Two-Dimensional Light

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.