Just what is a plasmon anyway?
Modern science, engineering, and medicine rely on accurate instrumentation that allows us to detect things of interest in ever-finer detail. In medicine, this involves seeing through the human body, often to find things that are almost indistinguishable from the surrounding tissue. In chemistry and environmental monitoring, it means detecting the presence of contaminants, pollutants, and reactants at very low concentrations. Then there is imaging, where we want to see details as fine as possible while disturbing the sample as little as possible. In recent years, a new technology has started to reshape all of these fields. It is the field of nanophotonics, and its products are coming to a hospital near you.
At a recent meeting of the MESA+ Research Institute for Nanotechnology, Ars sat down with Naomi Halas to get the low down on nanophotonics, and the strange beasts it enables, called plasmons.
What is nanophotonics?
Nanophotonics is research dedicated to understanding and using light on ever-smaller scales. Generally, we play with light that lies in the visible to the near-infrared (just slightly redder than the eye can see). These colors are convenient to generate and easy to manipulate, but their length scale is about one micrometer. This means that it is really difficult to manipulate this light (and get the light to manipulate objects) on the nanometer scale.
Nanophotonics is about understanding the fundamental physics involved in light fields with nanometer scale features and the development of techniques to generate those light fields. One key to these developments is an understanding of the interaction of light and metal.
Metals are special because their conductivity comes from a sea of electrons floating freely through a crystalline lattice of metal atoms. Light fields interact with these electrons by making them move and, when circumstances are right, this interaction can be extremely strong. An example of this interaction is a surface plasmon resonance, where the light field causes a huge number of free electrons to move together.
Because the electrons move as a group—something called coherence—and in large numbers, enormous electric fields build up and oscillate at the same frequency/color as the light that created the field. However, unlike the light field that created the surface plasmon resonance, these fields only extend a few nanometers from the plasmon. This creates, in principle, exactly what we want from nanophotonics: the ability to create light fields that have features just a few nanometers across and strong enough to be useful.
This glass is red because the gold nanoparticles absorb green light.
As you might expect, there is more to the story than simply making a plasmon. If plasmons are so easy to generate, why don't we see them forming naturally—metals behaving strangely and absorbing huge amounts of light and glowing with strange colors. Sometimes you do: the red color of some glasses is a result of the plasmon resonance from very small gold particles.
For the most part, however, this isn't the case. Like everything in physics, energy and momentum must be conserved, which is why the plasmon oscillates at the same frequency as the exciting light field—energy is conserved, and the universe can breathe a sigh of relief. Momentum, on the other hand, is more problematic. In fact, it is probably the hardest thing to explain to non-physicists. Momentum for waves is not about mass and velocity, its about wavelength and the medium through which the wave propagates. So, although energy conservation fixes the frequency, the medium (a metal) and the wavelength that the electrons want to oscillate at don't generally match up with that of the incident light.
Does this mean we can't really control the plasmon's properties? The wave nature of the phenomena come to our rescue again, because waves also interact with the shape and structure of the medium through which they propagate. This structure can make a significant difference, allowing light to excite plasmons that it otherwise couldn't. A consequence of this is that researchers can create structures that support the particular electric field shape and frequency that they desire. Much of nanophotonics involves using the right shaped metallic structure to confine light fields in the desired way.