Three-dimensional Optical Data Storage Research Articles

Reflection confocal microscope readout system for three-dimensional photochromic optical data storage

We propose to use a reflection confocal microscope (RCM) as a readout system for digital data stored in a three-dimensional (3D) photochromic optical memory. We describe the merits and the difficulties for 3D optical memory that are associated with the use of such a RCM.It is shown by means of 3D Fourier space analysis that successful reading of 3D data can be obtained by selection of the appropriate parameter for the RCM.The system parameters include the numerical aperture of the objective lens and the wavelength of the light used for analysis (reading). Experimental results of multilayer recording and reflection confocal reading in photochromic-molecule-doped poly(methyl methacrylate) are presented. Good-contrast images are obtained.

Optical data storage in C60 doped polystyrene film by photo-oxidation

A potential application of C60 for three-dimensional optical data storage in read only memory recorded by photo-oxidation is investigated. Photo-oxidation significantly enhances the fluorescence intensity of C60 molecules; therefore, one bit data can be recorded by the photo-oxidation of C60 doped polystyrene film at individual points by a focused laser beam and be read nondestructively as a brilliant fluorescence spot, in contrast to the nonoxidized areas after photo-oxidation is blocked. This enables three-dimensional recording using a laser scanning microscope. As a result, a pattern consisting of three layers of bit data was written and read in a C60 doped polystyrene film. We also discuss the time course of photo-oxidation of C60.

Fluorescence readout of near-field photochromic memory

The fluorescence- and transmittance-detection readout methods of a near-field photochromic optical memory by a scanning-probe technique were studied theoretically. Shot noise, as the principal noise, was taken into account in an analysis of the signal-to-noise ratio (SNR). Under most conditions, fluorescence-detection readout yielded a better SNR than did transmittance-detection readout. For a transmittance change of 0.9-1.0 as a result of recording, a readout light power of approximately 100 nW, and a system bandwidth of 1 MHz, only the fluorescence-detection readout method, under the condition that the fluorescent layer of the medium have a fluorescence quantum yield greater than 0.4, can produce a sufficiently large SNR.

Numerical simulations of scattering in a two-photon optical data storage system

Scattering effects in a two-photon optical data storage system are numerically studied. Surface scattering analysis with a scalar, beam propagation model is performed. We analyze the problem by modeling scattering from randomly varying surfaces and also by Fourier surface decomposition. Scattering induced by propagation through multiple pages of randomly recorded data marks is also studied with a hybrid finite-difference-time-domain/angular-spectrum model. Both surface and bulk scattering are shown to influence the spatial properties of the optical beam. Results and some possible implications are presented.

Photon-Induced Micro/Nano Fabrication, Manipulation, and Imaging with Unconventional Photo-Active Systems

We describe about the optical technologies of fabrication, control and imaging of micro/nano structures using unconventional photo-active systems. A focused beam of a near-infrared pulsed laser with a peak power of some ten kilowatts scans in three dimensions to form three- dimensional micro-structures such as micro-tubes and micro-coils with photo-polymerizable material, based on two-photon excitation/absorption process. Three-dimensional optical data-storage which contains data in a volumetric medium is also realized with two-photon process in a photorefractive crystal or with a photochromic molecule for recording and with confocal microscopic optics for phase-reading. We will also show our experimental result of self-growing of micro-fiber in photopolymer by the exposure of a stationary focused laser beam. These technologies can be coupled with the currently progressing near-filed optical technology for observation, fabrication and control of nanometric structures.

Three-dimensional optical storage inside a silica glass by using a focused femtosecond pulsed laser

We report on three-dimensional optical data storage inside a silica glass by using a focused 800 nm, 120 fs, 200 kHz pulsed laser. The glass is transparent in the wavelengths ranging from 250 to 1000 nm. After the irradiation of the 800 nm tightly focused femtosecond laser, a refractive-index bit was formed in the glass. The relations between the size of refractive-index bit versus average power and irradiation duration of the laser pulse have been examined. Absorption and ESR spectra showed a significant increase in the concentration of SiE′ color centers in the laser irradiated glass. A multi-photon absorption process is considered to have occurred in the glasses when irradiated by the laser pulses. We suggest that writing three-dimensional refractive-index bit inside the transparent glasses based on a multi-photon absorption process will become a promising method to fabricate optical memory with both an ultrahigh storage density and an ultrahigh recording speed.

Photochromism and Its Application to a High-density Optical Memory

In order to solve problems of photochromic memory, a super-low power readout method and a crosstalk reduction method for multi-wavelength recording were proposed. In the demonstration of 2-wavelength recording using photochromic diarylethenes, the crosstalk between multiplexed channels, which is caused from broad absorption bands of compounds, was effectively reduced by the method. A new high density optical memory, a super-resolution disk with a photochromic mask layer, was also introduced. The super-resolution method has enabled us to increase the recording density of conventional optical disks.

Theoretical study of the recording density limit of a near-field photochromic memory

The recording density limit of a near-field optical memory that uses a photochromic medium was theoretically studied by use of Shannon’s information theory. Shot noise and material noise were taken into account in the analysis of the signal-to-noise ratio. The conventional recording density limit, which is defined by the inverse of the minimum recorded mark area, and Shannon’s recording density limit were evaluated. The conventional recording density limit was 1011–1012 bits/cm2, and Shannon’s recording density limit was 1012–1013 bits/cm2.

