High-Index Materials for Optical Nanoantennas

Prototype of an optical nanoantenna, which can focus, direct, and effectively transmit light. (Source: MIPT / ITMO)

Optical nano­antennas, which can focus, direct, and effectively transmit light, have a wide range of appli­cations, including infor­mation trans­mission over optical channels, photo­detection, micro­scopy, bio­medical tech­nology, and even speeding up chemical reactions. For an antenna to pick up and transmit signals efficiently, its elements need to be resonant. In the radio band, such elements are pieces of wire. In the optical range, silver and gold nano­particles with plasmonic resonances have long been used for this purpose. Electro­magnetic fields in such particles can be localized on a scale of 10 nano­meters or less, but most of the energy of the field is wasted due to Joule heating of the conducting metal. There is an alternative to plasmonic nano­particles, which has been studied exten­sively for the last five years, namely particles of dielectric materials with high refractive indices at visible light frequencies, such as silicon. When the size of the dielectric particle and the wave­length of light are just right, the particle supports Mie resonances. Because the material pro­perties of dielec­trics are different from those of metals, it is possible to signi­ficantly reduce resistive heating by replacing plasmonic nano­antennas with dielec­tric analogs.

The key charac­teristic of a material determining Mie resonance para­meters is the refractive index. Particles made of materials with high refractive indices have resonances charac­terized by high quality factors. This means that in these materials electro­magnetic oscil­lations last longer without external excitation. In addition, higher refrac­tive indices correspond to smaller particle diameters, allowing for more miniature optical devices. These factors make high-index materials more suitable for the implemen­tation of dielectric nano­antennas. Now, researchers from the ITMO Uni­versity in Saint Peters­burg and the Moscow Institute of Physics and Tech­nology (MIPT) systema­tically examine the available high-index materials in terms of their resonances in the visible and infrared spectral ranges. Materials of this kind include semi­conductors and polar crystals, such as silicon carbide. To illustrate the behavior of various materials, they present their associated quality factors, which indicate how quickly oscil­lations excited by incident light die out. Theo­retical analysis enabled the researchers to identify 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 infrared band. In the mid-infrared part of the spectrum, which is of particular interest due to possible appli­cations, such as radiative cooling and thermal camouflage, the compound of germanium and tellurium took the pedestal.

There are funda­mental limi­tations on the value of the quality factor. It turns out that 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 in turn leads to a reduction of the quality factor, as well as heating, and that is precisely what the researchers are trying to get rid of. There is, therefore, a delicate balance between a high index of refrac­tion and energy losses. “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,” notes Dmitry Zuev, research scientist at the Metamaterials laboratory of the Faculty of Physics and Engi­neering, ITMO University. “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.”

According to the overview of manu­facturing techniques, silicon, germanium, and gallium arsenide are the most thoroughly studied high-index dielec­trics used in nano­photonics. A wide range of methods are available for manu­facturing resonant nano­antennas based on these materials, including litho­graphic, chemical, and laser-assisted methods. However, in the case of some materials, no tech­nology for fabrication of resonant nano­particles has been developed. For example, researchers have yet to come up with ways of making nano­antennas from germanium telluride, whose pro­perties in the mid-infrared range were deemed the most attractive by the theo­retical analysis.

“Silicon is currently, beyond any doubt, the most widely used material in dielectric nano­antenna manu­facturing,” says Denis Baranov, a PhD student at MIPT. “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.” The research findings obtained by the team could be used by nano­photonics engineers to develop new resonant nano­antennas based on high-index dielectric materials. Besides, the researchers suggest further theo­retical and experi­mental work devoted to the search for other high-index materials with superior pro­perties to be used in new improved dielectric nano­antennas. Such materials could, among other things, be used to considerably boost the effi­ciency of radiative cooling of solar cells, which would constitute an important techno­logical advance. (Source: MIPT)

Reference: D. G. Baranov et al.: All-dielectric nanophotonics: the quest for better materials and fabrication techniques, Optica 4, 814 (2017); DOI: 10.1364/OPTICA.4.000814

Link: Nanophotonics and Metamaterials, ITMO University, Saint Petersburg, Russia

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