Summary:Metamaterials are tailor-made photonic composites--combinations of materials designed to achieve optical properties not seen in nature. The properties stem from the unique structure of the composites, with features smaller than the wavelength of light separated by sub-wavelength distances. By fabricating such metamaterials, researchers have overcome fundamental limits tied to the wavelength of light. Light hitting a metamaterial is transformed into electromagnetic waves of a different variety—surface plasmon polaritons, which are shorter in wavelength than the incident light. This transformation leads to unusual and counterintuitive properties that might be harnessed for practical use. This project is developing new approaches to more simply fabricate metamaterials, and making new structures specifically designed to enable measurements of their strange properties. We are also exploring nanotechnology applications of these nanostructures, including microscopy beyond the diffraction limit. Description:Plasmonic materials are composed of metals and insulators that are ordered in geometric arrangements with dimensions that are fractions of the wavelength of light. Research groups are experimenting with a variety of geometric approaches, but all aim Plasmonic metamaterials are incarnations of materials first proposed by a Russian theorist in 1967. Also known as left-handed or negative index materials, the proposed materials were theorized to exhibit optical properties opposite to those of glass, air, and the other right-handed—or positive index—materials of our everyday world. In particular, energy is transported in a direction opposite to that of propagating wavefronts, rather than traveling in lockstep, as is the case in positive index materials. As a result, when juxtaposed with a positive index material, negative index materials were predicted to exhibit counterintuitive properties, like bending, or refracting, light in unnatural ways. Normally, light traveling from, say, air into water bends upon passing through the normal (a plane perpendicular to the surface) and entering the water. In contrast, light beaming from air toward a negative index material would not cross the normal. Rather, it would bend the opposite way, or the “wrong way,” as some have described the unnatural effect. Negative refraction was first reported for microwaves and infrared radiation. In 2007, in collaboration with the team of Harry Atwater at the California Institute of Technology, we were the first to report negative refraction of visible light in two dimensions. Our material platform is a sandwich-like construction with exceedingly thin layers. It consists of an insulating sheet of silicon nitride topped by a film of silver and underlain by gold. The critical dimension is the thickness of the layers, which taken together are only a fraction of the wavelength of blue and green light. By incorporating this metamaterial into an integrated-optics “lab on a chip,” we have been able to demonstrate negative refraction, in one plane, over a broad range of blue and green frequencies. Our system exploits the bulk materials properties of each component, but the collective result is an outsize response to light. Incident light couples with the undulating, gas-like charges normally on the surface of metals. This photon-plasmon interaction results in SPPs that generate intense, localized optical fields. The waves are confined to the interface between metal and insulator. This narrow channel serves as a transformative guide that, in effect, traps, squeezes, and compresses the wavelength of incoming light. We are using computer simulations to design plasmonic metamaterials with a negative index in three dimensions. The experimental composites will be made using a variety of fabrication methods, including multilayer thin-film deposition, and focused-ion-beam milling. Using these techniques we have recently engineered the first three-dimensional, all-angle metamaterial with a negative index of refraction in the ultraviolet. Besides characterizing 3D negative refraction at visible frequencies, we are fabricating nanomechanical systems incorporating metamaterials specifically designed to reveal one of the most unusual of the predicted properties of metamaterials, negative radiation pressure. Light falling on conventional materials, with a positive index of refraction, exerts a positive pressure, meaning that it can push an object away from the light source. In contrast, illuminating negative index metamaterials should generate a negative pressure that pulls an object toward light. Plasmonic negative-index metamaterials also have inspired efforts to achieve what once was dismissed as impossible: visible-light imaging of molecular and atomic scale objects. A theorized “superlens” could exceed the diffraction limit, which prevents positive-index lenses from resolving objects small than one-half of the wavelength of visible light. Because plasmonic materials can literally pinch light to a fraction of its original wavelength, a superlens would capture subwavelength spatial information that is beyond the view of conventional optical microscopes. We are exploring several approaches to building a non-diffraction-limited optical microscope based on the superlens concept. As a first step towards that goal we have recently demonstrated that a flat slab of our ultraviolet metamaterial is able to perform far-field (“Veselago”) imaging of arbitrarily-shaped two-dimensional objects. In conjunction, we plan to develop optical switches, modulators, photodetectors, and directional light emitters, also based on plasmonic metamaterials. Other proof-of-concept applications that we are currently exploring include high sensitivity biological and chemical sensing. To achieve this goal, we are developing optical sensors which exploit super-confinement of surface plasmons within high-quality-factor Fabry-Perot nano-resonators. This tailored confinement will allow efficient detection of specific binding of target chemical or biological analyte molecules because of the strong spatial overlap between the optical resonator mode and the analyte ligands bound to the cavity sidewalls. Structures are optimized using finite-difference-time-domain electromagnetic simulations, fabricated using a combination of electron-beam lithography and electroplating, and tested using both near-field and far-field optical microscopy and spectroscopy. In addition, a high-efficiency, fast optical plasmonic modulator based on an electrochemical switching of an electrochromic polymer has recently been developed. Selected Publications:
