Spontaneous light-induced Turing patterns in a dye-doped twisted nematic layer

Abstract

Optical pattern formation is usually due either to the combination of diffraction and nonlinearity in a Kerr medium or to the temporal modulation of light in a photosensitive chemical reaction. Here, we show a different mechanism by which light spontaneously induces stripe domains between nematic states in a twisted nematic liquid crystal layer doped with azo-dyes. Thanks to the photoisomerization process of the dopants, light in the absorption band of the dopants creates spontaneous patterns without the need of temporal modulation, diffraction, Kerr or other optical nonlinearity, but based on the different scales for dopant transport processes and nematic order parameter, which identifies a genuine Turing mechanism for this instability. Theoretically, the emergence of the stripe patterns is described on the basis of a model for the dopant concentration coupled with the nematic order parameter.

Extended stable equilibrium invaded by an unstable state

“…Theoretically was demostrated, recently, that an equivalent model to Eq. (2) describes the photo-isomerization process in a dye-doped nematic liquid crystal layer, where u stands for the Landau-De Gennes molecular scalar order parameter (see the details in 34 and the equivalence of the models in Supplementary Material).

Figure 5Concentric ring pattern propagation in photo-isomerization process in a dye-doped nematic liquid crystal layer illuminated by a laser beam with a Gaussian profile.

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Light-Induced Ring Pattern in a Dye-Doped Nematic Liquid Crystal

“…These fronts are characterized by being a circular spot that gathers in the center of the beam and spreads outwards and stops in the region where both states are energetically equivalent, Maxwell point. For intermediate light intensities, which do not induce isotropic liquid phase, the emergence of a pattern with a stripe shape has been reported [21,22]. In fact, these patterns correspond to regions that alternate higher and lower orientational molecular order.…”

“…The conventional phototropic transition detection is performed by sampling the excitation laser beam and extracting the reorientational order parameter with polarized optical microscopy [19][20][21][22][23]30]. The main inconvenience with those setups arises from the loss of information of the liquid crystal dynamics outside of the central Gaussian illuminated zone.…”

“…We note that one of the reasons for considering methyl red methyl ester is that it is more miscible in E7 than other dopants; for example, methyl red. To describe the dynamics of the photo-isomerization process in the dye-doped nematic layer with planar anchoring, let us consider the concentration of molecules in the cis-state C( r ⊥ , t) and the scalar order parameter S( r ⊥ , t), which satisfy the dimensionless rate equations [21,22]…”

Beauty in artistic expressions through the eyes of networks and physics

“…Patterns in nature are common and often beautiful and intriguing [47]. Pattern formation is thus, expectedly, also common in physics, chemistry and biology research, where emergent ordered and disordered structures often require quantification [48][49][50][51][52][53][54]. Indeed, the subject has been vibrant and popular ever since the seminal research by Alan Turing on the chemical basis of morphogenesis [55].…”