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Plasmonics propagates into new optical fields

04 Sep 2008

This month's topic is the promising field of plasmonics. Marie Freebody speaks to Niek van Hulst about the potential wealth of applications that are on the horizon.

Niek van Hulst heads up the Molecular Nano-Photonics group at the Institute of Photonic Sciences (ICFO) in Barcelona, Spain. His current research projects include optical field enhancement and probing light propagation on the nanoscale, using molecules and antennas as local probes. His work on nanoantennas has brought to light the interaction between plasmonic antennas and single molecules, where antenna-controlled emission could lead to simpler more-efficient biosensing systems.

Can you summarize what the field of plasmonics encompasses?
Plasmonics is the science, technology and application of plasmons, which are the collective oscillations of a free electron gas (plasma). Plasmons are mainly found in metals and often at optical frequencies. Fundamentally, a plasmon is a quasiparticle, a quantum of a plasma oscillation, as much as a photon is a quantization of light. In practice, the connection to classical plasma oscillations allows us to describe most of the properties of plasmons directly by Maxwell's Equations. The coupling between photons and plasmons is particularly interesting. The resulting particle is called a plasma polariton, which can propagate along the surface of a metal until it decays by absorption or radiatively into a photon. These surface plasma polaritons, or surface plasmons, allow light energy to be carried along a surface in the form of collective electron motions.

Why is plasmonics an important area of research?
Plasmons carry light energy as a package of electron oscillations. This means that their behaviour deviates from the normal rules associated with photons. Plasmonic structures can exert huge control over electromagnetic waves at the nanoscale. The peculiar dispersion of plasmons enables excitation of modes with very large wavevectors in only a narrow frequency range. As a result, energy carried by plasmons allows for light localization in ultrasmall volumes, far beyond the diffraction limit of light. At the same time, the very flat dispersion allows for extremely slow light when relying on plasmon propagation. The localized nanoscale fields come together with large field enhancements, which is a major advantage for sensing, imaging and spectroscopy applications.

What are the main applications and when do you expect them to occur?
Applications mainly depend on controlling the losses and the cost of nanofabrication techniques. Plasmonic biosensors and surface plasmon array biosensors (sensor chips) do exist. Novel sensors will exploit nanoscale dispersion control and nanometric volumes, allowing improved sensitivity even at higher background levels. Other promising areas include optical imaging systems with nanometre-scale resolution, hybrid photonic–plasmonic devices and negative-index metamaterials. Enhanced and directed emission of semiconductor luminescence (e.g. quantum dots) may well find commercial application in plasmon-assisted lighting in a couple of years, while plasmon antennas that enhance light capture could play a role in the harvesting of sunlight. Finally plasmonic nanocircuits combine a large bandwidth with a high level of compaction and make plasmonic components promising for all-optical circuits.

What is the most important recent advance in the field of plasmonics?
The most important recent advances are the coherent control of nanoscale localized optical fields and the efficient local trapping of particles by plasmonic forces. Other important advances include demonstrators of functional plasmonic circuits and the applications of optical antennas in nanoscale sensing and imaging.

During the 1960s and 1970s it was found that resonant surface plasma oscillations are very sensitive to any change at the interface, such as the adsorption of molecules to the metal surface, paving the way to surface plasmon resonance (SPR) sensing. Today SPR is still the basis of many standard tools for measuring adsorption of material onto surfaces, particularly in commercial biosensor applications and various lab-on-a-chip sensors. With the advent of nanotechnology in the late 1990s, plasmons regained interest. Thanks to nanostructuring methods, such as e-beam lithography, ion-beam milling and nanoimprinting, we can engineer local plasmon resonances at will. We can also guide surface plasmons, tune plasmon dispersion, create localized nanoscale fields and focus light energy.

What are the key challenges left to overcome in this field?
Plasmonic resonances depend on the type, shape and size of the material on the nanometre level for optical frequencies. The plasmonic properties suffer appreciably from intrinsic losses of metals, which is the main limitation. This means that reproducible nanofabrication techniques, crystalline materials and inexpensive replication are all key issues.

What do you think the next big breakthrough will be?
We may soon witness interesting scientific progress in plasmonic lasers and plasmon-assisted quantum optics. In the long-term we will see wider applications thanks to nanocircuitry, energy harvesting, nanoscale imaging and sensing.

• This article originally appeared in the September 2008 issue of Optics & Laser Europe magazine.

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