Nanoplasmonics

Scanning-electron micrograph of plasmonic nano-resonator milled by focused-ion-beam in single-crystalline gold
(C. Ross et al., Nano Letters 7, 3612 (2007))

Collective charge oscillations at the boundary between an insulating dielectric medium (such as air or glass) and a metal (such as gold, silver or copper) are able to sustain the propagation of infrared- or visible-frequency electromagnetic waves known as surface-plasmon-polaritons (SPPs). SPPs are guided along metal-dielectric interfaces much in the same way light can be guided by an optical fiber, with the unique characteristic of subwavelength-scale confinement normal to the interface.

Nanofabricated systems which exploit SPPs offer fascinating opportunities for crafting and controlling the propagation of light in matter. In particular, SPPs can be used to channel light efficiently into nanometer-scale volumes, leading to direct modification of mode dispersion properties (substantially shrinking the wavelength of light and the speed of light pulses for example), as well as huge field enhancements suitable for enabling strong interactions with nonlinear materials. The resulting enhanced sensitivity of light to external parameters (such as, for example, an applied electric field or the dielectric constant of an adsorbed molecular layer) shows great promise for applications in sensing and switching.

Our program involves the design and fabrication of novel components for measurement and communications based on nanoscale plasmonic effects. These devices include ultra-compact plasmonic interferometers for applications such as bio-sensing, optical positioning, and optical switching, as well as individual building blocks (plasmon source, waveguide, and detector) of an exploratory high-bandwidth infrared-frequency plasmonic communications link fully integrated on a silicon chip.

Two-colour all-optical plasmonic modulator based on CdSe quantum dots
(D. Pacifici, H.J. Lezec and H. Atwater, Nature Photonics 1, 402 (2007)).

In addition to building functional devices based on SPPs, we also plan to exploit the dispersion characteristics of surface-plasmon polaritons traveling in confined metallo-dielectric spaces to create photonic materials with artificially tailored bulk optical characteristics, otherwise known as "metamaterials".

When a beam of light enters a material from a vacuum or air at non-normal incidence, it undergoes refraction – a change in its direction of propagation. The angle of refraction depends on the absolute value of the refractive index of the medium, according to Snell’s law. For all naturally occurring substances, the beam is deflected to the opposite side of the normal and the refractive index is taken to be positive. In 1968, V. Veselago studied a theoretical material with simultaneously negative electric permittivity and magnetic permeability, and predicted that it would have a negative index of refraction n. Light crossing a boundary between such a medium and one with a positive index of refraction would display the exotic property of refracting to the same side of the normal. Such a deflection, termed negative refraction, was predicted to lead to a variety of surprising effects, for example focusing with a flat lens. Recently, J. Pendry predicted that a negative-index flat lens could lead to imaging with resolution well below the diffraction limit.

Considerable research is now being devoted to developing negative-index metamaterials. Such metamaterials were first implemented at microwave frequencies by means of periodic assemblies of millimeter-scale split ring resonators and wires. Recently, we demonstrated the first direct geometric visualization of negative refraction at visible (blue and green) frequencies in a two-dimensional metamaterial consisting of a plasmonic gold-cladded slot waveguide with nanometer-scale dielectric core thickness. Now we are working to extend the concept and engineer a plasmonic three-dimensional, visible-frequency negative-index metamaterial suitable for advanced measurement applications such as optical microscopy beyond the diffraction limit.

Implementation of the above dimension-critical structures will be enabled by optimization of advanced top-down, direct write nanofabrication techniques such as focused-ion-beam milling. In parallel, a new suite of optical measurement and metrology techniques will be developed to handle the specific challenges of characterizing nanometer-scale plasmonic structures and systems.

Plasmonic waveguide device for demonstration of negative refraction at visible frequencies
(H.J. Lezec, J.A. Dionne, H. Atwater, Science 316, 430 (2007))

Plasmon-interferometers for sensing and switching
Integrated plasmonic communications links on a silicon chip
Three-dimensional plasmonic metamaterial with a negative index of refraction
Focused-ion beam nanofabrication, construction analysis and metrology

Henri Lezec - NIST

Online: December 2007
Last Updated: January 2008

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