Na Pohaku ~ Opal: A Photonic Crystal

This opal from South Australia is beautiful… but why does it look like this? It’s actually a photonic crystal!

Opal is made of small spheres of silicon dioxide, the same stuff as quartz. These spheres are between 150 to 300 nanometers in diameter, and they’re arranged in a regular pattern.

When light waves pass through an opal, funny stuff happens when the distance between these spheres is half the wavelength of the light. Imagine water waves trying to fight their way through a regular pattern of round logs standing in the water and you’ll get the picture. The waves start to bounce back and interfere with themselves.

So, light of certain wavelengths winds up bouncing back off the opal instead of going through! Since the opal is not perfectly the same everywhere, different parts reflect different colors of light.

When a repeating pattern strongly affects the behavior of light, it’s called a photonic crystal. Nowadays people are making artificial photonic crystals. Way back in 1887, the English physicist Lord Rayleigh experimented with stacking layers of different materials to make light of certain wavelengths interfere with itself and bounce back. When this happens, we now call it a photonic band gap. But Rayleigh only got this effect to occur in a single direction – at right angles to the layers.

Now people can make 3d arrays of tiny spheres that act as photonic crystals. We might be able to use them to make color-changing paints and inks. But the first commercial products involving photonic crystals are photonic-crystal fibers – a bit like ordinary optical fibers, but different.

Photonic crystals are one example of a metamaterial – a material whose optical or electronic properties depend not just on their composition, but from carefully designed tiny structures.

In the Middle Ages, some thought an opal could make you invisible if you wrapped it in a fresh bay leaf and held it in your hand. Now people are designing metamaterials that can act like a ‘cloak of invisibility’. But more about that later.

Puzzle: I said opal is made of tiny spheres of silicon dioxide (called silica), but what’s in the spaces between these spheres?

I don’t know – I only have a clue. Wikipedia says opal is a hydrated form of amorphous silica, usually containing between 6% and 10% water. Maybe the spaces between the spheres is a mix of silica and water?

For more see:

https://en.wikipedia.org/wiki/Opal
https://en.wikipedia.org/wiki/Photonic_crystal
https://en.wikipedia.org/wiki/Metamaterial

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