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Structure and Nonlinear Index of Refraction of Sunset Yellow Lyotropic Chromonic Liquid Crystal in the Isotropic and Nematic Phases

Dennys Reis, Renato Mafra Moysés, Lino Misoguti, Antônio Martins Figueiredo Neto

TL;DR

The study investigates nonlinear optical properties of a prototypical lyotropic chromonic liquid crystal, Sunset Yellow (SSY), by applying nonlinear ellipse rotation (NER) to measure the nonlinear refractive index $n_2$ in isotropic and nematic phases. By combining SWAXS with NER across a range of concentrations ($w_{SSY}$) and temperatures, the work decomposes $n_2$ into fast ($n_{2,\mathrm{fast}}$) and slow ($n_{2,\mathrm{slow}}$) contributions, linking them to electronic and reorientational dynamics and to nanoscale stacking in chromonic assemblies. Key findings show that $n_{2,\mathrm{fast}}$ increases with SSY content and temperature and is enhanced in the nematic phase due to orientational order, while $n_{2,\mathrm{slow}}$ correlates with stack size and remains similar across phases; no anisotropy is detected in $n_{2,\mathrm{fast}}$ with director orientation. This work demonstrates how nanoscale aggregation and phase behavior govern nonlinear optical responses in chromonics, informing design of optoelectronic devices using these materials.

Abstract

Lyotropic chromonic liquid crystals are formed by the self-assembly of aromatic compounds in concentrated solutions. Despite numerous applications of chromonic systems in optical and photonic devices, they all make use of the anisotropic linear optical properties of the nematic or columnar liquid crystalline phases. This paper extends the investigations of chromonic systems to the domain of nonlinear optics. For this purpose, the magnitude and sign of the nonlinear refractive indices, $n_2,$ were measured by the nonlinear ellipse rotation (NER) technique. This was performed on aqueous solutions of sunset yellow azo dye, the prototypical chromonic system. Samples with different concentrations and temperatures were used, both in the isotropic and nematic phases. In addition, the molecular aggregation states of the chromonic samples as a function of temperature and concentration were investigated by wide angle X-ray scattering. NER measurements as a function of the laser pulse width from $65\,fs$ to $\sim 5\,ps$ allowed the decomposition of $n_2$ into a fast contribution, $n_{2,fast},$ associated with molecular electronic processes, and a slow one $n_{2,slow},$ associated with molecular reorientational processes. It was shown that $n_{2,fast}$ doubled from the isotropic phases of the $15$ to the $30\,\%\,\text{w/w}$ samples, proportionally to the increase in mass fraction. However, $n_{2,fast}$ for the aligned nematic phase of $30\,\%\,\text{w/w}$ sample was higher than the double of the corresponding value for the $15\,\%\,\text{w/w}$ sample, showing an effect associated to the orientational order of this phase. Also, $n_{2,fast}$ was shown to depend linearly on temperature.

Structure and Nonlinear Index of Refraction of Sunset Yellow Lyotropic Chromonic Liquid Crystal in the Isotropic and Nematic Phases

TL;DR

The study investigates nonlinear optical properties of a prototypical lyotropic chromonic liquid crystal, Sunset Yellow (SSY), by applying nonlinear ellipse rotation (NER) to measure the nonlinear refractive index in isotropic and nematic phases. By combining SWAXS with NER across a range of concentrations () and temperatures, the work decomposes into fast () and slow () contributions, linking them to electronic and reorientational dynamics and to nanoscale stacking in chromonic assemblies. Key findings show that increases with SSY content and temperature and is enhanced in the nematic phase due to orientational order, while correlates with stack size and remains similar across phases; no anisotropy is detected in with director orientation. This work demonstrates how nanoscale aggregation and phase behavior govern nonlinear optical responses in chromonics, informing design of optoelectronic devices using these materials.

