Nematic liquid crystals and smectic liquid crystals can be infiltrated into a three-dimensional periodic array of interconnected nanosize voids in opal films prepared by sedimentation of ${\mathrm{SiO}}_{\mathrm{2}}$ spheres of various diameters. The optical stop band in the transmission spectra of opals shifts drastically by the infiltration of liquid crystals. The stop band is also found to shift at the phase transition points with changing temperature. This effect enables the tuning of optical properties of opals as a prototype tunable photonic crystal. This phenomenon can also be used as a measurement method for the refractive index.

We have measured the quadratic electroabsorption (EA) spectrum of a variety of soluble luminescent and nonluminescent π-conjugated polymer films in the spectral range of 1.5–4.5 eV. The luminescent polymers include MEH and DOO derivatives of poly(phenylene-vinylene), poly(phenylene ethylene), and polythiophene; the nonluminescent polymers include poly(diethynyl silane) and monosubstituted polyacetylene. All EA spectra show a Stark shift of the low-lying odd-parity exciton ${(1B}_u)$ and imply the presence of phonon sidebands. There are also higher-energy bands due to transfer of oscillator strength to even-parity exciton states ${(A}_g),$ the strongest of which ${(mA}_g)$ is located at an energy about 1.3 times that of the ${1B}_u$ exciton in both luminescent and nonluminescent polymers; in the luminescent polymers the EA spectra also show a second prominent $A_g$ state ${(kA}_g)$ at an energy of about 1.6 times that of the ${1B}_u.$ We have successfully fitted the EA spectra by calculating the imaginary part of the third order optical susceptibility, $\mathrm{Im}\left[{\chi }^3\right(-\omega ;\omega ,0,0\left)\right],$ using a summation over states model dominated by the ground state, the ${1B}_u$ exciton, two strongly coupled $A_g$ states (${\mathrm{mA}}_g$ and ${\mathrm{kA}}_g$), and their most strongly coupled vibrations, using Frank-Condon overlap integrals. A distribution of conjugation lengths, which results in a distribution of excited state energies, was also incorporated into the model. The decomposition of the EA spectra due to the conjugation length distribution was then used to calculate the ${1B}_u$ exciton polarizability $\left(\Delta p\right)$ using first derivative analysis. For the longest conjugation lengths in our films, we found $\Delta p$ to be of order ${10}^4(\mathrm{\AA }{)}^3$ in luminescent polymers and ${10}^3{\mathrm{\AA }}^3$ in nonluminescent polymers, respectively, in good agreement with recent subnanosecond transient photoconductivity measurements. We also found that the Huang-Rhys parameter of the ${1B}_u$ exciton varies between 0.25 and 0.9, being in general smaller for the luminescent polymers. The consequent exciton relaxation energies were calculated to be of order 100 meV.

Electrical tuning of the wavelength of the defect-mode lasing in a one-dimensional periodic structure has been demonstrated using a dye-doped nematic liquid crystal as a defect layer in the periodic structure. Lasing wavelength is widely tuned upon applying an electric field, which is due to the refractive index change in the defect layer caused by the field-induced realignment of the liquid crystal molecules.