Holographic recording of information

Optical storage systems

Did you know that your entire video library could be contained on a single disc? The information age in our society is the result of the ability to efficiently generate, transmit and store data. A key element in the success of this revolution has been the development of high-capacity data storage systems, with fast access to information and low cost.

Storing more information in less space

CDs were first introduced to the audio market in 1982 as an alternative to vinyl records and cassettes. Two years later the CD-ROM was born and in 1990 the CD-R, rewritable compact discs that have been changing the way we listen to music and store data. It was not until 1995 that a unified DVD standard format was created, which offers the advantage of having at least 7 times the capacity of a CD.

Do you know how a CD or DVD is recorded?

In current storage systems, information is recorded in the form of binary digits or bits (ones and zeros), through local changes in the physical properties of a surface.

Magnetic systems, such as our computer’s hard disk, use changes in the magnetic properties of the surface of the recording material, which a read head is then able to interpret and retrieve the information.

Optical storage systems, such as CDs and DVDs, record information through changes in the optical properties of the media used. These discs are made up of several layers, including a light-sensitive film, in which, by concentrating the laser on a point, absorption changes are produced in the dyes, or phase changes between crystalline and amorphous in the alloys, which can be detected later and lead to the recording. To read the music, image or file data, another laser beam detects these changes in the optical properties of the surface and retrieves the information.

Holographic information storage

Optical and magnetic systems are reaching the physical limits whereby it is not possible to reduce the size of the information unit, bit, or it is too complex to record or read it. Recording and accessing information in today’s storage systems is done bit by bit, which represents a limitation in the speed of information processing.

A different way of storing and reading information is holographic and in particular volume recording. In volume holography, the entire volume of the material is recorded, not just its surface as in current CDs and DVDs.

This makes it possible to store several holograms on the same area of the material, so that it is possible to record more than 1 Terabyte (1,000 Gigabytes) on a disc the size of a DVD, where currently only about 5 Gigabytes can be recorded. Furthermore, holographic information is not recorded bit by bit, but with a single flash of light we can record or retrieve a complete image that can contain millions of bits. This increases recording, access and data transfer rates to billions of bits per second.

How is holographic recording done?

An alternative form of information storage in which capacity increases dramatically is based on holographic recording.

Although holography was conceived in the late 1940s, it was not until the development of lasers in the 1960s that its technological storage potential began to be seriously considered. The rapid development of holography to represent three-dimensional images started to make people think about data storage: given a typical laser, with a wavelength of 500 nm, one Terabit, one million bits, could be stored per cubic centimetre… or more.

In holographic storage, information is not recorded bit by bit, but a complete object is recorded at each point. What acts as an object is a Spatial Light Modulator (SLM), that is a 2D element where the information has been previously encoded in the form of light and dark pixels, which let light through or block it. For registration, the laser beam is split into two beams, one that is carried over the SLM and a reference beam. By superimposing the beam transmitted by the SLM and the reference beam on the light-sensitive recording medium, the hologram is produced and the image of the object is stored. The reference beam is also used to read out the stored information, retrieving the entire object at once, which significantly increases the speed of access to the data.

It is also possible to store several holograms in the same area of the recording medium, for example by changing the direction of one of the two light beams, so that storage capacities far higher than those of current DVDs are foreseeable. It is expected to be possible to store 1,000 Gigabytes on a DVD-sized disc and to achieve access speeds of billions of bits per second. In comparison, a DVD player reads data 100 times slower.

What do we do at INMA?

Polymers for optical information storage

One of our lines of research focuses on the study of polymers for optical information storage. A polymer is a very large molecule formed by the repetition of a single unit, the monomer. Polymers have some very attractive characteristics:

Many degrees of freedom in their synthesis
Their properties can be modified in a controlled way
Ease of processing into different forms; in particular, thin films can be prepared.

Depending on the nature of the monomers, light can modify some of the properties of the polymers. In the polymers studied by our group, light is able to orient the molecules by modifying, among other things, the refractive index of the material. In other words, this allows information to be digitally recorded. When the polymer is subsequently illuminated, the light “sees” a different material if it is oriented or unoriented. This allows the information contained therein to be read.

These processes are reversible and the information can be recorded and erased repeatedly.

Characteristics of polymers for optical data storage

Polymers for holographic storage must have good optical quality, high sensitivity and fast recording times.

How are they studied?

In a first step, very simple holograms are recorded by interfering two equal beams of laser light on the material. This produces a non-uniform illumination, which generates areas on the film in which the molecules are oriented and others in which they are not. A periodic modulation of the refractive index of the polymer therefore appears: we have recorded a diffraction grating. Non-uniform illumination generates a diffraction grating in the material.

What happens when we subsequently illuminate the polymer?

The light reaching the film sees the lattice associated with the fringes of oriented and unoriented material. If a single beam is sent over the lattice, the light is redistributed into several diffracted beams. In other words, the information we have recorded can be “read”. The aim is to find systems in which the gratings are recorded in a short time and the diffracted intensity is large.

In our work on photo-oriented polymers for holographic storage, we studied recording processes with short illumination times, with a nanosecond pulse laser, as well as the recording of several holograms at the same point.

More information

Instituto de Nanociencia y Materiales de Aragón