Seeing light by electrons
"Fiat Lux - he saw the light and it was good.......". Indeed light is at the basis of our sensations and observations. Yet, oddly enough, light has its limits in seeing how light and matter interact. Specifically, the behaviour of light at the nanoscale is hard to discern by light microscopy. Here we show that electrons are an effective alternative to image light at the true nanoscale. Exciting by electrons, while still detecting light, we have probed the inside of photonic crystals and mapped how light behaves with a spatial resolution of 30 nanometer, far below the wavelength of light.
Photonic crystals are nanostructures in which two materials with different refractive index are arranged in a regular pattern, giving rise to exotic optical properties. Natural photonic crystals can be found in certain species of butterflies, birds and beetles as well as in opal gemstones where they give rise to beautiful iridescent colors. Due to major advances in nanofabrication techniques it has become possible to fabricate artificial photonic crystals with optical properties that can be accurately engineered. These structures can be used to make high-quality nanoscale optical waveguides and cavities, which are important in telecommunication and sensing applications.
We constructed 2-dimensional photonic crystals by etching a hexagonal pattern of holes in a very thin silicon nitride membrane. The photonic crystal inhibits light propagation for certain colors of light, which leads to strong reflection of those colors. By leaving out one hole a very small cavity can be defined where the surrounding crystal acts as a mirror for the light, making it possible to strongly confine light within such a "crystal defect cavity".
Using the electron approach we can now see the finest details of photonic crystals that were simply inaccessible before. Seeing where the various colours are trapped in the cavity provides direct insight into the light - matter interaction on the nanoscale. Such understanding is crucial for the development of enhanced optical devices such as bio-sensors for healthcare and more efficient solar cells and displays.