High refractive indices have been achieved either by introducing substituents with high molar refractions (intrinsic HRIPs) or by combining high-n nanoparticles with polymer matrixes (HRIP nanocomposites).
Intrinsic HRIP Sulfur-containing substituents including linear
thioether and
sulfone, cyclic
thiophene, thiadiazole and
thianthrene are the most commonly used groups for increasing refractive index of a polymer. Polymers with sulfur-rich thianthrene and tetrathiaanthracene moieties exhibit n values above 1.72, depending on the degree of molecular packing.
Halogen elements, especially
bromine and
iodine, were the earliest components used for developing HRIPs. In 1992, Gaudiana
et al. reported a series of
polymethylacrylate compounds containing lateral brominated and iodinated
carbazole rings. They had refractive indices of 1.67–1.77 depending on the components and numbers of the halogen substituents. However, recent applications of halogen elements in
microelectronics have been severely limited by the
WEEE directive and
RoHS legislation adopted by the
European Union to reduce potential pollution of the environment. moiety even if they have chemical structures analogous to
polycarbonates. Shaver
et al. reported a series of polyphosphonates with varying backbones, reaching the highest refractive index reported for polyphosphonates at 1.66.
Organometallic components result in HRIPs with good
film forming ability and relatively low optical dispersion. Polyferrocenylsilanes and
polyferrocenes containing phosphorus
spacers and
phenyl side chains show unusually high n values (n=1.74 and n=1.72). They might be good candidates for all-polymer photonic devices because of their intermediate optical dispersion between organic polymers and
inorganic glasses.
HRIP nanocomposite Hybrid techniques which combine an organic polymer matrix with highly refractive inorganic nanoparticles could result in high n values. The factors affecting the refractive index of a high-n nanocomposite include the characteristics of the polymer matrix, nanoparticles and the hybrid technology between inorganic and organic components. The refractive index of a nanocomposite can be estimated as {n_{comp}} = {\Phi_p}{n_p} + {\Phi_{org}}{n_{org}}, where {n_{comp}}, {n_p} and {n_{org}} stand for the refractive indices of the nanocomposite, nanoparticle and organic matrix, respectively. {\Phi_p} and {\Phi_{org}} represent the volume fractions of the nanoparticles and organic matrix, respectively. The nanoparticle load is also important in designing HRIP nanocomposites for optical applications, because excessive concentrations increase the optical loss and decrease the processability of the nanocomposites. The choice of nanoparticles is often influenced by their size and surface characteristics. In order to increase optical transparency and reduce
Rayleigh scattering of the nanocomposite, the diameter of the nanoparticle should be below 25 nm. Direct mixing of nanoparticles with the polymer matrix often results in the undesirable aggregation of nanoparticles – this is avoided by modifying their surface. The most commonly used nanoparticles for HRIPs include TiO2 (
anatase, n=2.45;
rutile, n=2.70), ZrO2 (n=2.10),
amorphous silicon (n=4.23),
PbS (n=4.20) and
ZnS (n=2.36). Polyimides have high refractive indexes and thus are often used as the matrix for high-n nanoparticles. The resulting nanocomposites exhibit a tunable refractive index ranging from 1.57 to 1.99. ==Applications==