A scheme of three-dimensional optical data storage based on single-beam femotsecond (fs) laser two-photon recording and one-photon fluorescent readout has been developed. A mode-locked Ti: sapphire fs laser beam of pulse width 80fs at a repetition rate of 80MHz and a wavelength of 800nm was used for two-photon recording and a CW laser with a wavelength of 532nm was used for one-photon fluorescent readout. Thus, data bits can be recorded and retrieved inside a photochromic molecule doped polymethylmethacrylate (PMMA) storage medium. With a 0.65 NA objective used for recording and readout, the effects of writing power and exposure time on the spatial resolution of the system were explored. Bit sizes were observed to increase exponentially in size with excitation power and linearly in size with exposure time approximately. The multilayer capability of the system for three-dimensional optical memory was demonstrated and experimental results of eight-layer data inside a diarylethene derivative doped PMMA storage medium were presented.
Multi-layer data storage based on nonlinear effect caused by two-photon absorption is an attractive approach in the field of mass data storage. A two-photon multi-layer optical disc storage system with disc rotation structure has been proposed. The multi-layer fluorescent disc used in this system consists of three layers. A transparent substrate (under layer) and a thin reflective layer (middle layer) are bonded together forming a kind of structure similar to DVD disc, which is necessary to servo the vertical and radial deviation. Two-photon bits are recorded in top layer. The storage system has two modules: servo module and confocal module. The former keeps following the vertical and radial deviations by means of focusing and tracking servo technologies used in current two-dimensional optical storage devices, so the system can be compatible with CD/DVD. According to the driving signal of actuators in servo pick-up, the confocal module can also follow the disc deviation in both recording and reading processes. The servo module has been finished and the result of preliminary experiment is presented. Using the actuator and the objective lens (NA 0.6) in SANYO pick-up, we successfully recorded and read three data layers in photobleaching material with a homemade femtosecond laser. The layer separation was 15μm and the transverse bit separation was 4 μm.
A potential application of the novel 4, 4'-bis(diphenylamino-trans-styryl)-biphenyl (BPSBP) for three-dimensional optical data storage by two-photon induced photobleaching was investigated. The fluorescence of BPSBP molecules under two-photon absorption excited by a focused femtosecond(fs) pulsed laser was quenched; therefore, bit data can be recorded by two-photon bleaching in a BPSBP doped Polymethylmethacrylate (PMMA) film and be read nondestructively as darker spots than the non bleaching area emitting fluorescence under low power fs pulsed laser illumination. This enables three-dimensional data recording and reading using a two-photon scanning fluorescence microscope. Experimental results of recording and reading in three layers of three-dimensional data bits with a transverse bit separation of 5μm and an axial layer separation of 8μm (0.5x10<sup>10</sup>bits/cm<sup>3</sup>) were presented, and 10<sup>7</sup> readout cycles were achieved possibly.
We have demonstrated three-dimensional optical data storage in a novel photochromic material with two-photon excitation. The data are read out with a homemade confocal fluorescence microscope combined with the three-dimensional writing system. Since the degree of destruction is closely related with the total dwell time of the readout laser, we try to read out the data through an optimization among the doped concentration of photochromic material of the PMMA film, the readout power, and the dwell time of the laser at each data bit. Using the optimized readout condition, it is possible to achieve 107 readout cycles with very little erasure of the data at the scanning speed of a 50× CD driver.
Three-dimensional optical data storage realized by microexplosion based on a multi-photon absorption process is a promising method to fabricate optical read-only memory with large recording capacity. The writing of multi-layered data bits inside a PMMA block doped with rare-earth ions (Sm<sup>3+</sup> and Ce<sup>3+</sup>) under multi-photon absorption excited by a 800nm femtosecond (fs) pulsed laser was reported. The fs pulsed laser beam was focused into the sample with a microscope objective (N.A.= 0.65 40X), and bits were recorded as structurally altered regions which have high contrast in refraction index and fluorescence intensity. The recorded bits could be retrieved parallel by transmission using a conventional optical microscope or be read out serially by fluorescence signal of the structurally altered region excited by a 514.5nm laser using a reflect-type confocal microscope. Experimental results of three-dimensional recording and reading with 4-um in-plane bit spacing and 8-um inter-plane spacing in eight layers were presented. The structure changed in the material before and after the laser irradiation was discussed.