Nondestructive readout of a three-dimensional photochromic optical memory with a near-infrared differential phase-contrast microscope

A three-dimensional (3D) rewritable optical memory using photochromic material is described for high-density memory. The bits are recorded in a 3D volume of photochromic material. A transformation of the photochromic molecule between two isomers with different absorption spectra can be stimulated by irradiation with appropriate wavelengths. We show that a nondestructive readout of photochromic memory is possible by use of a small difference in the refractive index of the photochromic isomers in the near-IR range. For this purpose a near-IR laser-scan differential phase-contrast microscope is used. Experimental results of 3D recording and nondestructive reading are presented.

New Photopolymers Based on Two-Photon Absorbing Chromophores and Application to Three-Dimensional Microfabrication and Optical Storage

AbstractMolecules exhibiting strong two-photon absorption hold great potential for a wide range of applications including two-photon fluorescence imaging, three-dimensional (3D) optical data storage, and 3D microfabrication. We have observed two-photon absorptivities as high as 1500×10− 50 cm4 s/photon in bis-donor diphenylpolyene derivatives that are correlated to simultaneous charge transfer from the end groups to the polyene bridge in the molecule. Many of these molecules are also excellent photoexcitable electron donors that can initiate charge-transfer reactions with acrylate monomers. Marcus theory is used to describe the efficiency of these charge-transfer reactions. Polymerization rates have also been measured and we show that these twophoton chromophores display increased sensitivity and recording speed over conventional UV photo-initiators. The fabrication of complex, three-dimensional structures by twophoton polymerization is demonstrated and discussed in the context of advanced photonic applications.

Photochemical and Spectroscopic Properties of Naphthacenequinones as Candidates for 3D Optical Data Storage

More than 30 peri-aryloxy-p-quinone derivatives have been synthesized for possible application in 3D optical memory devices. Extinction coefficients, fluorescence, photochromism and photo-decomposition quantum yields of 15 naphthacenequinone derivatives for both isomeric forms have been measured.

Three-dimensional optical storage inside transparent materials

We present a novel method for three-dimensional optical data storage that has submicrometer size resolution, provides a large contrast in index of refraction, and is applicable to a wide range of transparent materials. Bits are recorded by use of a 0.65-N.A. objective to focus 100-fs laser pulses inside the material. The laser pulse produces a submicrometer-diameter structurally altered region with high contrast in index of refraction. We record binary information by writing such bits in multiple planes and read it out with a microscope objective with a short depth of field. We demonstrate data storage and retrieval with 2-microm in-plane bit spacing and 15-microm interplane spacing (17 Gbits/cm(3)). Scanning electron microscopy and atomic force microscopy show structural changes confined to an area 200 nm in diameter.

Optical Density Dependence of Write/Read Characteristics in Photon-Mode Photochromic Memory

When photobleaching reactions are used for writing in a photochromic optical memory, the initial optical density in the recording layer plays an important role in the various write/read characteristics; i.e., secondary harmonics, repeatedly readout characteristics, and crosstalks in a multi-wavelength recording. Better characteristics, that is, lower secondary harmonics, higher readout repeatability and lower nonlinear component of crosstalks, were obtained for media with higher optical density.

Superlow-Power Readout Characteristics of Photochromic Memory

Readout characteristics have been investigated with a superlow-power readout method in order to solve the problem of information destruction caused by repeated readout operations in photon-mode photochromic memories. For ordinary photochromic diarylethene media, it was possible to read signals of about 105–106 cycles with this method. This method was also applied successfully to wavelength multiplexed recording.

A photochromic memory with a non-destructive read-out property

Isomerizations of [2.2](4,4′)-azobenzenophane ( 1) in polycarbonate films were investigated spectrophotometrically, and the light-intensity dependent efficiency of the photoisomerization of macrocyclic azobenzene dimers, including 1, was analyzed theoretically. It is demonstrated that the efficiency of photoisomerization of 1 is dependent on the light intensity, even in a polymer film. A correlation between the lifetimes of the trans-cis isomers of macrocyclic azobenzene dimers and light intensities, where the efficiency of photoisomerization becomes dependent on light intensity, is also described. A photochromic memory composed of 1 and polycarbonate shows a non-destructive read-out property.

Three-dimensional optical data storage in refractive media by two-photon point excitation

What is to our knowledge the first high-density (>10(12) bits/cm(3)) optical recording of digital information in a multilayered, three-dimensional format is reported. Information is written as submicrometer volume elements of increased refractive index in a photopolymer by two-photon excitation of a photoinitiator at the waist of a highly focused beam from a colliding-pulse mode-locked laser. Quadratic dependence of two-photon excitation on intensity confines polymerization to the focal volume. Information is read with sufficient axial resolution by differential interference contrast microscopy. This write-once, read-many technique should increase the capacity of the spinning disk format by 100-fold.

Photochromic memory devices

Photochromic memory devices

Dynamic range of photochromic memories

Dynamic range definitions for conventional photographic films emphasize the primary noise mechanisms of such films, namely grain noise. Dynamic range is redefined and noise mechanisms are discussed for photochromic and other grainless films. Measurements indicate peak signal-to-noise ratios in excess of 60 dB for photochromic memories.