What is volumetric optical data storage?


Background:

This year, commercial applications will store more than two exabytes of information in digital media.  Approximately 10 % of the information will be stored on magnetic disk drives, with the remainder on tapes and optical disks.  This increasing capacity demand has thus far been met through steady increases in areal density of magnetic and optical recording media (e.g. hard drives, compact disks), where data are stored on a planar (2-D) surface.  While the limits of magnetic recording are still being debated, the limits of conventional optical storage are well understood.  Future increases in density are possible by taking advantage of shorter wavelength lasers, higher lens numerical aperture, or by employing near-field techniques.

The three-dimensional (3-D) volumetric approach to increasing effective storage capacity is quite unique for optical memory technologies.  Three-dimensional storage is envisioned as a cubic storage element, with bit spacing having dimensions of the writing/reading laser wavelength.  Instead of recording only on a plane (2-D), bits are stored throughout the volume of the material (3-D). With a wavelength of 650 nm, storage of one terabit per cubic centimeter is possible.

Three-dimensional (3-D) optical memory is a revolutionary technology that has the benefits of lower cost (tens of dollars/Gbit), low risk, and an order of magnitude smaller size and mass, as compared to existing optical data storage technologies.


The Process:

We are investigating and characterizing a 3-D volumetric optical memory device based on a new class of light-absorbing (photochromic) compounds that, when pulsed with lasers, absorb photons two at a time and can trigger chemical and physical changes (such as molecular structure, or fluorescence) with micrometer-sized resolution in three dimensions.

With a tightly focused laser beam, the photochromic process can be initiated and controlled within micrometer-size spaces.  A data mark is written within the volume only at points of sufficiently high intensity.  At these points, two-photon absorption occurs, resulting in a bond dissociation.  Thus, the molecular structure is changed into a new, ‘written’, molecule with a different absorption and emission spectrum.  To “read” the information written within the volume, the approach exploits the fact that the written form absorbs at longer wavelengths than the unwritten form.  Excitation of written molecules is followed by fluorescence at lower wavelengths, which returns the molecule to its ground state.  The presence or absence of this fluorescence is detected and classified as a physical ‘1’ or ‘0’ for the stored data mark.  Since the decay lifetime is ~5 nanoseconds and the concentration of molecules is high, it is possible to excite the written molecules many times in a single read cycle and increase the total light collected at the detector.

The advantage of a 2-photon absorption process is based upon its ability to selectively excite molecules inside a volume, without populating molecules on the surface of the device.  This may be achieved because the laser photons have less energy than the energy gap between the ground state and first allowed electronic level.  Therefore, photons propagate through the medium without being absorbed by a one-photon process.  However, in the vicinity of the laser beam focus, the intensity is high enough so that two photons can combine to excite carriers across the energy gap.  The transition probability of a 2-photon absorption process partly depends upon the writing beam intensity, so lasers emitting high intensity light in short pulses (i.e. picosecond and sub-picosecond pulses) must be used.

The recording material is dispersed in a polymer host, which can then be shaped to produce disks with integrated structures for alignment and mounting.  This project uses 25mm x 3mm PMMA disks with homogeneously dispersed storage materials.  Polymerization molding, compression molding and polishing have been utilized to produce the desirable optical quality polymer for 3D optical memory disks - these are well-established manufacturing processes.

In comparison to existing technologies, consider that a compact disc holds roughly 650 Mbytes of information.  A DVD holds roughly 9 Gbytes of data, by using both sides of the disc.  Now consider our 25 mm diameter test samples, if used with 500 layers and 1 Gbyte per layer, can produce a disk containing 500 Gbytes!  With parallel readout beams, a high data rate retrieval can also be achieved.




Note: this text is taken from various Milster Group publications.  Do not use any of it without permission from the authors.


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