Optical Storage Inside Transparent Materials

storage offers the potential for very large recording capacity. In addition to extensive ongoing research in volume holo-graphic data storage, a number of recent papers report 'point-like' or 'bit-wise' binary 3-D optical storage in photopolymers 1,2 and photorefractive materials. 3,4 Using a nonlinear optical process in the medium, the optical interaction can be confined in all three dimensions to a micron-sized focal volume. Pioneering experiments demonstrated 3-D optical recording at a density of up to 1.6 × 10 12 bits/cm 3 in a photopolymer using two-photon absorption and extremely tight focusing. 2 The stored information consisted of less than 1% changes in the local index of refraction, and was read out serially with a Nomarski differential interference contrast (DIC) laser microscope. Experiments in photorefractive materials 3,4 used linear absorption to record data at a density of up to 4.2 × 10 9 bits/cm 3. The authors suggested that the recording density could be increased using two-photon absorption. The stored data was read out with a phase-contrast microscope. 3,4 Here we report a novel method for creating sub-micron-sized bits that have a large contrast in index of refraction and that can be read out with transmitted or scattered light under a standard microscope. The method can be used for permanent 3-D optical data storage in a wide range of materials including fused silica, fused quartz, sapphire, and various glasses and plastics, thus allowing for a storage medium that is mechanically, chemically and thermally very stable, and inexpensive. Unlike a photo-polymer gel, there are no problems of distortion due to shrinkage and flow, and of isomerization due to ultraviolet light. And unlike photorefractive materials , the difficulties of fixing the recorded data are entirely avoided. We tightly focus ultrashort laser pulses inside a transparent material to create localized structural changes, thereby altering the index of refraction. This process is used to record digital information in three dimensions by writing multiple planes as illustrated in Fig. 1. In our experiments, we translate the sample in the transverse plane, and move the focus-ing objective along the beam axis. Figure 2 shows an example of a random binary pattern stored inside fused silica, recorded using 0.5-µ J, 100-fs, 780-nm pulses from a regeneratively amplified Ti:Sapphire laser, focused by a 0.65 numerical aperture (NA) microscope objective. With this focusing, the threshold for observable structural change is 0.3 µ J. The pattern is read out in parallel …