Avtar Matharu and colleagues from the University of York, UK, explain how, when it comes to data, size matters

Modern-day society lives in an information-obsessed world. The volume of information produced and stored annually is exponential in growth. The information age has arrived in all its glory with on-line access to vast resources of electronic information never imagined less than one decade ago. On a daily basis we are bombarded with high resolution digitised images utilising mega- and giga-bytes of information. In the area of home entertainment we can now purchase the latest state-of-the-art BluRay DVD player capable of storing a staggering 27 GB, surpassing its counterparts, the CD (750 MB or 75 minutes of music) and DVD (4.7 GB or a 2 hour movie). However, this is still not enough as the societal needs of the 21st century for increased information content with even faster processing speeds continue to grow at breakneck speed.

The world's information could be stored and retrieved within a liquid crystal holographic box

There's one big problem - storage space. Although magnetic disk is still the best medium for storage of large amounts of information, it has severe limitations. Magnetic disk, and conventional magneto-optical data storage technology, use the surface of the medium to store and retrieve bits of data. The available area on a magnetic disk cannot be compressed indefinitely to record even smaller bits of information due to the superparamagnetic effect. The diameter of the magnetic domains is limited to around 10 nm: smaller than this and thermal self-erasure occurs. The magnetised bit flips randomly, finding it difficult to attain a stable state.

New, alternative technologies need to be considered. Holography, an optical 3-D volumetric approach, may be the answer to meet the societal needs of the 21st century rather than the current 2-D surface approach. The concept of holography was first introduced by Gabor in 1948 but remained dormant for many years due to a lack of technological advances in complementary optics and image processing instrumentation. Holographic storage provides the potential to store in excess of one terabyte of information with transfer rates exceeding 1 GB/s and data access time of less than 100 µs.

In holographic storage, a whole page of information in the form of a bit-map is recorded at once, instead of storing single bits as in digital storage. Typically, two laser beams derived from a single laser source are overlapped on a photosensitive material. One of the beams, called the object beam, passes through the object (bit-map) of interest, and the other beam is a plane wave providing a phase reference. In conventional holography, the two beams have the same polarization. They create a complicated interference pattern in the film that is characteristic of the object. The recorded information is read out with a conjugate of the reference wave. By the process of angular multiplexing several holograms can be recorded in the same volume.

Significant advances have been made in holography, as exemplified by InPhase Technologies' tapestryT holographic media, capable of storing and retrieving 200 GB on a standard 120 mm CD format at high speed, equivalent to a near ten-fold improvement on optical BluRay DVD technology. There are still drawbacks, however, as the ideal material is still to be found. Any material suitable for holographic storage should possess fast optical switching between two states, high thermal stability over a wide temperature range and non-destructive read-out. As the search for holographic materials continues, liquid crystals may play an important role. Liquid crystals constitute the fourth state of matter intermediate between the solid and the liquid states. The intermediate ordering of molecules between order and disorder may enhance storage properties. Although liquid crystals are not a pre-requisite for holographic storage, their anisotropic shape is important.

Imagine one day we may all be carrying a credit card with a 1 x 1 cm square hologram capable of storing all our personal details including medical records with X-ray scans. In the event of sudden serious illness the hologram may be read immediately to display your full medical history rather than having to wait for information to be sent by email or post. The time saved may ultimately save your life.

Read the full Tutorial Review 'Liquid crystals for holographic optical data storage' in Issue 12 of Chemical Society Reviews.