Physicists Look for Materials with High Refractive Index for Developing Nanoantennas

Physicists from MIPT and ITMO University have performed a comparative analysis of a wide array of materials to ascertain whether they can be used for dielectric nanophotonics. The outcomes of this systematic study have the ability to optimize the application of familiar materials for developing optical nanodevices and also to stimulate the hunt for innovative materials that have exceptional characteristics.

Optical nanoantenna. (Credit: MIPT)

An antenna—used to send, receive and process electromagnetic signals—has the ability to efficaciously transmit, receive and redirect electromagnetic radiation. Normally, antennas are considered to be macroscopic devices that operate in the radio as well as microwave range. Yet, similar optical devices do exist. As visible light has a wavelength of a few hundred nanometers, optical antennas are necessarily nanosized. Optical nanoantennas with the ability to focus, direct and efficaciously transmit light can be used for different applications such as microscopy, transmitting information over optical channels, biomedical technology, photodetection and also to accelerate chemical reactions.

If an antenna has to receive and transmit signals in a highly efficient manner, the elements of the antenna have to be resonant. These are pieces of wire in the radio band. In the case of optical range, for many years, gold and silver nanoparticles that have plasmonic resonances have been used. Although the electromagnetic fields in these particles can be localized of the order of less than or equal to 10 nm, much of the field’s energy is dissipated as a result of Joule heating of the conducting metal. However, dielectric material particles that have high refractive index at visible light frequencies (e.g. silicon) have been considered as substitutes for plasmonic nanoparticles.

In the past 5 years, much research has gone into extensive investigation of the use of dielectric material particles. Upon using the dielectric particle of correct size and the precise light wavelength, the particle has the ability to support optical resonances of a specific type, known as Mie resonances. As the material characteristics of dielectrics differ from that of metals, resistive heating can be considerably minimized by substituting plasmonic nanoantennas with dielectric analogs.

The most important property of a material that determines Mie resonance parameters is its refractive index. Resonances of particles formed of materials that have higher refractive index are characterized by high-quality factors, indicating that the electromagnetic oscillations in such materials stay for a very long period of time even without external excitation. Moreover, if the refractive indices are high then the particle diameters must be smaller, enabling the development of miniature optical devices. Due to such factors, high-index materials or materials that have high refractive index, tend to be more appropriate for developing dielectric nanoantennas.

The Researchers have published their paper in the journal Optica, in which they have systematically investigated the high-index materials at hand with regard to their resonances in the infrared and visible spectral ranges. Materials of this kindinclude semiconductors and polar crystals, such as silicon carbide. In order to illustrate the characteristics of different materials, the Researchers list out their corresponding quality factors, specifying how swiftly oscillations stimulated by incident light are weakened.

At the end of the theoretical investigation, the research team recognized that crystalline silicon is the best prevalent material for developing dielectric antennas functioning in the visible range. In the infrared band, germanium proved to be better than other materials. In the mid-infrared range, a compound formed of germanium and tellurium was found to outperform other materials. The mid-infrared range is of specific interest because of prospective applications such as radiative cooling and thermal camouflage. Radiative cooling is the cooling of a heated body by radiating heat from the body into the environment in the form of electromagnetic waves. Thermal camouflage is the process by which thermal radiation released by an object is reduced, thereby rendering it invisible to infrared cameras.

There are basic restrictions on the value of the quality factor. As it turns out, high refractive index of semiconductors are caused due to interband transitions of electrons, which necessarily involve the attenuation of energy from the incident light. This attenuation consequently results in the decrease of the quality factor, and also heating and this is exactly what the Physicists are attempting to do away with. Hence there is a weak balance between loss of energy and high refractive index.

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. 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.

Dmitry Zuev, Research Scientist, the Metamaterials Laboratory of the Faculty of Physics and Engineering, ITMO University

In relation to the outline of manufacturing methods, germanium, silicon and gallium arsenide are the most extensively investigated high-index dielectrics applied in the field of nanophotonics. There are various techniques—such as chemical, lithographic and laser-assisted techniques—for producing resonant nanoantennas from these materials. Yet in the case of some materials, no technology for fabrication of resonant nanoparticles has been developed. For instance, technique for producing germanium telluride nanoantennas is lacking, where characteristics of germanium telluride in the mid-infrared range were regarded to be the most fascinating at the end of the theoretical investigation.

Silicon is currently, beyond any doubt, the most widely used material in dielectric nanoantenna manufacturing. 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. 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.

Denis Baranov, a PhD student at MIPT

The outcomes of the research can be applied by nanophotonics Engineers to create innovative resonant nanoantennas using high-index dielectric materials. In addition, the paper indicates that further experimental and theoretical research is needed for finding out other high-index materials with exceptional characteristics that can be applied in new advanced dielectric nanoantennas. Apart from other things, these materials can be used to significantly boost the efficaciousness of radiative cooling of solar cells, which will be a significant technological breakthrough.

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