Now, Heykal Aouani and colleagues at Imperial College London have boosted the efficiency of up-conversion of three infrared photons to a green photon. They use a nonlinear indium tin oxide (ITO) nanoparticle embedded in a nano-antenna system designed to enhance the effect of an incident electromagnetic field. "We designed a system that can improve the efficiency by a factor of a million," says Aouani.

Conventional linear optical systems can be described as harmonic oscillators; an incident electromagnetic field at a certain frequency will cause the electrons in the material to move at a specific frequency. In contrast, in a nonlinear optical system the incident light can activate responses at higher harmonic frequencies.

In second-harmonic up-conversion, two low-energy photons from the incident light are converted into one photon with double the energy, while third-harmonic up-conversion converts three photons into one at triple their energy, and so on. Such higher harmonics can be increasingly weak and difficult to detect, but excited plasmons – resonant oscillations in a material’s electrons – within a nano-antenna have been shown to enhance the electromagnetic field, strengthening the nonlinear interactions and hence the signals from higher harmonics.

With their latest research Aouani and colleagues demonstrate that it is possible to hugely enhance the nonlinear optical response of the ITO nanoparticle, not just the plasmonic metal in the nano-antenna itself. Their work also demonstrates the potential use of a nonlinear optical nanoparticle to probe the electric field enhancements.

"The two interactions can be very complementary," explains Aouani. "These field enhancements cannot be measured directly, but systems to characterize plasmonic nanoantennas are needed to determine their physical properties."

It takes two

The Imperial College team produced the plasmonic nano-antenna system from a so-called plasmonic dimer – two metal nanorods separated by a nano-sized gap. The dimer system presented the researchers with the advantages of controllable fabrication and a high-field enhancement known from theory.

Since ITO sputtering is a non-directional deposition process, the researchers fabricated the ITO nanoparticle first and then decorated it with the dimer system. The particle diameter was 25 nm and optimum enhancement was achieved with a 35 nm gap.

Exceedingly high enhancements

"Usually people working in plasmonics are enhancing nonlinear nanoparticle second-harmonic generation," points out Aouani. Third-harmonic up-conversion signals are generally more difficult to work with so the researchers were surprised to find how successful the field enhancements were.

"We did the experiments and the enhancements were huge, so we made some simulations of the system," he explains. Finite different time domain calculations of the field intensity enhancement agreed closely with those determined from measured enhancements to the nonlinear response of the ITO. "The work bridges the gap between calculations and the real world," he adds.

Next steps

Despite the factor of a million enhancement in the nonlinear response of the ITO nanoparticle, the overall efficiency is still just 0.0007%. Aouani emphasizes that while the figure looks low, it equates to an up-conversion for every ten thousand photons rather than for every billion, as would usually be the case.

Even so, the researchers are already looking into demonstrating the effect with an array of dimers, which should improve the efficiency of the process even further.

Details of the research are reported in Nature Nanotechnology doi:10.1038/nnano.2014.27