Mystery of How Light Beams Become Frozen in Azobenzene

In many ways, azobenzene is an ordinary kind of organic molecule. It consists of two benzene rings joined together by a pair of double-bonded nitrogen atoms. This gives it a kind of butterfly-like appearance—a pair of benzene ‘wings’ with the nitrogens acting as a hinge.

What’s unusual about azobenzene is that when it is bombarded with light of just the right frequency, the wings really do flap. The light causes the relative orientation of the two wings to change, producing a kind of flapping motion. Light of another frequency switches the molecule back to its original state.

Chemists call this kind of change cis-trans isomerisation and it is relatively common in organic molecules. But in azobenzene, it leads to a unique and extraordinary effect.

When a beam of laser light hits a thin film of azobenzene, it disrupts the surface, rather like the surface of water blown with a straw. Because of this, when the light is turned off, its field structure ends up imprinted on the surface.

Chemists have exploited this effect to create a variety of shapes and structures on the surface of azo-polymer films. These shapes can later be wiped out by bathing the surface in ordinary white light, which allows the surface to relax. Chemists are excited by all this because it has important potential applications in the fields such as photolithography.

But here’s the thing: nobody is quite sure how light influences the bulk properties of the film in this way. Most polymer engineers think it must be related to cis-trans isomerisation. But how does the simple flapping of individual molecules cause the mass movement of the substance as a whole?

Today, the mystery deepens thanks to the work of Alexandre Dubrovkin at the University of Angers in France and a few pals. These guys have exploited this effect to drill regular holes through a thin azopolymer film and have even created doughnut-shaped structures that could serve as photon traps in nanophotonic devices.

But here’s the puzzle. These guys have achieved their trick using ordinary white light rather than laser light tuned to the frequency that causes the trans-cis isomerisation.

That’s strange because in previous experiments, chemists have used white light to erase the surface structures generated with laser light, as if white light allows the polymer to relax back into its original state.

So how white light might also push the polymer is unknown. Indeed, it has left chemists scratching their heads over exactly how light interacts with azopolymers.

The technique for creating the holes is relatively simple. These guys create a thin film of azopolymer by placing a drop on a disc and spinning it until a thin film has spread out across it.

They then zap the film with a thin beam of white light from a xenon halogen lamp. They show that this produces holes of specific size for a given depth of film. The holes are about 400 nanometres across and 10 nanometres deep. That’s important because it means the technique is easily repeatable.

One interesting feature of the light-pushing process is that the volume of the film is preserved. So this produces doughnut-like structures around the holes.

These nanowells and nanodoughnuts should be useful. Dubrovnik and co say there is “a rich field for applications in nanophotonics and photochemistry.”

And these structures are certainly easier to produce in this way than with other techniques such as electron beam bombardment, which requires a vacuum, or techniques that rely on laser light which are sensitive to polarisation.

But one question remains outstanding: how does white light do this to azopolymer films? Until chemists and physicists can work this out, the ability to exploit the phenomenon will always be limited.

Answers in the comments sections please.

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