ITMO & Australia National University: Invisible Hobbits & Harry Potter? Creating Invisible Objects Without Metamaterial Cloaking

Invisible Meta Cloaking 041415 id39742Physicists from ITMO University, Ioffe Institute and Australian National University managed to make homogenous cylindrical objects completely invisible in the microwave range. Contrary to the now prevailing notion of invisibility that relies on metamaterial coatings, the scientists achieved the result using a homogenous object without any additional coating layers. The method is based on a new understanding of electromagnetic wave scattering. The results of the study were published in Scientific Reports (“Switching from Visibility to Invisibility via Fano Resonances: Theory and Experiment”).
Radio Anechoic Chamber at Metamaterials Laboratory
This is the radio-frequency anechoic chamber used for the experiment. (Research: ITMO University)
The scientists studied light scattering from a glass cylinder filled with water. In essence, such an experiment represents a two-dimensional analog of a classical problem of scattering from a homogeneous sphere (Mie scattering), the solution to which is known for almost a century. However, this classical problem contains unusual physics that manifests itself when materials with high values of refractive index are involved. In the study, the scientists used ordinary water whose refractive index can be regulated by changing temperature.
As it turned out, high refractive index is associated with two scattering mechanisms: resonant scattering, which is related to the localization of light inside the cylinder, and non-resonant, which is characterized by smooth dependence on the wave frequency. The interaction between these mechanisms is referred to as Fano resonances. The researchers discovered that at certain frequencies waves scattered via resonant and non-resonant mechanisms have opposite phases and are mutually destroyed, thus making the object invisible.
The work led to the first experimental observation of an invisible homogeneous object by means of scattering cancellation. Importantly, the developed technique made it possible to switch from visibility to invisibility regimes at the same frequency of 1.9 GHz by simply changing the temperature of the water in the cylinder from 90 °C to 50 °C.
“Our theoretical calculations were successfully tested in microwave experiments. What matters is that the invisibility idea we implemented in our work can be applied to other electromagnetic wave ranges, including to the visible range. Materials with corresponding refractive index are either long known or can be developed at will,” said Mikhail Rybin, first author of the paper and senior researcher at the Metamaterials Laboratory in ITMO University.
The discovery of invisibility phenomenon in a homogenous object and not an object covered with additional coating layers is also important from the engineering point of view. Because it is much easier to produce a homogeneous cylinder, the discovery could prompt further development of nanoantennas, wherein invisible structural elements could help reduce disturbances. For instance, invisible rods could be used as supports for a miniature antenna complex connecting two optical chips.
The subject of invisibility came into prominence with the development of metamaterials – artificially designed structures with optical properties that are not encountered elsewhere in nature. Metamaterials are capable of changing the direction of light in exotic ways, including making light curve around the cloaked object. Nevertheless, coating layers based on metamaterials are extremely hard to fabricate and are not compatible with many other invisibility ideas. The method developed by the group is based on a new understanding of scattering processes and leaves behind the existing ones in simplicity and cost-effectiveness.
Source: ITMO University

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Stacked nanoparticle layers shine new light on optical thin films

Posted: November 5, 2012

Stacked nanoparticle layers shine new light on optical thin films(Nanowerk Spotlight) The refractive index is the property of a material that changes the speed of light and describes how light propagates through the material. The refractive index is an important property of solar cells – the higher it is, the more incident light gets reflected and is not converted to a photocurrent.