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domingo, 9 de agosto de 2015
Nanoplasmonics and Three-Dimensional Plasmonic Metamaterials
domingo, 18 de janeiro de 2015
Breakthrough lights up metamaterials
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This is a schematic of active metamaterial which shows enhanced light emission and extraction. A City College of New York led-team has successfully demonstrated how to both enhance light emission and capture light from metamaterials embedded with light emitting nanocrystals. The breakthrough, headed by physicist Dr. Vinod Menon, could lead to a range of applications including ultrafast LEDs, nanoscale lasers and efficient single photon sources. In the demonstration, the team used metamaterials having hyperbolic dispersion to enhance the light emission properties of the nanocrystals and simultaneously engineered an efficient light extraction scheme. "The idea of metamaterials in the context of optics is that you can manipulate light and decide how you want it to behave in this medium," said Professor Menon a photonics expert whose specialty includes control of light-matter interaction at the nanoscale. While enhancement in light emission from such systems was shown by different groups, including Dr. Menon's, in the past, they were not useful since light did not come out easily from them, making their practical application an issue. The present work alleviates this issue and takes the first step towards developing practical light emitters based on metamaterials. "We've shown both an increase in light emission and were able to extract light." added Professor Menon, whose team included City College PhD students Tal Galfsky and H.N.S. Krishnamoothy Also part of the research team were scientists from the University of Alberta (Canada), Purdue University in Indiana. Part of the research was also carried out at the Center for Functional Nanomaterials at Brookhaven National Laboratory. The paper appears in the latest issue of Optica. Story Source: The above story is based on materials provided by City College of New York. Note: Materials may be edited for content and length. Journal Reference:
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sábado, 10 de maio de 2014
Making it big: Metamaterial applications a step closer to reality
May 9, 2014
The Agency for Science, Technology and Research (A*STAR)
The use of a fabrication technique borrowed from the semiconductor industry brings metamaterial applications a step closer to reality. Artificial materials engineered to have properties not found in nature, such as a negative refractive index are engineered to interact with light and sound waves in ways that natural materials cannot. They thus have the potential to be used in exciting new applications, such as invisibility cloaks, high-resolution lenses, efficient and compact antennas, and highly sensitive sensors.
Metamaterials are engineered to interact with light and sound waves in ways that natural materials cannot. They thus have the potential to be used in exciting new applications, such as invisibility cloaks, high-resolution lenses, efficient and compact antennas, and highly sensitive sensors.
While the theory of this interaction is relatively well understood, it has been challenging to fabricate metamaterials that are large enough to be practical. Now, Yi Zhou and colleagues at the A*STAR Data Storage Institute in Singapore have demonstrated a promising new fabrication technique that can produce large areas of an important class of metamaterial, known as fishnet metamaterials1.
Most optical metamaterials consist of tiny repeated metallic structures. When light of a particular frequency falls on them, it establishes oscillating fields inside each structure. These fields can resonate with each other and thereby produce desirable collective behavior. Fishnet metamaterials usually have several vertically stacked repeat units spread out over much larger lateral dimensions. Because they are structured both vertically and laterally, they are called three-dimensional materials.
Fishnet metamaterials are usually made in one of two ways. They can be fabricated by carefully patterning individual films and then stacking these films on top of each other. However, this multilayer process is difficult, as it requires careful alignment of the films.
The second approach is to pattern a sacrificial substrate and then deposit repeated layers onto it. This 'pattern-first' process suffers from its own difficulties, the most important of which is that the total thickness of the final fishnet material is typically limited to tens of nanometers or less. This restricts the kind of resonances that can be achieved and, in turn, the functionality of the final film.
Zhou and colleagues were able to increase the total thickness of pattern-first fishnet films to around 300 nanometers, allowing five bilayers of film to be deposited and resulting in a strong characteristic resonance and pronounced metamaterial behavior. To achieve this, they adopted a technique called trilayer lift-off, which is commonly used in industry but seldom applied in research laboratories. It involves patterning a sacrificial layer of a photoresist resting on a layer of silicon dioxide under which lies a second photoresist layer.
By alternating the patterning and etching steps, the A*STAR team could achieve a film thickness greatly exceeding the size of the lateral patterns etched into the film. "This technique will help researchers design large-area three-dimensional nanodevices more easily," says Zhou, "and help bring the science of metamaterials to reality."
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The above story is based on materials provided by The Agency for Science, Technology and Research (A*STAR). Note: Materials may be edited for content and length.