Tuning the polymer refractive index with nanosized organic dopants

Technologies like DVDs, Blu-ray Disc media, and plastic electronics increasingly rely on high-performance polymers with special optical and thermomechanical properties.^1,2 To be useful, such polymers need improved thermal stability and an individually customized refractive index and transmittance.³ Before they can be commercialized, these new components must be easy to process and mass produce with low-cost replication methods.⁴ Accordingly, new coatings for micro-optical components must be developed. For telecommunications and consumer electronic parts, these materials should have a refractive index customized for either the near-infrared wavelengths—between 1300 and 1600nm—or the visible range—between 380 and 780nm.

Two main strategies can be pursued to modify the refractive index of polymers. The first option is chemical synthesis of new polymers. It requires that at least one of the monomers contain an electron-rich, easily polarizable aromatic moiety like benzene or fused aromatic systems. In the second option, dopant molecules can be dissolved in a polymer, thereby creating a guest-host system. Yet this approach can lead to plasticizing and miscibility problems.

To further this goal, we have developed multiple nanosize organic dopants that tune the refractive index of commercial polymers at a visible wavelength (see Figure 1). This approach allows the index to vary linearly with dopant concentration. Figure 2 shows this linear relationship for different phenanthrene-type molecules dissolved in a commercial reactive polymer resin of methyl methacrylate/polymethyl methacrylate (PMMA). After curing the solid polymer, solutions containing benzochinoline show the largest refractive index—around 1.58—at a dopant content of 45wt%. At constant dopant content, substituting bromine for hydrogen in the 9-position of the phenanthrene ring system results in a lower refractive index than that accessible with pure phenanthrene. A different molecule, triphenylmethane, can be dissolved up to 25wt% and has an effect on the refractive index similar to phenanthrene.

Figure 1. Chemical structures of all investigated organic dopants.

We found comparable shifts using an epoxide (Epofi, Struers A/S) as a polymer matrix. Figure 3 shows the refractive index change with phenanthrene or benzochinoline concentration. At 633nm, the dopants can significantly raise the refractive index from the pure epoxide's value of 1.57. Both dopants show an almost identical influence on the shift. While the solubility of phenanthrene is limited to 10wt%, benzochinoline can be dissolved up to 50wt% in the epoxide matrix. With this combination, we can achieve a refractive index of almost 1.63.

Figure 2. The refractive index in PMMA varies linearly with dopant concentration.

In addition to developing a guest-host-system, we doped benzylmethacrylate with phenanthrene or triphenylmethane and then polymerized the mixture. For the pure polymer, substituting the benzyl moiety for the methyl side chain in PMMA boosts the refractive index from 1.49 to 1.506. Dissolving phenanthrene or triphenylmethane in the polymer increases its value even more. The literature describes the results for guest-host and composite systems^5,6 for passive and active micro-optical devices, including beam splitters and a Pockels modulator.

In another approach, we added nanosized inorganic ceramics as high refractive fillers. While the organics can be dissolved in the matrix up to the solubility limit, the inorganic material must be dispersed. Rayleigh's scattering law dictates that scattering, and hence loss of optical transmittance, can only be avoided if the particles are smaller than a tenth of the optical wavelength. Previous investigations showed that the dispersion of commercially available nanosized ceramics like SiO₂, Al₂O₃, or ZrO₂—which have primary particle sizes between 12 and 40nm—can only be used for small refractive index changes. This limitation exists because particle agglomeration leads to pronounced scattering at larger filler contents.^7,8

Figure 3. Doping benzylmethacrylate and epoxide with organic molecules can also boost the refractive index.

The study demonstrates the possibility of modifying the refractive index of polymers using nanosized organic dopants. Current research efforts are focused on minimizing the plasticizing effect. We are also researching the use of the new polymers, using micro-injection molding to create optical components.