Air for instance, has a low refractive index very close to 1.0; but silicon, still the most common material used in today’s commercial solar cells, has a high refractive index which causes more than 30% of incident light to be reflected back from the surface of the silicon crystals.Solar cell manufacturers have therefore developed various kinds of anti reflection coatings to reduce the unwanted reflective losses (read more in our Nanowerk Spotlight“Moth eyes inspire self-cleaning antireflection nanotechnology coatings”). The purpose of these optical thin-films is to minimize the differences in the refractive indices between the ambient medium and the solar cells (or other opto-electronic devices).”For both solar cells and LEDs, coating with nano-particles can enhance the performance without harming the electrical properties of the devices, as can occur with etching or lithographic processing,” Hsuen-Li Chen, a professor in the Department of Materials Science and Engineering at National Taiwan University, tells Nanowerk.

nanoparticle multilayer stacksSchematic representation of: a) graded-refractive-index nanoparticle multilayer stacks, and b) scattering particles on graded-refractive-index nanoparticle stacks. (Reprinted with permission from Wiley-VCH Verlag)

In new work, reported in the October 16, 2012 online edition of Advanced Functional Materials (“Nanoparticle Stacks with Graded Refractive Indices Enhance the Omnidirectional Light Harvesting of Solar Cells and the Light Extraction of Light-Emitting Diodes”), Chen and his team have not only demonstrated this advantageous feature but also provided a strategy for optimizing the types and sizes of nanoparticles for use in both solar cells and LED’s. “Previous research did not mainly focus on the refractive indices of nanoparticles” says Chen. “Therefore, we wanted to know how these nanoparticles behave if they were spin-coated onto substrates. We assumed that nanoparticle stacks can be seen as optical thin films with refractive indices because of their little roughness and we successfully used both simulation and experimental measurement to prove our hypothesis.”The team’s main motivation has been to develop an easy and inexpensive method to construct optical thin films.

Traditionally, multi-layer optical thin-films with graded refractive indices were fabricated by PVD (physical vapor deposition) or CVD (chemical vapor deposition). However, using vacuum systems is both time consuming and expensive. In order to save money and processing time, Chen’s team therefore decided to spin-coat dielectric nanoparticle stacks with suitable refractive index to fabricate graded refractive indices multi-layers.”Our assumption was that, if the sizes of the nanoparticles are far less than the wavelength,they can be treated as optical thin films with effective refractive indices after they have been spin-coated onto the substrate,” Chen explains.

“Our rapid, low-cost, solution-based method allows the construction of graded-refractive-index nanoparticle stacks that function as broadband, omnidirectional antireflection coatings. This technique can minimize the reflectance of the silicon-air interface and increase the efficiency of silicon solar cells.””Moreover” he says, “if the sizes of the particles were to be close to the wavelength of incident light, these particles will behave as scattering centers, changing the direction of incident light and roughening the surface.

Different optoelectronic devices require different surface morphologies; some need a flat surface to avoid scattering and retain their electrical properties, while others need a moderately rough surface to enhance the light extraction or increase the optical path through a scattering effect.”Therefore, in their experiments, the researchers prepared both types of system readily through the selection of the types and size of the nanoparticles and their subsequent spincoating onto the substrates.Chen points out that the thickness of each spin-coated layer is critical and can be controlled by the rotational speed. “However, it took time to optimize them because we constructed multi-layers. The interference between individual layers as well as the solvent we chose would influence the thickness and roughness significantly. Fortunately, we were able to overcome these problems.”Because spin-coating is a rapid and cheap process, the main issue in this work was to choose nanoparticles with suitable refractive indices.

As long the refractive indices of the materials of optoelectronic devices are known, it is not difficult to coat nanoparticle stacks onto them to reduce the interfacial reflectance.The result of the team’s study is a novel strategy for arranging dielectric nanoparticles of various types and sizes to enhance both the omnidirectional light harvesting of solar cells and the light extraction of LEDs; and because the fabrication approach involves rapid and simple spin-coating and does not require any etching, the electrical properties of the devices will remain unimpaired.

“The underlying strategy of our process is to match the optical constants while considering the effects of scattering under different wavelengths,” Chen sums up the study. “We are convinced that our nanoparticle-based technique has great potential for application to other optoelectronic devices, including thin-film solar cells, organic solar cells, transparent conductors, and OLEDs.

By Michael Berger. Copyright © Nanowerk

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