Friday, November 5, 2010

Plasmonics at the bottom? Part one.

According to Thomas Friedman, one reason why the world got flat is the mass installation of optical fibers that now span the globe; fibers that guide light signals conveying voluminous streams of voice communications and gigantic amounts of data.  Some researchers believe that such colossal capacity in photonic devices (able to channel and manipulate electromagnetic radiation like light) could someday replace electronic circuits in microprocessors and other computer chips.  Why haven't they already?  Because of the diffraction limit, which constrains the size and performance of photonic devices due to interference between closely-spaced light waves.  The width of an optical fiber carrying closely space light waves must be at least half the light's wavelength inside the material.  Chip-based optical signals usually employ near infared wavelengths of 1.5 ┬Ám (micrometers, or millionths of meter–a common unit in cell biology), which far exceeds typical dimensions of base components in most electronic devices being used today–certainly in my computer.

However, scientists have kept busy.  In the 1980s, researhers experimentally confirmed that directing light waves at the interface between a meter and a dielectric (a nonconductive material such as air, glass, or highly pure water) can, if conditions are right, induce a resonant interaction between the waves and the mobile electrons at the metal's surface.  This means that the oscillations of electrons at the surface match those of the electromagnetic field (read: waves) outside the metal.  We now have surface plasmons:  density waves of electrons that propagate along the interface like the ripples that spread across the surface of a pond after a rock splashes into it. 

Recently, scientists discovered that a fine-tuned metal-dielectric interface can generate surface plasmons with the same frequency as the outside electromagnetic (EM) waves, but with a much shorter wavelength.  This effect may enable the plasmons to travel along nanoscale wires called interconnects, carrying information from one section of a microprocessor to another.  I could sense chip designers salivating over these interconnects; they're still making ever smaller and faster transistors, but it's now harder to to build minute electronic circuits that can move data quickly across the chip.

Ten years ago, Professor Atwater and colleagues at Caltech named this emerging discipline "plasmonics", sensing a research pathway may lead to a new class of devices.  One day, plasmonic components might be vital in a diverse set of instruments, using them to improve the resolution of microscopes, LED efficiency, and the sensitivity for chemical and biological detectors.  Plasmonics are even being considered for medical applications; e.g., tiny particles designed for plasmon resonance absorbion to kill cancerous tissues.  Then there's the invisibility cloaks, but that'll be discussed in a future entry, which will be quite soon.

In meantime, here's a nice artist's rendering of a light beam striking a metal surface, thereby generating a plasmon, which is also awkwardly called an electron density wave.  If the light beam is focused on a surface with a circular grove, as shown here, it produces concentric waves, organizing the electrons into high- and low-density rings.