Better Materials Could Advance Design of Dielectric Nanophotonic Devices

A team of physicists has conducted a comparative analysis of available high-index materials and existing fabrication techniques, examining their performance as optical nanoresonators. The systematic study has produced results that could be used to optimize the use of known materials for building optical nanodevices, as well as encourage the search for new materials with superior properties.

A research team from the Moscow Institute of Physics and Technology (MIPT) and the Saint Petersburg State University of Information Technology, Mechanics and Optics (ITMO) analyzed high-index materials, including semiconductors and polar crystals, to determine their resonances in the visible and IR spectral ranges. Resonant behavior was analyzed in terms of linear characteristics of spherical nanoparticles.

An optical nanoantenna. Courtesy of the researchers from MIPT and ITMO University.

Materials were examined for their scattering efficiencies and Q factors of the magnetic Mie resonance. Quality factors associated with the behavior of various materials were also investigated.

The team identified crystalline silicon as the best currently available material for the realization of dielectric antennas operating in the visible range, with germanium outperforming other materials in the IR band. In the mid-IR portion of the spectrum, a germanium-tellurium compound demonstrated the best performance.

The team also looked at methods for all-dielectric nanostructure fabrication. In the case of some materials, researchers found that no technology for fabrication of resonant nanoparticles had yet been developed. For example, a way to make nanoantennas from germanium-telluride has yet to be developed.

Optical resonances in plasmonic (a) and dielectric (b) nanoparticles. Image courtesy of the researchers from MIPT and ITMO University and MIPT Press Office.

“Silicon is currently, beyond any doubt, the most widely used material in dielectric nanoantenna manufacturing,” said MIPT researcher Denis Baranov. “It is affordable, and silicon-based fabrication techniques are well-established. Also, and this is important, it is compatible with the CMOS technology, an industry standard in semiconductor engineering.

“But silicon is not the only option," according to Baranov. "Other materials with even higher refractive indices in the optical range might exist. If they are discovered, this would mean great news for dielectric nanophotonics.”

Comparison of various high-index materials in terms of their quality factors, which reflect how long it takes for the Mie resonance of the particle to fade. Higher Q factors indicate longer fading times and a more pronounced resonant behavior of the particle. Image courtesy of the researchers from MIPT and ITMO University.

In the course of their study, researchers considered the need to balance a high index of refraction with energy loss. High refractive indices in semiconductors are associated with interband transitions of electrons, which inevitably entail the absorption of energy carried by the incident light. This absorption can lead to a reduction of the quality factor.

“This study is special both because it offers the most complete picture of high-index materials, showing which of them is optimal for fabricating a nanoantenna operating in this spectral range, and because it provides an analysis of the manufacturing processes involved,” said ITMO researcher Dmitry Zuev. “This enables a researcher to select a material, as well as the desired manufacturing technique, taking into account the requirements imposed by their specific situation. This is a powerful tool furthering the design and experimental realization of a wide range of dielectric nanophotonic devices.”

The research findings could be used by nanophotonics engineers to develop novel resonant nanoantennas based on high-index dielectric materials. The research also presents an outlook for the search for better materials with higher refractive indices and novel fabrication methods that could enable low-cost manufacturing of optically resonant high-index nanoparticles.

The research was published in Optica, a publication of the Optical Society (doi: 10.1364/OPTICA.4.000814).