5D Data Storage
The 21st century has seen rapid growth in our ability to store and access data. The Internet is increasingly bringing all forms of information technology to everyone’s fingertips, making life faster, more informative and more enjoyable than ever. However, with individuals and organizations generating ever-larger datasets, we are in desperate need of more efficient forms of data storage that have a high capacity, low energy consumption and long lifetime.
In order to increase the data capacity of optical storage, there is the potential of storing more than one bit in a single voxel by implementing multiplex technology. While CDs only have two dimensions in which to store information: tiny pits on the CD surface either reflect or do not reflect laser light to convey the 1s and 0s of binary data in a single layer of plastic; and in DVDs the data is stored by burning pits on multiple layers, adding a third storage dimension, the recently developed 5D optical storage technique uses birefringence as an extra degree of freedom – the property of a medium whereby its refractive index varies depending on the polarization and direction of incident light. Birefringence generated by the orientation and size of optical nano-gratings offers two extra dimensions, providing much higher storage capacities.
The optical structures required for 5D information storage are produced by femtosecond laser-assisted micromachining, whereby short pulses of intense laser light are fired at a glass substrate where they lead to nonlinear absorption. If they have high enough intensity, linearly polarized femtosecond laser pulses interacting with fused silica create self-assembled nanostructures with stripe-like oxygen deficient regions about 20 nanometers across and oriented perpendicular to the incident beam polarization (see figure below, showing a magnified region of a device containing pits with different orientations and strengths).
5D optical memory is far superior, especially when applied to fused silica which has a high chemical and thermal stability. The lifetime of 5D memory is 10^20 years at room temperature, indicating unprecedented stability among all techniques. Also, 5D optical data based on nanogratings can be erased and rewritten - which are two key features when considering data storage.
The subwavelength periodicity of these nanostructures behaves as a uniaxial anisotropic material where the optical axis is parallel to the direction of laser polarization. Since the optical anisotropy (called “form birefringence”) is of the same order of magnitude as positive birefringence in crystalline quartz, the perpendicular components of the light possess different propagation constants. This changes the way light travels through the glass, thus modifying its polarization, which can then be read by a combination of an optical microscope and a polarizer similar to that found in Polaroid sunglasses to represent multi-bit information.
Analizing polarization and intensity maps provided by the polarization sensitive microscope the coded information is retrieved.