The magic of intensified nonlinear optical activity

Nonlinear optics exploit media that respond in a nonlinear way to incident electromagnetic radiation, changing its wavelength (and frequency) and thus colour. Industrially relevant nonlinear optical effects include Raman scattering, two-photon absorption and high harmonic generation.

Recently, composite materials consisting of carefully designed polymers and metal nanoparticles have been shown to produce nonlinear optical activity orders of magnitude better than conventional materials. The EU-funded project ‘Polymer / metal nanoparticles composites with enhanced non-linear optical properties’ (COMPONLO) was launched to conduct a systematic evaluation of these materials.

Scientists chose two co-polymer systems with different glass transition temperatures, one of the most important properties of any epoxy. The glass transition temperature is actually a range of temperatures over which the polymer transitions from brittle and glass-like to soft and rubbery. Both copolymer systems were synthesised with and without gold (Au) nanoparticles.

Researchers used a technique called corona poling to evaluate induction or increase in nonlinear optical properties. Corona poling aligns molecules in a polymer film or polymer such that its index of refraction will change when subjected to an external electric field.

They found that polymers in the low transition temperature family exhibited nonlinear optical behaviour (second harmonic generation or SHG) even before corona poling. Corona poling temporarily decreased the signal but it was recovered after aging at room temperature. Au nanoparticles significantly enhanced the SHG relative to that of pristine polymers.

Pure co-polymers in the higher glass transition temperature family exhibited nonlinear optical properties only after poling. Addition of Au nanoparticles induced nonlinearities even without poling but reduced the SHG signal after relative to that of pure co-polymers regardless Au content. It is possible that the Au nanoparticles block further molecular alignment after poling, in essence locking the macromolecular configuration. In addition, Au nanorod or nanoshell structures interfered with development of nonlinear optical activity, likely due to scattering of the SHG-produced light by over-sized Au particles.

COMPONLO outcomes are among the first characterising in a systematic way a promising new class of materials with enhanced nonlinear optical activity. They lay the foundations for exploitation of the phenomena in exciting new devices.