Nonlinear refractive index measurements of discotic and calamitic nematic lyotropic phases

In this work, through the Z-scan technique, we report on measurements of the nonlinear refractive index $\left(n_2\right)$ in discotic and calamitic nematic phases at room temperature in lyotropic mixtures of potassium laurate, decanol and ${\mathrm{D}}_2\mathrm{O}$. This technique presents high sensitivity when compared to conventional interferometry. The nonlinear optical birefringence $\left(\mathrm{\Delta }{n}_2\right)$ of these nematic phases was also determined. The sign and absolute value of this relevant nonlinear parameter are discussed in terms of structural changes in the micellar configuration which takes place in each nematic lyotropic phase.

Figure 1

Experimental configurations of the nematic director, magnetic field and polarization of the laser beam for nonlinear refractive index measurements of (a) $n_{2\mid \mid }$ and (b) $n_{2\perp }$ on the $N_C$ phase; (c) $n_{2\mid \mid }$ and (d) $n_{2\perp }$ on the $N_D$ phase.

Figure 2

Sketch of the Z-scan apparatus. ${\mathrm{L}}_1$, ${\mathrm{L}}_2$, and ${\mathrm{L}}_3$ are lenses, chopper (Ch), sample (S), iris (I), and detector (D).

Figure 3

Typical curves of Z-scan measurements on lyotropic mixtures for both configurations between the polarization of the laser beam and nematic director: (a) $\widehat{n}\mid \mid \vec{E}$ and (b) $\widehat{n}\perp \vec{E}$ on the $N_C$ phase; (c) $\widehat{n}\mid \mid \vec{E}$ and (d) $\widehat{n}\perp \vec{E}$ on the $N_D$ phase. The solid line corresponds to the fitting [16] of Eq. (1) with $t\sim 10t_{co}$.