Though DVDs have been consumer-grade tech for years, it wasn’t until yesterday that scientists announced they were certain why they worked. Now we know that burning a DVD is really more like freezing it.
DVD burning image by Filipe La Rotta.
It’s been well known for decades that if you train a laser onto a specially-crafted layer of metal alloys on an optical disk, you create a storage medium that can hold an enormous amount of data for decades – and possibly centuries. But what’s going on when the laser hits metal?
An international group of scientists based in Germany have been puzzling out that problem for several years. They’re using one of the world’s biggest supercomputers, JUGENE, to simulate what happens – nanosecond by nanosecond – when a laser hits the thin layer that gleams on the surface of a pristine DVD. For their most recent experiment, the group focused on one of the most popular alloys, or metal mixes, used to make DVDs. This alloy is nicknamed AIST, after its elements – AIST contains small amounts of silver (Ag), indium (In), antimony (Sb), and tellurium (Te). Like antimony (pictured at left), all these elements are very reflective, and together they give your DVDs that lustrous shine.
These metals also share a very special property. AIST alloys begin as what physicists call “amorphous,” or “disorganized,” meaning that their structure is very haphazard. In the diagram below, you can see AIST’s amorphous structure in the bottom left. But when you zap them with a laser, AIST molecules align into a perfect, crystalline lattice in a nanosecond (bottom right). It turns out that the element antimony can switch its molecular bonds around quickly, despite having no extra space to move (top right and top left in the diagram show the fast bond switches, after the wavy red laser beam hits).
In nature, of course, we see phase changes like this all the time. When water freezes, it goes from a disordered liquid to a crystalline solid. What makes AIST special is that it moves between two solid states: when you burn a DVD, you’re turning its surface from an amorphous solid into a crystalline solid. Your DVD is frozen into a crystalline order with the heat of a laser.
Though it’s hard to get more awesome than that, this breakthrough in our understanding of DVDs is just the beginning. The researchers say their work – which is also applicable to the alloys used in Blu-Rays – is going to pave the way for even better forms of optical storage, with longer life, larger storage capacities, and shorter access times.