Two-Photon Technology 
  
Mempile’s technology is based on two-photon physics, the key to commercially reliable, economical, and high capacity optical storage.

The Limits of Current Optical Disc Technology 
 
Quantum mechanics dictates that light can only be focused down to a certain sized spot (known as the diffraction limit). This barrier limits the amount of data that can be stored in a fixed area, and is the reason that optical media moved from using infrared (CD) to red ( DVD) to blue lasers (since blue light allows tighter focusing than red). An additional way to increase storage capacity is to introduce multiple layers in a single disc, as is the case with DVD-9 and DVD-18. However, with current technology, which uses physically separated layers, the reflections from different layers interfere with each other.   It is generally accepted that the number of reflective layers in an optical disc, based on linear optical processes, cannot be larger then 10.   To increase the number of layers beyond this limit there is a need for a recording and readout method that is insensitive to the number of layers. Two-photon recording and reading allow the addressing laser to interact substantially only with the layer that is addressed due to the nonlinear relationship of power and excitation at any position within the medium, providing for true 3D optical storage.

Linear Optical Processes

Current optical media utilize linear processes such as reflection and one-photon absorption. In a disc with multiple layers, every active layer interacts with the incoming laser beam and it becomes impossible to efficiently focus through many data layers onto a particular layer. While this problem can be minimized and accounted for in discs with just a few data layers (e.g. DVD-9), it can not form the basis for a next generation media designed to hold hundreds of layers of information and TB of data.
 
Two-Photon Technology: The Key to Three-Dimensional Optical Storage 
 
As viewed from a quantum mechanical perspective, light consists of innumerable tiny packets (quanta) of energy called photons. In linear optical processes, one photon interacts with a molecule to excite some process. In two-photon processes however, individual photons are too small to interact with the molecules – they just pass by unaltered. In this case, photochemical processes are only excited when two photons interact with a molecule at almost exactly the same time. Their combined energy is sufficient to make a change where the energy of one photon alone is not.   Simplistically, non-resonant two-photon absorption is a process in which two-photons interact simultaneously with a molecule wherein the absorbed energy is the combined energy of the two photons.
The chance of two photons arriving at the same molecule at the same time is very low when the photons are spread out, and so under normal conditions two-photon processes simply do not happen. However, when the light is brought to a very tight focus, the photons are concentrated enough at the focal point that two-photon photochemistry can take place. Thus, two-photon photochemistry provides what is required to access hundreds of data layers, three-dimensional resolution.   With two-photon excitation, the laser light only interacts with the layer that it is focused on, passing unperturbed through the others without affecting them.
Until now, two-photon photochemistry has mainly been in the realm of research labs and academia. Mempile has spent years of concentrated research to molecularly engineer chromophores that are optimized for two-photon reading and writing processes, and has two-photon sensitivity that is orders of magnitude higher than those of most chromophores. Thus, Mempile’s two-photon technology can be used for volumetric or three-dimensional optical recording no longer restricting recording to 1, 2 or even 10 data layers per disc.

Mempile’s TeraDisc technology enables the recording of hundreds of virtual layers!