Patent ID: 7269118

Claim:
A method for retrieving information from a three dimensional storage medium, the method comprising: using a three dimensional storage medium comprising an active medium capable of being in two states, wherein a data unit is represented by the ratio between the concentration of the first and second of said two states in a given volume portion of said medium and a data sequence is represented by a sequence of such data units; irradiating said active medium with light as to concentrate light flux through a volume portion of said storage medium so as to generate in said volume portion a detectable non-linear optical response characteristic of said concentration ratio, the non-linear optical response being related to a χ (n) process, where n is greater than 2, allowing for separating the non-linear optical response from other light signals due to a propagation direction characteristic of the non-linear optical response; detecting said non-linear optical response to retrieve information stored in said volume portion; and tracking a data sequence for retrieving said data sequence in a reproducible manner.