PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 770 March 23, 2006 by Phillip F. Schewe, Ben Stein
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 relatively large circuit dimensions---1 micron) and electronics (relatively low throughput but tiny dimensions---tens of nm). 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 APS March Meeting, which took place last week in Baltimore.
- Plasmons in biosensors and cancer therapy: Naomi Halas (Rice Univ) 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's 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.
- Plasmon microscope: Igor Smolyaninov (Univ. Maryland) 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.
- Photon-polariton superlensing and giant transmission: Gennady Shvets (Univ. Texas) 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% of the total surface area, 30% of the light got through, courtesy of plasmon activity at the holes.
- Future plasmon circuits at optical frequencies: Nader Engheta (Univ. Pennsylvania) 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) 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.
NANOPORES AND ZEPTOMOLE BIOLOGY. Some proteins naturally form nanometer-scale pores that serve as channels for useful biochemical ions. Through this ionic communication, nanopores enable many functions in cells, such as allowing nerve cells to communicate
(they are even responsible for twitching the frog leg in Galvani's famous discovery in the 1700s). Nanopores can be destructive too. When the proteins of bacteria and viruses attach to a cell, their
nanopores can facilitate infection, for example by shooting viral DNA through them into the cell. At the APS March Meeting, NIST's John J. Kasianowicz showed how single biological nanopores can be used to detect and characterize individual molecules of RNA and DNA. He also demonstrated constructive uses for anthrax-related nanopores in diagnosing anthrax infections and testing anti-anthrax drugs. Anthrax bacteria secrete a protein called "protective antigen" that attaches to an organic membrane such as a cell wall. The protein forms a nanopore that penetrates the membrane. When another anthrax protein called "lethal factor" attaches to the protective antigen nanopore, it prevents ionic current from flowing through the pore (and out of the
organic membrane). By monitoring animal blood samples for changes in ion current, Kasianowicz and his colleagues at the National Cancer Institute and the United States Army Medical Research Institute for Infectious Diseases electronically detected a complex of two anthrax proteins in less than an hour, as opposed to the existing methods which can take up to several days. Also, they demonstrated a method for screening potential therapeutic agents against anthrax toxins
using the anthrax nanopore (see Anthrax at NIST for a picture and more information).
A Brown University group led by Sean Ling was among those reporting progress in developing a nanopore-based method for sequencing DNA faster and more cheaply than traditional
biochemical techniques. In one scenario the change in ion current as DNA moves through the nanopore could yield the sequence of bases (letters) in the DNA. However, the letters in DNA are so close to each other (about 4 angstroms) and the DNA moves so quickly through the nanopore that researchers have had to come up with creative solutions for reading the individual letters. For example, the Brown group attaches complementary blocks of DNA, about 6 letters long, to the DNA sequence of interest, so that the researchers would read blocks of multiple letters at a time, while slowing down the passage of the DNA by attaching a magnetic bead to it. Other researchers are finding value in developing nanopores for fundamental biology studies. Discussing his group's latest work with artificial, silicon-based nanopores, Cees Dekker of the Delft University of Technology showed how lasers and other manipulations with the artificial pores are enabling new
single-molecule (zeptomolar) biophysics studies on the properties of DNA, RNA, and proteins by studying how they pass through the pores (see www.aip.org/png for an artist's rendering of DNA traversing through a nanopore)