Photochemical switching between colloidal photonic crystals at the nematic-air interface

A direct observation of the photochemical switching between colloidal crystals with different lattice constants in a liquid-crystal (LC) emulsion is reported. Glycerol droplets introduced in a nematic liquid crystal form two-dimensional hexagonal colloidal crystal at the nematic-air interface with a lattice constant depending on the surface tension ${\sigma }_{\mathit{LCA}}$. We dope an azobenzene derivative into the LC emulsion to modulate the colloidal structures by using cis-trans photoisomerization of the doped dye. The photoisomerization changes ${\sigma }_{\mathit{LCA}}$ and the lattice constant of colloidal crystals with a relaxation time $T\sim 10\mathrm{s}$. A simple theoretical description, which qualitatively agrees with experimental results, has been proposed. Possible applications in infrared range photonic crystals are discussed. The number of different kinds of dyes $N$ set the number of different possible photonic crystals as $2^N$, which may be transformed one into another by corresponding $2N$ light beams inducing cis-trans photoisomerization of the doped dyes.

Figure 1

Photochemical condensation of glycerol droplets in the LC emulsion (a) before and (b) after photoirradiation. A diameter of droplets is approximately $3\mu \mathrm{m}$. Irradiation of ultraviolet ($\lambda =365\mathrm{nm}$, $50\mathrm{mW}∕{\mathrm{cm}}^2$) light was performed at $30°\mathrm{C}$ where the LC exhibits a nematic phase.

Figure 2

Photochemical fabrication of patterned colloidal crystals: (a) linear grating and (b) hexagonal structure. Photoirradiation was performed at $30°\mathrm{C}$.

Figure 3

Photonic manipulation of the lattice constants under the illumination of UV ($\lambda =365\mathrm{nm}$, $50\mathrm{mW}∕{\mathrm{cm}}^2$) or visible ($\lambda =435\mathrm{nm}$, $100\mathrm{mW}∕{\mathrm{cm}}^2$) light at $30°\mathrm{C}$.