Discovery Of Plasmon

The discovery of plasmon opened up a whole new physical field of research coined “plasmonics” by Harry A. Atwater, a Howard Hughes Professor and Professor of Applied Physics and Materials Science at the California Institute of Technology and his research group and it was in the year 2000 that they officially gave the name to this emerging discipline, sensing that research in this area could lead to an entirely new class of devices.

The field of plasmonics , which is even younger than spintronics, involves the transfer of light electromagnetic energy into a tiny volume, thus creating intense electric fields — a phenomenon that has many scientists rethinking the laws of electromagnetics on a nanoscale. The plasmonics field has many wide-ranging applications, from guiding light through metal wires, to bio-sensing, to making objects invisible to the eye.

Plasmon is one of the 7 members of the Quasiparticles (Davydov soliton, Exciton, Magnon, Phonon, Plasmon, Polariton and Polaron), an idea originated by the Soviet Physicist Lev Davidovich Landau in his theory of Fermi liquids which was introduced in 1956, which was originally invented for studying liquid helium-3.

The dynamics of Landau’s theory is defined by a kinetic equation of the mean-field type. A similar equation, the Vlasov Equation, is valid for a plasma in the so-called plasma approximation, in which charged particles are considered moving in the electromagnetic field collectively generated by all other particles, and hard collisions between the charged particles are neglected. When a kinetic equation of the mean-field type is a valid first-order description of a system, second-order corrections determine the entropy production, and generally take the form of a Boltzmann-type collision term, in which figure only “far collisions” between virtual particles. In other words, every type of mean-field kinetic equation, and in fact every mean-field theory, involves a quasi-particle concept.

Phonons are the quanta of classical sound waves and sound waves do not need the notion of atoms. Magnons are the quanta of classical spinwaves, which also do not need elementary spins. Photons inside an isolator are the quanta of classical dressed electromagnetic waves and do not need the notion of electrons for the definition of the refractive index. Plasmons are the quanta of the plasma oscillations and they only need charge density and mass density and no electrons or ions. Polarons are the quanta of the oscillating polarization in a lightly doped semiconductor and also do not need elementary charge or mass.

The phenomena of two-dimensional light, or plasmons, can be triggered when light strikes a patterned metallic surface. Plasmons may well serve as a proxy for bridging the divide between photonics (high throughput of data but also at the relatively large circuit dimensions of one micron, or one thousandth of a millimeter) and electronics (relatively low throughput but tiny dimensions of tens of nanometers, or millionths of a millimeter).

One might be able to establish a hybrid discipline, plasmonics, in which light is first converted into plasmons, which then propagate in a metallic surface but with a wavelength smaller than the original light; the plasmons could then be processed with their own two-dimensional optical components (mirrors, waveguides, lenses, etc.), and later plasmons could be turned back into light or into electric signals.

To show how this field is shaping up, here are a few plasmon results from that great international physics bazaar, the March Meeting of the American Physical Society, which took place last week in Baltimore.

1. Plasmons in biosensors and cancer therapy:

Naomi Halas (Rice University, halas@rice.edu) described how plasmons excited in the surface of tiny gold-coated, rice-grain-shaped particles can act as powerful, localized sources of light for doing spectroscopy on nearby bio-molecules.

The plasmons’ electric fields at the curved ends of the rice are much more intense than those of the laser light used to excite the plasmons, and this greatly improves the speed and accuracy of the spectroscopy. Tuned a different way, plasmons on nanoparticles can be used not just for identification but also for the eradication of cancer cells in rats.

2. Plasmon microscope:

Igor Smolyaninov (University of Maryland, smoly@eng.umd.edu) reported that he and his colleagues were able to image tiny objects lying in a plane with spatial resolution as good as 60 nm (when mathematical tricks are applied, the resolution becomes 30 nm) using plasmons that had been excited in that plane by laser light at a wavelength of 515 nm. In other words, they achieve microscopy with a spatial resolution much better than diffraction would normally allow; furthermore, this is far-field microscopy — the light source doesn’t have to be located less than a light-wavelength away from the object.

This work is essentially a Flatland version of optics. They use 2D plasmon mirrors and lenses to help in the imaging and then conduct plasmons away by a waveguide.

3. Photon-polariton superlensing and giant transmission:

Gennady Shvets (University of Texas, gena@physics.utexas.edu) reported on his use of surface phonons excited by light to achieve super-lens (lensing with flat-panel materials) microscope resolutions as good as one-twentieth of a wavelength in the mid-infrared range of light. He and his colleagues could image subsurface features in a sample, and they observed what they call “giant transmission,” in which light falls on a surface covered with holes much smaller than the wavelength of the light.

Even though the total area of the holes is only 6 percent of the total surface area, 30 percent of the light got through, courtesy of plasmon activity at the holes.

4. Future plasmon circuits at optical frequencies:

Nader Engheta (University of Pennsylvania, engheta@ee.upenn.edu) argued that nano-particles, some supporting plasmon excitations, could be configured to act as nm-sized capacitors, resistors, and inductors — the basic elements of any electrical circuit.

The circuit in this case would be able to operate not at radio (1010 Hz) or microwave (1012 Hz) frequencies but at optical (1015 Hz) frequencies. This would make possible the miniaturization and direct processing of optical signals with nano-antennas, nano-circuit-filters, nano-waveguides, nano-resonators, and may lead to possible applications in nano-computing, nano-storage, molecular signaling, and molecular-optical interfacing.