Abstract

Lyotropic chromonic liquid crystals are formed by the self-assembly of aromatic compounds in concentrated solutions. Despite numerous applications of chromonic systems in optical and photonic devices, they all make use of the anisotropic linear optical properties of the nematic or columnar liquid crystalline phases. This paper extends the investigations of chromonic systems to the domain of nonlinear optics. For this purpose, the magnitude and sign of the nonlinear refractive indices, were measured by the nonlinear ellipse rotation (NER) technique. This was performed on aqueous solutions of sunset yellow azo dye, the prototypical chromonic system. Samples with different concentrations and temperatures were used, both in the isotropic and nematic phases. In addition, the molecular aggregation states of the chromonic samples as a function of temperature and concentration were investigated by wide angle X-ray scattering. NER measurements as a function of the laser pulse width from to allowed the decomposition of into a fast contribution, associated with molecular electronic processes, and a slow one associated with molecular reorientational processes. It was shown that doubled from the isotropic phases of the to the samples, proportionally to the increase in mass fraction. However, for the aligned nematic phase of sample was higher than the double of the corresponding value for the sample, showing an effect associated to the orientational order of this phase. Also, was shown to depend linearly on temperature.

Paper Structure

This paper contains 11 sections, 4 equations, 12 figures, 1 table.

Table of Contents

  1. Introduction
  2. Materials and Methods
  3. Samples
  4. Experimental Techniques
  5. Polarized Light Optical Microscopy
  6. 2D Small- and Wide-Angle X-ray Scattering
  7. Nonlinear Ellipse Rotation
  8. Results and Discussion
  9. Structure
  10. Nonlinear Index of Refraction
  11. Conclusions

Figures (12)

  • Figure 1: Diagram of the nonlinear ellipse rotation (NER) setup. In the bottom left, there is an illustration of the rotation of the polarization ellipse of the transmitted beam (orange) with respect to the polarization ellipse of the incident beam (blue), due to the nonlinear rotation effect. The eccentricities of the polarization ellipses are the same in both cases. The polarization ellipse rotation angle, $< \alpha(z) >,$ is measured as a function of sample position, $z$, in beam propagation direction, around the focus position $z=0.$
  • Figure 2: Textures of the chromonic phases of sample 30SSY, observed between crossed polarizers by polarized light microscopy on heating. (a) N phase, at $T = 25.0\,^{\circ} C;$ (b) N and I phases coexistence, at $T = 36.0 \,^{\circ} C;$ (c) N and I phases coexistence, at $T = 45.0 \,^{\circ} C;$ and (d) I phase, at $T = 48.0\,^{\circ} C.$ P and A represent the directions of the polarizer and the analyzer, respectively.
  • Figure 3: Phase diagram for the mixture of dye sunset yellow and water, where $w_{SSY}$ is the mass fraction, in $\%\,\text{w/w SSY}.$ Samples with $w_{SSY}$ from $5$ to $35\,\%\,\text{w/w},$ in steps of $5\,\%\,\text{w/w},$ were investigated. $N+I$ represents the phase coexistence region. The dashed lines were interpolated into the data as a guide. Temperatures and concentrations of the samples measured by WAXS are marked in red.
  • Figure 4: X-ray scattering images. (a) Isotropic phase of sample 20SSY at $T = 25.0\,^{\circ} C;$ (b) oriented nematic phase of sample 35SSY at $T = 30.0\,^{\circ} C.$ The bands are identified in each image. Perpendicular, parallel and diagonal refer to the stacking direction, e.g., parallel refers to X-ray scattering band due to the structures along, or parallel to, the stacking direction. Corresponding bands in both images have the same name. In (b), $\hat{n}$ is the N phase director.
  • Figure 5: X-ray scattering curves from the images of Figure \ref{['fig:waxs:2D']}. (a) Isotropic phase of sample 20SSY at $T = 25.0\,^{\circ} C;$ (b) oriented nematic phase of sample 35SSY at $T = 30.0\,^{\circ} C.$ Inset: For the oriented N phase, the sectors used for the azimuthal averages are highlighted in white over the WAXS image. The white arrow represent the N phase director, $\hat{n}.$
  • ...and 7 more figures