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.
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
to exploit surface plasmons, which are light-induced packets of electrical charges that collectively oscillate at the surfaces of metals at optical frequencies. Under specific conditions, the incident light couples with the surface plasmons to create self-sustaining, propagating electromagnetic waves known as surface plasmon polaritons (SPPs). Once launched, the SPPs ripple along the metal-dielectric interface and do not stray from this narrow path. Compared with the incident light that triggered the transformation, the SPPs can be much shorter in wavelength.
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.