Nonlinear Refraction in Photoinduced Isotropic State of Liquid Crystalline Azobenzenes

The photoinduced isotropic state of liquid crystalline azobenzenes exhibits one of the strongest nonlinear optical refraction known for isotropic materials. The optical nonlinearity of these materials is large in a wide band of the visible spectrum and is accompanied with high, over $90\%$, transmission for red wavelengths.

Figure 1

Self-phase modulation patterns in the photoinduced isotropic state of an azo LC for a laser beam of $\lambda =633\mathrm{n}\mathrm{m}$ wavelength: (a) $20\mu \mathrm{m}$-thick azo LC 1205 in a beam of $42\mu \mathrm{W}$ power focused by a lens of 35 mm focal length; (b) $10\mu \mathrm{m}$-thick azo LC in a beam of 1.75 mW power focused by a lens of 75 mm focal length.

Figure 2

Spontaneous (dark) relaxation dynamics of photoinduced isotropic phases of $4,4^{\prime }$-disubstituted azobenzene compositions characterized by the change in the isotropic-nematic phase transition temperature, $\delta T=T_{iso}^{trans}-T_{iso}^{cis}$, due to changing concentration of cis isomers: (1) azo LC 8721; (2) azo LC 1007.

Figure 3

The constant of nonlinear refraction of azo LC 1205 as a function of (a) the UV-exposure time at the wavelengths $\lambda =633$ and $\lambda =532\mathrm{n}\mathrm{m}$, and (b) the waist radius of a red laser beam. The power of the laser beam in these measurements was $20\mu \mathrm{W}$ and 280 nW for the wavelengths $\lambda =633\mathrm{n}\mathrm{m}$ and $\lambda =532\mathrm{n}\mathrm{m}$, correspondingly.

Figure 4

The response time vs laser beam power for (a) $\lambda =633\mathrm{n}\mathrm{m}$ and (b) $\lambda =532\mathrm{n}\mathrm{m}$ and different UV-exposure times.