Photoaddressable polymers will enable discs to store molecular-level data.

ANIMAL IMAGES: COREL

In its central research division laboratories in Leverkusen, Germany, Bayer AG is using new molecules called photoaddressable polymers (PAPs) to develop holographic data discs capable of storing 1000 gigabytes (GB) of information at a molecular level. With these polymers, Bayer’s chemists plan to produce two new kinds of data storage systems:

PAP-DVD, a 50-GB storage medium in CD format with about 60 times the storage capacity of conventional CDs and 10 times that of current DVDs. Expected by 2002.

HoloCD, a 1000-GB system that stores holographic information in a PAP coating with about 1500 times the storage capacity of a conventional CD. Potentially ready by 2005.

Holograms
We see holograms nearly every day on credit cards. If you move the card back and forth in good light, you can see the “MC” or the eagle move like a 3-D object. That is one type of hologram.

To understand how holograms work, first let’s consider how conventional photographs work. When we look at an object, we see the light that is reflected off the object. A conventional photograph records the light using photosensitive film. In contrast, a hologram records, not the direct reflection, but an interference pattern of light coming from the object. A traditional hologram is produced when a beam of laser light, the reference beam, interferes with another laser beam reflected from an object that is to be recorded. One analogy is to think of the interference pattern as a complex mathematical summation of many different beams of light reflected, from a range of angles, off the object.

Photographic film, a light-sensitive crystal, a polymer, or some other optical material captures the interference pattern. Re-illuminating this recorded interference pattern with the reference beam reproduces a 3-D image of the object. When you look at a traditional hologram which has been illuminated with a laser, the laser serves as the reference beam to reproduce the image. Continuing the analogy from the previous paragraph, the image looks 3-D because each eye sees a beam of light from a different angle. As you move your head or turn the hologram, you see light beams reflected from the hologram at different angles, the way they would reflect off a 3-D object. When you look at the special credit card holograms, normal white light serves as the reference beam.

The object beam of the laser reflects off the original object or information, then intersects with the reference beam to form an interference pattern on the CD or other optical material. Later, when a reference laser illuminates the interference pattern on the CD, a hologram of the original will appear.

Holographic recording has the advantage of being inherently parallel. It reads and stores a page at a time. The technology permits data transfer rates of up to 1 gigabit (or 125 megabytes) per second, making it ideal for storing image data. Another advantage of holographic storage lies in its use as associative memory. Illuminating a hologram with a reference beam recovers stored information. Conversely, illuminating a hologram with a pattern of information reproduces the corresponding reference beam and angle, identifying the page where the information is stored. In other words, show it a partial pattern, and Pop! the entire answer appears. To use an analogy, if a person sees a leg and a tail, he can associate those pictures with a dog, retrieving that image from memory. Similarly, a hologram can retrieve full information using partial patterns. Thus, holographic memories can be searched very quickly for data patterns. Another use, holographic computing, is discussed at the end of this article.

CD and DVD versus PAP
A strong laser burns small pits into the surfaces of conventional CDs and DVDs to represent digitized data. These pits are burned into a substrate or on the synthetic Makrolon material on the disc. Makrolon is the polycarbonate Lexan resin used as the foundation for many CDs. A weaker laser reads the pits, recognizing information by the changes in the light reflection. A conventional CD stores approximately 75 minutes of music. The difference between CD and DVD storage is that a DVD uses shorter-wave lasers, enabling seven times the storage capacity of a CD because the laser has a smaller wavelength, resulting in the ability to write more information in a smaller area.

The structure and principles of a PAP-DVD are somewhat different from those of a conventional DVD. The Makrolon layer of the information-dense PAP-DVD is coated with another layer, up to 2 mm of PAP, and then written on, dot by dot, by a laser beam. Each dot contains pigments formed by the molecular side chains in the PAP layer. The side chains are photosensitive and absorb the laser energy, becoming aligned in specific spatial directions.

Holographic storage is achieved by shining two laser beams onto the PAP layer. The holographic information is stored in this layer when the two beams meet on the polymer, and an interference pattern indicating the difference between their phases is etched into the substance. One beam passes through the data page, representing the contents of the page, and the other beam is a reference. The resulting interference pattern causes the alignment of the polymer side chains in the PAP layer to change, storing the image of the data page. The alignment is stable and can be read again with a weaker laser. The information is read by illuminating the layer, so that the reflected light pattern is identical to the image on the data page.

Varying the intensity of the laser beam controls the alignment. Variable alignment enables higher storage capacity. Changing the angle of the beam slightly creates an entirely new pattern that can be recorded on the same substance without affecting any of the information already recorded. Therefore, more data can be stored in a given laser-recorded area than is possible using the “pit principle” of CDs or DVDs. The alignment of the side chains can then be read any number of times by a weaker, low-power, reading laser.

