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Instant insight: Holography speaks volumes

29 July 2009

As the world generates more and more data, scientists strive to develop technologies to store all the information. Sřren Hvilsted, at the Technical University of Denmark, Kongens Lyngby, and colleagues at the Materials Science Institute of Aragón, Zaragoza, Spain, explain how holograms could be the answer.

We are immersed in an information age. We demand efficient data generation, transmission and storage, as well as quick access to these data. The development of cheap, high capacity data storage media has been of key relevance in this information revolution, which is exemplified by the ubiquitous world wide web. But the never-ending growth of information content in our society pushes researchers to develop new storage technologies with increasing capacities and faster data access.

holographic storage

Many pages of information can be stored in the same volume of azobenzene films

"Holographic recording can overcome some of the limitations in optical storage capacity and writing and reading rates"
Current commercial optical media use lasers to record information bit by bit as a local property change in 2D films. Their storage capacity is determined by their ability to record and read these changes, which depends on the laser's spot size. The pioneering CD has been surpassed by the DVD and, more recently, by the Blue-ray disc. This increasing information density has predominantly been achieved by reducing the laser's wavelength and increasing the laser lens' numerical aperture (the range of angles over which it can accept light). This allows more data to be tightly packed on to the same size of disc.

"When irradiated with linearly polarised light, azobenzene molecules change their orientation to align perpendicular to the light and this photoinduced change can be used to write information into the material"
Holographic recording can overcome some of the limitations in optical storage capacity and writing and reading rates by recording complete pages of bits simultaneously as an optical interference pattern. Two coherent beams, called the signal and reference beams, interfere in the storage material. The signal beam carries the information to be stored while reference beam is designed to reproduce it. The light in the interference pattern modifies some of the optical properties of the storage material, such as its refractive index or absorption. Irradiating the material with the reference beam for a second time causes part of the light to be diffracted, reconstructing the signal beam and recovering a complete page of information. This dramatically boosts the information transfer rate. By increasing the thickness of the recording film (known as volume holographic storage), scientists have achieved huge improvements in storage capacity. 1000 pages of information can be stored in, and recovered independently from, the same small volume of the disc. Although the concept of volume holography was proposed some decades ago, the associated technology has developed slowly, partly due to the lack of suitable materials that can act as recording media.

Azobenzene-containing polymers have drawn the attention of many research groups interested in optical storage. When irradiated with linearly polarised light (light that travels in a single plane), azobenzene molecules change their orientation to align perpendicular to the light, a property known as macroscopic optical anisotropy. This photoinduced change can be used to write information into the material, either bit by bit or by holography. But the recording laser light cannot penetrate right through thick azobenzene films because azobenzene units absorb strongly at this wavelength. Therefore, most studies have been limited to recording in films only a few micrometres thick. To fully exploit the advantages of volume holography, scientists have investigated diluting the azobenzene molecules in a polymeric matrix that is transparent at the recording wavelength. The recording light can penetrate through the samples, which can be several hundreds of micrometres thick. But the reduced azobenzene content can result in a decrease in the sensitivity and stability of the recorded holograms. Different approaches based on random copolymers, block copolymers and blends of polymers are being examined to try to fulfil application requirements.

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Link to journal article

The volume holographic optical storage potential in azobenzene containing polymers
Sřren Hvilsted, Carlos Sánchez and Rafael Alcalá., J. Mater. Chem., 2009, 19, 6641
DOI: 10.1039/b900930m

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