Centre d’Élaboration de Matériaux et d’Etudes Structurales (UPR 8011)


Accueil > Recherche > NeO : Nano-Optique et Nanomatériaux pour l’Optique > Cristaux liquides pour l’optique

Physics of complex twisted liquid crystal structures

Cholesteric liquid crystals offer a multitude of possibilities for shaping light, many of which are poorly understood or still unexplored. Shaping of ultrafast laser pulses (20 fs), optical materials whose function depends on the length scale for light-matter interactions, broadening of the reflection bandgap, solutions for the interlacing of opposite helicity senses when a single sense is the current situation, which leads to hyper-reflectors : this selection of our contributions is summarized in this section.

Cholesteric liquid crystals for ultrafast optics

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Chirped pulse amplification is a technique for amplifying an ultrashort laser pulse with the laser pulse being stretched out temporally and spectrally prior to amplification. Then, the amplified pulse is recompressed back to the original pulse width through reversal of the process of stretching. There are several ways to construct stretchers and compressors. Using cholesteric liquid crystals with a constant pitch or a pitch gradient is a solution that has just appeared in the research domain.

We present a novel statistical approach conducted on a large number of samples that reveals the existence of different groups (clusters) in the optical response. This quantitative approach highlights the possibility of stretching or compressing ultra-short pulses. We show that the profile of the Bragg band allows tuning the dispersion of 20 fs pulses. The negative or positive sign of the Group Velocity Dispersion (GVD) — leading to compression or stretching of the pulses — depends on the relative position of the pulse spectrum versus the Bragg band. A pitch-gradient cholesteric makes it possible to minimize the third order of the spectral phase that adds adverse temporal satellites to the pulse (pre-pulses and post-pulses). Novel and promising opportunities to shape ultrashort (sub-100 fs) pulses are offered.

Reference paper :

M. Neradovskiy, A. Scarangella, A. Jullien, and M. Mitov, Dispersion of 20 fs pulses through band edges of cholesteric liquid crystals, Optics Express, 27, 21794 (2019).

 

Funding : ANR (Agence Nationale de la Recherche, France), COLEOPTIX (Grant : ANR-17-CE30-0025).

 

Tilted cholesteric liquid crystal structures

Ongoing research on cholesteric liquid crystals takes advantage of the peculiar behavior of twisted structures subject to curvature. In tilted or oblique cholesterics, the orientation of the helix axis is not everywhere perpendicular to the film surfaces. The tilted structure may periodically repeat by giving rise to specific deformed twists, like in the polygonal texture. The tunability of color and polarization properties is made available, which is not possible in regular planar textures. Such a variability of optical properties is reached by changing the parameters of the material design, like the surface anchoring energy, without applying any external field.

By hyperspectral imaging we vizualize the transmission and reflection of tilted cholesteric structures with a spectral resolution of 6 nm over 400-1000 nm against a few tens of nm that would be achieved by the available techniques. A correlation between spectral shifts and spatial twists is thus made possible.

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(a) Polygonal textures observed by optical microscopy in transmission (unpolarized light) and reflection (crossed polarizers) modes. (b) 3D structure of the polygonal texture by combining different microscopy methods : atomic force microscopy (AFM), optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

 

Selection of papers :

  • A. Jullien, A. Scarangella, U. Bortolozzo, S. Residori and M. Mitov, Nanoscale hyperspectral imaging of tilted cholesteric liquid crystal structures, Soft Matter, 15, 3256-3263 (2019).
  • G. Agez, R. Bitar and M. Mitov, Color selectivity lent to a cholesteric liquid crystal by monitoring interface-induced deformations, Soft Matter, 7, 2841-2847 (2011).

 

Funding : ANR (Agence Nationale de la Recherche, France), COLEOPTIX (Grant : ANR-17-CE30-0025).

 

Chiral microlenses

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Here are multifunctional optical films whose function depends on the scale at which the interaction with light occurs. At the macroscopic scale, the progressive inclination of the helical axis with the annealing time provides a new scenario for adjusting the color of the cholesteric by the fine control of the surface tension. At the micrometer and nanometer scales, it appears that the polygonal texture of the film is a network of microlenses that focus and guide the light with a pattern (spot or ring) depending on the incident wavelength. The peculiar structure of this chiral lens, elucidated by means of complementary microscopies, is responsible for this behavior. We name it Bragg lens.

