholesteric lcs
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| At CEMES, we found the elaboration procedure of a cholesteric
gel whose the optical characteristics go beyond the 50% reflectance
limit. Photopolymerizable monomers are introduced in the volume
of a clc exhibiting a thermally induced helicity inversion and
the blend is then cured with UV light when the helix is right-handed.
The reflectance exceeds 50% when measured at the temperature assigned
at a cholesteric helix with the same pitch but a left-handed sense
before reaction. The reflection properties are investigated in
the infrared region. From scanning electron microscopy investigations,
we have shown that the organization of the mesophase is transferred
onto the structure of the network. The gel structure was discussed
as consisting of a polymer network with a helical structure containing
two populations of low molar mass LC molecules. Each of them was
characterized by a band of circularly polarized light which is
selectively reflected. The monitoring of the optical response
with the temperature offers the opportunity to discriminate the
respective contributions of bound and free fraction of lc molecules
to the reflectance and to give evidence of the progressive increase
of the reflected flux when the temperature decreases from the
curing temperature. Novel opportunities to modulate the reflection
over the whole range are offered. Potential applications are related
to the light management for smart windows or reflective polarizer-free
displays with a larger scale of reflectivity levels. |
 Scanning electron microscopy image of the polymer network formed in the cholesteric phase. The helical structure has been transferred onto the polymer morphology. The polymer fibrils draw stacked rows of parallel arcs which appear when the cut direction is oblique to the helical axis; these arcs are enhanced in white on the left part of the picture. Such arced patterns are also visible in thin sections of biological materials: crustacean integuments, dermal plates of fishes, insect cuticles, collageneous matrices of bone tissue, plant cell walls and chromatin organizations for DNA. The case of Plusiotis Resplendens |
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