One of the first research programs I was working on in at the Institute of Microstructure Technology (IMT)
involved nano-structures. One nano meter is one thousand millionth of a meter, and loosely speaking structures up to a
millionth of a meter are still classed as nano - this is the regime in which I worked; big nano-structures. This size scale is
interesting because it is similar to that of the wavelength of visible or/and infrared light,
and so producing many strange optical phenomena. The butterfly's wing and the piece of opal are great examples of how
structuring on this size scale leads to strong optical effects. The butterfly's wing acts as an optical filter, only
reflecting blue light, producing the vibrant colour. It is important to note that the better the ordering of the structure
(shown in the insets) the stronger the optical effect will be. The opal acts in much the same way as the wing, but has a more
random structure, leading to the reflection of different colours from different places. To study these optical effects it is
important to have a as close to perfect ordering - repeating of a simple shape - as possible.
Another nice example of surface plasmon polaritons (or surface plasmons for short) is the Lycurgus Cup (picture).
It was made in the 4th century AD and appears green under normal lighting. However, when illuminated from the inside it
becomes red in colour. This is a similar effect to the butterfly's wing, in that the colours are being filtered. However, it
was found that the size of the particles of silver and gold inside the glass are approx. 100 times smaller than the wavelength
of light. Something different must be happening compared to the case of the wing. For the wing it was only the size of the
structuring that was important, in the cup, as in all stain glass windows, it is the material that is important.
From everyday experience we know without thinking that metals interact strongly with light - a mirror is maybe a good example:
the light is forced to turn around and travel in the opposite direction whenever it meets a mirrored surface. The reason for this is that
metals have free electrons floating around in them. Light and electrons have an intimate connection due to their electric
fields. For a mirror the oscillation of the light field causes the electrons in the metal to oscillate too. But because the
electrons have some mass (and photons are traditionally said to be massless) it takes some time to get them movin. In the simple case of a mirror, they tend to
oscillate in anti-phase with the optical field (if the field is moving left the electrons are moving right).
This movement stops the light from entering the material. The electrons create a screening field that the light cannot pass
through, and so, since the light has to go somewhere, it is reflected. Here in the case of the cup the surface plasmon
resonances of the metal particles efficiently scatter the green light, giving the cup its usual colour.
This allows only red light to be transmitted, providing the strong contrast in colours that must have amazed the people of Roman
times. The free electrons moving around the atomic nuclei of the metallic particals can only oscillate at certain frequencies corresponding to the allowed
standing waves - this is the simplest definition of a surface plasmon, an oscillation of the free
electrons at the surface of a metal at a certain frequency. At these frequencies the light is highly absorbed and scattered,
leading to a reduced transmission and enhanced reflection, giving the cup its properties. By altering the size of the
metal particle the scattered colours (and therefore transmitted colours) are altered. For surfaces things are a little
different, but the same general rules apply: structuring can cause standing waves in the free electrons of a metal, and this
can create surface plasmons. Since transmission is impossible in this case (there is a block of metal under the surface),
the plasmon will be observable as an absorption of a certain wavelength/colour of the reflected light.
For a more detailed analysis go here:
- Surface Plasmon Experiment
- Surface Plasmon and ğradiativeĞ Surface Waves for the NANO to Terahertz Region
...to be continued...