Storing Digital Data Inside Glass

21 March 2013

Interview with

Eric Mazur, Harvard University

The so called 'digital dark age' that archivists are warning of does mean that the search is on for a store and ignore technology to allow us to preserve digital data into the long term. One team from Harvard University led by Professor Eric Mazur has invented a process that can, they say, "permanently store digital data inside glass"

Dave - So Eric, how does this process actually work?

Eric - It works by taking extremely short, but intense laser pulses and focusing them into the bulk of the glass. Not on the surface, but in the bulk of it and essentially, creating a tiny void that can serve the same role as changing the dye at the surface of a DVD or a CD.

Chris - But if you vaporise the glass on the inside, I don't understand where the glass that you vaporise goes to make the hole.

Eric - Well, if you take any material, it's made up of atoms and atoms themselves are mostly nothing. The nucleus of an atom is tiny, tiny compared to the actual size of the atom. So, in the lattice of any material, glass happens not to have a lattice. It's amorphous, but there's plenty of interstitial, another place to drive additional materials. So basically, what happens is that in that tiny little void that you've created with this super intense laser pulse, the atoms have basically been driven into the surrounding material.

Chris - So, you get a little sort of hole which behaves like you said the pit that you would write into a DVD or a CD. So, do you use the same sort of data encoding then to put the data into these little pits or you're using your own proprietary way of encoding the data?

Eric - No, we would use the standard technique and in fact, we've only demonstrated the technique. We've not gone as far as commercialising it, but you would, I think as a first step simply use the same method that you use at the surface of a commercial CD or DVD. But now, instead of writing just a single layer, or in the case of a multi-layer DVD, maybe 2 or 3, you could write in the thickness of a standard CD or DVD 100 or more layers.

Dave - So, how do you cause it to form this little pits inside the glass at a certain depth and not higher up because surely the lights got to go through some glass to get there?

Eric - Exactly, so the trick is in exploiting this extremely high intensity of light that occurs with pulses that have durations that are measured in what are called femtoseconds and millionth of a billionth of a second. When you make your laser pulse that short, the intensity of the light becomes phenomenal because you're basically focusing the light in time if you wish. If you now also focus it in space using a lens, you essentially put a whole lot of light on top of each other, you basically bunch up the light at the focus of this lens both in space, transverse as well as in time in the longitudinal direction. And as a consequence, the interaction of the light and matter becomes very different from the one we're normally accustomed to.

If you take glass for example, we use glass as windows because it's Argon-ion and He-Ne laser beamstransparent which means by definition, the light is not interacting with the glass. It goes through it. But when you get to this exceptionally high intensity, and I'm talking an intensity where the electric field in the light - because light is electromagnetic wave - becomes larger than the electric field that binds electrons to atoms. When you get to that regime, you basically change the nature of the interaction between the light and the material completely and you are actually able to rip apart the bonds between the atoms and deposit energy in that tiny little volume. It's tiny because it is completely limited to the volume where you focus the light. So, if I place the focus of my laser beam, not at the surface but inside the glass, I could not do this with a material that's not transparent obviously because then absorbtion would start at the surface. But if I think glass or plastic or any other transparent material, the absorption will only happen in a very tiny volume at the focus of the laser beam.

Chris - Eric, some people say that glass is a bit like a super cooled liquid so it flows over time. Does this mean that the glass could move over time and occlude these tiny holes you've made and it's then your data would disappear?

Eric - That is definitely, true. Nothing lasts forever, right? I mean, we know that. However, there are certainly glass specimens that have lasted over thousand years. We still have glass objects from the Romans that is in very good shape. I think it would extend the lifetime of most recording media by several orders of magnitude and I think a hundred or several hundred years would be probably a safe estimate for a reliable readout of any data stored in this way. 

Chris - And I presume that the really good thing here is that because the data is being stored inside the glass rather than on the surface of the glass, like on the surface of a record, it means that surface abrasion or damage does not damage the integrity of the data because you can read through that and see the message still written inside.

Eric - Correct. You could always repolish the outside to get a clean surface again.

Chris - So, how much data can you pack into a certain area or unit area of glass?

Eric - Well, as I said, I mean, you can easily pack in a hundred - in a standard platter, the size of a CD or DVD, you could easily put 100 to 200 layers. So imagine, putting the equivalent of 200 DVDs in a single DVD.

Chris - And if you had much interest from anyone saying that they like to try and use this commercially or is this literally just rooted in the academic world at the moment as an interesting thing you can do?

Eric - No, we certainly have had interest. There are still a number of technological hurdles. One is that the laser that is used to write is not something that is the size of a laser that you would find in a DVD or CD writer. So, this would not easily become a consumer item, but it could become an item for archival data storage. The other one is that writing the data is a lengthy process, given the current state of femtosecond lasers. Although, that is improving dramatically over the past few years and I could very well see that in a few years' time we could actually have a laser that could write 200 DVDs in a relatively short amount of time, meaning, something like minutes rather than hours or days.

The other problem is of course, that right now, magnetic and other type of semiconductor media are so cheap that writing the data in magnetic media and then copying them if you want to preserve them, often is a more cost effective technique than trying to exploit a new and as yet unproven technique.

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