Tuesday, November 21, 2017

The Medium Shrinks the Message: New 4-Color Optical Data Storage

Last weekend I had a second encounter with a popular blurb about a new optical-data-storage technology originating at Case Western Reserve University (my alma mater). A polymer chemist, Emily Pentzer (and co-authors), discovered a way to combine a thermochromic and a photochromic chemical in a polymer film to make four possible colors depending on the stimulation. You can see the essence of the invention in the figure below [1].  The refereed-journal publication is [2].

The promise of the invention is to enable a twofold shrinkage of data storage because you can extract two bits of information (four colors) where previous technologies had enabled only one bit (0 or 1) per storage location.  The medium could be said to shrink the message.

The blurb’s description is a bit cryptic relative to the above goal, so I began a line of investigation that began with pure imagination and ended with obtaining the paper and discussing the matter with its main author.

From [1], I learned that the invention involves a polymer layer that contains two kinds of small molecules in low concentration. Call the two additives P (photochromic, actually o-nitrobenzyl ester of benzoic acid) and T (thermochromic, actually cyano-substituted oligo(p-phenylene vinylene)). When a layer containing P and T (which is tough enough to resist even abrasion by sandpaper) is exposed to no light, it is colorless (black). When that layer is exposed to UV, it fluoresces ultramarine.   When the layer is exposed to heat (perhaps via IR), it fluoresces green. Finally, when the layer is exposed to both heat and uv, it fluoresces cyan.  The colors appear in the figure below. 

To me the text doesn’t describe an encoding system (to which information can be deposited and then retrieved at leisure).  I imagined the following variation of the technology for writing and subsequent reading.  Suppose the stimulation is always a mixture of heat and UV.  If the layer contains neither T nor P, the color is black; if it contains T but not P, the color is green; if it contains P but not T, the color is blue; and if it contains both P and T, the color is light blue.  In this explanation, the information is contained in whether P or T (or both or neither) is applied to the information-storage site.  The stimulating radiation at the moment of retrieval is always the same, because we have no way of knowing in advance which information-laden color will be retrieved.

My departure from [1] was a bad guess, and in retrospect pasting together all those little P and T fragments would be very expensive. I was led to the original paper [2], whose abstract clarifies the mechanism for the write and read algorithms for numbers (0) to (3): “The as-prepared film is non-fluorescent (0), and can be written through a wooden or metal mask with thermal treatment (1), light treatment (2), or both (3), giving three different colours of fluorescence under UV irradiation.”

This explanation still left me wondering how UV can write on the mixture of polymers and yet stops perturbing the medium during the reading process. When you are “reading” the written medium with a UV beam, how do you ensure you don’t change symbol (1) into symbol (3) and symbol (0) into symbol (2)?  In other words, how do you arrange for the medium to be write-once, read-many-times? Accordingly, I conducted a brief e-mail interview with Dr. Pentzer. Here was the essence of it:
Hue Angles: “Am I correct in assuming that the light treatment is UV, and that UV spectrum peaks at 365 nm?”
Pentzer: “Yes, we use a typical hand-held UV lamp like that used to visualize TLC plates.”

Hue Angles: “Is the thermal treatment done with a beam of IR radiation, or do you deliver localized heat with another technology?”

Pentzer: “We actually use a little heat pen. We are currently trying to start a collaboration with engineers/other scientists who can use an IR beam.  We want to combine engineering approaches with our chemistry.”

Hue Angles: “When you are reading the written medium with a UV beam, how do you ensure you don’t change symbol (1) into symbol (3) and symbol (0) into symbol (2)?”

Pentzer: “It really depends on the strength of the UV source. With a hand-held lamp, we can read about 20 minutes before we start to have issues with visibility. So, if we pattern with a strong UV light source, we can read with a handheld lamp---no problem. We also have to ensure we don't expose the patterned films to sunlight for too long...so, it's really a game of reading it only when you need to.”

I’m sure we’ll hear more about this new technology as it develops. With all my speculations gone, the medium will shrink the message still further.

[1]. CWRU researchers find a chemical solution shrinks digital data storage, http://thedaily.case.edu/cwru-researchers-find-chemical-solution-shrinks-digital-data-storage/ . July 5, 2017.
[2] P. Wei, B. Li, A. de Leon, and E. Pentzer, Beyond binary: optical data storage with 0, 1, 2, and 3 in polymer films. J. Mater. Chem. C, 2017, 5, 5780-5786. http://pubs.rsc.org/en/content/articlelanding/2017/tc/c7tc00929a#!divAbstract

Michael H. Brill

        Graphical summary of the new CWRU technology (copied from the website in Ref. 1)