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  • Ultrafast Lasers Enable High-Density Data Storage

Photonics.com
Jun 2016
OTTAWA, Ontario, June 22, 2016 — A laser-based data storage and retrieval technique could increase the storage capacity of conventional optical discs, such as DVD and Blu-ray, without the need for special preparatory steps. The scalable technique stores information bits in a multilayer structure using the 3D volume of the disc, and ultrafast lasers are used to keep the changes in the disc on a micron scale.

Conventional discs can currently be transformed into ultrahigh capacity storage media by encoding multilevel and multiplexed information within the 3D volume of the recording medium. However, in most cases the recording medium must be photosensitive, which requires doping with photochromic molecules or nanoparticles.

5-bit (32 grey-level) image fabrication
5-bit (32 grey-level) image fabrication. (a) Multi-grey image of Richard Feynman, Nobel laureate in physics in 1965 (105 × 147 pixels). (b) Processed image after assigning 32 grey levels corresponding to write laser pulse energy range of 25 to 130 nJ. A femtosecond laser operating at 800 nm was used as write laser. (c) Confocal fluorescence microscope image of (b) obtained by a CW, 488 nm read laser at 100 μW of power. (d) Reconstructed negative image of (c). Courtesy of Mary Evans Picture Library/The Canadian Press.

To exploit 3D optical data storage capabilities without the need for dopants, researchers at the University of Ottawa used a pulsed laser to record data in micron-sized modified regions of a disc. Upon excitation by the read laser, each modified region emitted fluorescence. The fluorescence moieties induced by the laser demonstrated different emission profiles upon excitation at different wavelengths. The researchers correlated the intensity of the fluorescence signal with the energy of the recording laser and used this to assign 32 grey levels, corresponding to five bits of data.

The research team demonstrated that up to 20 layers of embedded data were possible. By tailoring the read laser power and detector sensitivity, the number of layers in a conventional optical disc that could be imaged without loss of information could be extended up to 30 (up to 60, if data on both sides of the disc was read). Similar results were obtained using different excitation lasers and discs made from different types of plastic, including polydimethylsilozane (PDMS), polystyrene (PS) and polycarbonate (PC).

3D stacked 5-bit images
3D stacked 5-bit images. (a) Set of three 32 grey level images of a Canada goose (113 pixels x 75 pixels), Albert Einstein, Nobel laureate in Physics in 1921 (145 × 87 pixels), and Gerhard Herzberg, Nobel laureate in Chemistry in 1971 (81 × 113 pixels) stacked on top of each other with a spacing of 20 µm. The left column shows the individual processed images and the right one shows the images recovered by confocal fluorescence microscopy at an excitation wavelength of 488 nm. (b) Lateral view of 3D stack of confocal fluorescence microscopy images shown in (a). Images courtesy of Nature Mapping Foundation, Getty Images, Mary Evans Picture Library/The Canadian Press.

The flexibility provided by this technique enables data to be stored in commonly available plastics and makes data accessible at any excitation within the visible spectrum. The stored data can be embedded in the bulk material and is thermally stable up to the glass transition temperature of the recording medium, thereby offering a long shelf life and eliminating the need for the recording medium to be photosensitive.

Storage capacities of up to 0.2 terabytes, scalable up to 0.5 terabytes, were demonstrated. Higher storage capacities could potentially be achieved by overcoming the diffraction limit of light to record data on a submicron scale.

Reduction of fluorescence signal can lead to loss of information, but this can be easily recovered by increasing the detector sensitivity and/or the power of the read laser.

Optical discs are a practical medium for long-term data storage, which requires stability and integrity at relatively low cost. Storing information bits in a multilayer structure using the 3D volume of a disc can overcome the limitations of the planar technology of optical discs and provide ultrahigh storage capacities.

The research was published in Scientific Reports (doi: 10.1038/srep26163)


GLOSSARY
optical
Pertaining to optics and the phenomena of light.
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
fluorescence
The emission of light or other electromagnetic radiation of longer wavelengths by a substance as a result of the absorption of some other radiation of shorter wavelengths, provided the emission continues only as long as the stimulus producing it is maintained. In other words, fluorescence is the luminescence that persists for less than about 10-8 s after excitation.
detector
1. A device designed to convert the energy of incident radiation into another form for the determination of the presence of the radiation. The device may function by electrical, photographic or visual means. 2. A device that provides an electric output that is a useful measure of the radiation that is incident on the device.
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