Related applications of chiral microlenses are in the field of wavelength-tunable chiro-optical devices and lab-on-a-chip optical systems that offer the combined benefits of multiple light manipulation capabilities, seamless integration, and mechanical stability.

 

Selection of papers :

  • C. Bayon, G. Agez and M. Mitov, Wavelength-tunable light shaping with cholesteric liquid crystal microlenses, Lab Chip,14, 2063-2071 (2014).
  • C. Bayon, G. Agez and M. Mitov, Size-effect of oligomeric cholesteric liquid-crystal microlenses on the optical specifications, Optics Lett., 40, 4763-4766 (2015).

 

 Highlighted in Nature Photonics :

 

 

Highlighted in Liquid Crystals Today :

 

 

 

Mechanical origin of the broadening the reflection bandgap in cholesteric gels

Cholesterics with a broadened bandgap — a few hundreds of nm against a tens of nm — address fundamental questions about the fine tuning of the mesoscopic structural chirality with the help of the physical parameters related to the design procedure. Potential applications are smart windows to regulate the solar light and heating or reflective polarizer-free displays operating in a low light ambiance.

In cholesteric liquid-crystalline gels or Polymer-Stabilized Cholesteric Liquid Crystals, the mechanical role of the polymer network over the structure of the whole gel has been ignored. We show that it is the stress gradient exerted by the network over the helical structure that drives the broadening of the optical band gap, as evidenced by the absence of a gradient in chiral species. Model calculations and finite-difference time-domain simulations show that the network acts as a spring with a stiffness gradient. The present results indicate a revision to the common understanding of the physical properties of liquid-crystalline gels is necessary when a concentration gradient in a polymer network is present.

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Model of coil spring with a gradient of stiffness as inspired by cross-sectional views of the polymer network elucidated by TEM. Left column : The network appears dark (respectively, light grey). (a) The network concentration is homogeneously distributed in the bulk (symmetrical irradiation conditions). (b) A network concentration gradient is evident (asymmetrical conditions). (c) In this peculiar case, the network detached from one side of the cell and relaxed into the volume of the liquid crystal (asymmetrical conditions). A small fraction of polymer is visible close to the left substrate. Right column : Model composed of N subsprings placed end to end, which represent the helical network. (a) All the subsprings have the same spring constant ki and the same length. (b) The more concentrated the network, the larger the constant of the local subspring becomes. Due to the network that models the liquid crystal structure, the pitch of the gel is proportionally distorted. (c) The subsprings relaxed and attained their unstressed configuration.

Reference paper : G. Agez, S. Relaix and M. Mitov, Cholesteric liquid crystal gels with a graded mechanical stress, Phys. Rev. E, 89, 022513 (2014).

 

Double helicity cholesteric liquid crystals

A cholesteric liquid crystal selectively reflects light because of its helical structure. The reflectance is equal to at most 50% for unpolarized incident light, which is a consequence of the polarization-selectivity rule. These limits must be exceeded for some innovative applications like hyper-reflectors or in stealth technology. Fundamentally the topic of double helicity cholesterics addresses the quest for solutions to efficiently imbricate inverse helices (generating double-handed circularly polarized reflection bands) inside defect-free and single-layer structures, and to obtain smart materials with field-switchable physical properties.

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Selection of papers :

  • M. Mitov, Cholesteric Liquid Crystals with a Broad Light Reflection Band, Adv. Mater., 24, 6260-6276 (2012).
  • M. Mitov and N. Dessaud, Going beyond the reflectance limit of cholesteric liquid crystals, Nature Materials, 5, 361-364 (2006).
  • A. C. Tasolamprou, M. Mitov, D. C. Zografopoulos and E. E. Kriezis, Theoretical and experimental studies of hyperreflective polymer-network cholesteric liquid crystal structures with helicity inversion, Optics Commun., 282, 903-907 (2009).
  • G. Agez and M. Mitov, Cholesteric Liquid Crystalline Materials with a Dual Circularly Polarized Light Reflection Band Fixed at Room Temperature, J. Phys. Chem. B, 115, 6421-6426 (2011).