September 29, 2015: A team of researchers has found a way to solve the disappearing memory problem in rewritable CDs and DVDs. They have created the first permanent optical memory on a chip. It is in fact a critical step towards photonic chips.
Electrical engineer Wolfram Pernice at the Karlsruhe Institute of Technology in Germany, and Harish Bhaskaran, a nanoengineering expert at the University of Oxford are behind this study.
Rewritable material like CD and DVD is made up of a thin layer of alloy of germanium, antimony and tellurium. When zapped with an intense pulse of laser light, GST film changes its atomic structure from an ordered crystalline lattice to an ?amorphous? jumble.
These two structures reflect light in different ways, and CDs and DVDs use this difference to store data. To read out the data?stored as patterns of tiny spots with a crystalline or amorphous order?a CD or DVD drive shines low-intensity laser light on a disk and tracks the way the light bounces off.
The researchers realized that the material gets affected by how light reflects off the film, how much of it is absorbed. When a transparent material lay underneath the GST film, spots with a crystalline order absorbed more light than did spots with an amorphous structure.
Using standard chip-making technology, the researchers built a chip with a silicon nitride device, which contains and channels pulses of light. They placed a nanoscale patch of GST atop this waveguide. To write data in this layer, the researchers piped an intense pulse of light into the waveguide. The high intensity of the light?s electromagnetic field melted the GST, turning its crystalline atomic structure amorphous. A second, slightly less intense pulse could then cause the material to revert back to its original crystalline structure.
The researchers then increased the amount of data they could store and read. For starters, they sent multiple wavelengths of light through the waveguide at the same time, allowing them to write and read multiple bits of data simultaneously, something you can?t do with electrical data storage devices.
The researchers believe that the resulting chips could be 50 to 100 times the speed of today?s computer processors.