Physical Mechanism
PAPs consist of long-chain molecules whose primary backbones have a large number of shorter side chains attached to them. In their normal state, these side chains are disordered and pointing in all directions. When the laser hits the polymer, the side chains of the molecule chains align perpendicularly to the polarization of the light. This is the basis for the storage capacity of PAPs.

Information can be deposited within the polymer according to the orientation of the side chains. The advantage of this is that the arrangement of the side chains is very stable. The alignment of the side chains can be read any number of times by a weaker, low-power, reading laser beam. The retrieved light pattern is identical to the image on the data page. The reading beam must be weaker than the writing beam, because it could alter the orientation of the side chains. A stronger laser is used to erase and rewrite information. Clearly, this storage medium is more efficient, in terms of information density, than CDs and DVDs.

Chemical Mechanism
Researchers at Bayer are studying a class of PAPs that were synthesized in 1985. These materials are based on copolymers with azobenzene side chains. The polar side chains can be reoriented with visible light, yielding stable birefringence without the need of further processing.

Visible light shining on azo-copolymers induces isomerization cycles between the trans- and the cis form of the azobenzene molecule. This catalyzes a molecular reorientation of the chromophore. The molecule tends to align its transition dipole moment perpendicular to the polarization of the incident light. The transition dipole moment is aligned along the molecule’s long axis; therefore, the azobenzene molecule will be aligned perpendicular to the laser polarization in the bright regions of the interference patterns caused by the two intersecting laser beams. The molecules are randomly oriented in the dark regions.

Applications
The storage potential and rapid access of holographic storage make it ideal for recording high-quality video images for applications in many industries. Holographic storage is a natural for genetic information and protein molecules. Imagine the convenience of having customized Chemical Abstracts searches stored on a holographic CD.

Almost every industry can potentially use holographic CDs for information storage; however, new techniques based on PAPs may change computing. When the logistics surrounding holographic memory are worked out, computing can take an evolutionary leap into holographic processing. In contrast to the motto of Sun Microsystems, “The network is the computer,” photorefractive holographic processing (PHP) suggests that “the memory is the computer.”

The underlying premise is that a photorefractive polymer can be used for more than simple storage; it can also be used for associative retrieval. With conventional memory, the user supplies an address to locate stored data. With holographic storage, as discussed above, the user supplies a reference laser beam to address the stored object data.

However, an object beam can be used to reconstruct reference beams used to record data. Let’s look at a simple example. Imagine that we store a picture of a dog. Then, rather than access the full picture, we use a partial picture of the dog’s leg to retrieve the full picture. Or say we use a black and white color segment and retrieve all similar pictures of animals such as Dalmatians and zebras.

Imagine that the patterns comprising stored pages correspond to the various fields of a database. If each stored page represents a data record, then this reconstruction process can be used to simultaneously compare the entire database against the query. This method suggests a content-addressable holographic data storage method, using parallel retrieval. Imagine a PHP protein database. Conceivably, a database search with a receptor query would retrieve the correct protein shapes that match it. This is retrieval; now let’s look at processing.

Rather than a database or a picture, let’s search for a solution to a path analysis problem. For example, if you want to drive from your house to the Grand Canyon, then you take out a map and draw a couple of lines by hand or by computer to indicate a set of paths. Some paths may be fast and efficient; other paths may be scenic and less direct. You then analyze the paths and choose the combination that best suits your needs.

Searching a maze is a similar idea. In this case, you might start from the end and from the beginning and then draw a set of lines until they connect from both directions. PHP can help with these kinds of path analysis problems. It can search through a large set of successive partial solutions until a complete path is constructed. The result is a near-parallel computation of a path analysis algorithm.

Ultimately, path analysis could prove to be useful in finding proteins for specific biological receptors. This method could forgo the need for formidable amounts of computing power and instead use the PHP method to find the solution from a set of retrievals, just like finding a way out of a maze.

Conclusion
PAPs for holographic storage hold significant promise. Clearly, the increase in storage capacity over the next couple of years provides a definite benefit. Moreover, a new computing paradigm, like PHP, will help to solve many complex problems. Although associative computing is still in its infancy, the possibilities are tremendous. PHP computers have as much potential as DNA and quantum computers. A little polymer chemistry may result in some massively parallel computing.

Further Reading

• Ashley, J., et al. Holographic Data Storage. IBM Journal of Research & Development 2000, 44.
• Photoaddressable Polymers http://www.ep4.phy.uni-bayreuth.de/ag_haarer/research/photo_poly/index.html (accessed June 2001).
• Psaltis, D.; Burr, G. W. Holographic Data Storage. Computer 1998, 31, 52–60.
• Psaltis, D.; Mok, F. Holographic Memories. Scientific American 1995, 273, 70.

Hank Simon received an M.S. in nuclear chemistry and a Ph.D. in information technology, both from Texas A&M University. He has worked with IT architecture and data mining for 25 years. Send your comments or questions regarding this article to tcaw@acs.org or the Editorial Office 1155 16th St N.W., Washington, DC 20036.