In recent years, optical discs and hard disc drives have been widely used as storage media. However, the lifetime of recorded data in these media is about 100 years. On the other hand, a permanent storage system that can store data for more than 1,000 years is strongly required, especially for historically valuable data. One candidate system for permanent storage is a system using fused silica, which is thermally and chemically stable. In this paper, we reported simultaneous multi-bit recording in fused silica with a femtosecond laser and a spatial light modulator. The recording quality was evaluated using signal-to-noise ratio with an optical microscope. We recorded a four-layer sample with a dot pitch of 2.8 µm and obtained a signal-to-noise ratio greater than 15 dB. Furthermore, we confirmed that the sample had good thermal resistance at 1,000 °C for 120 min, which indicates a lifetime of over 319 million years.

A driveless, multilayer read system was demonstrated as applicable to a permanent digital-storage using fused silica. The combination of a low-magnification microscope and signal processing were simple enough to be emulated in the distant future. Test data were recorded in a 2-mm-thick fused-silica plate by a femtosecond laser to form four layers with dot pitch of 2.8 µm and interlayer distance of 60 µm. The total recording density was 40 Mbytes/in.2, which is as high as that of a conventional compact disc. This system achieved a bit-error rate in the order of 10-3 when reading the test data (without error-correction code) from images taken at a 19.5-times-magnification. Signal processing using unsharp masking and subtraction of images at different focal points effectively contribute to read data from blurred image with the low bit-error rate.

Illuminating a transparent mold under total internal reflection condition generates evanescent light. Near-field photolithography uses such light, and we simulated this exposure in two dimensions using the finite-difference time-domain (FDTD) method. Our simulation suggests the feasibility of resolving a 130 nm pitch grating pattern, which is finer than the diffraction limit of light. The simulation results showed fair agreement with our experimental results, confirming the strong influence of exposure light polarization to the distribution of optical near fields in photoresist films. This indicates that the FDTD simulation is promising to predict exposure results for designing molds. We further extended the simulation varying the thickness and refractive index of a photoresist film. Based on the simulation results, showing the good exposure contrast in the thin surface layer of the photoresist film, we suggested two methods to resolve a thick resist film: the multilayer resist method which allows us to use a sufficiently thin photoresist film, and the surface imaging technology which can completely dry develop a thick photoresist film even if its exposure area is confined in the surface layer.

We construct a kink solution on a non-BPS D-brane using Berkovits' formulation of superstring field theory in the level truncation scheme. The tension of the kink reproduces 95% of the expected BPS D-brane tension. We also find a lump-like solution which is interpreted as a kink-antikink pair, and investigate some of its properties. These results may be considered as successful tests of Berkovits' superstring field theory combined with the modified level truncation scheme.

We investigated data recording for permanent data storage using an ultrafast pulse laser with nanojoule-class pulse energy and megahertz-class repetition rate in transparent materials, and driveless reading based on a simple imaging system. A transparent ceramics called Lumicera Ò , manufactured by Murata Mfg. Co., Ltd., was used as the recording medium. Lumicera Ò has a lower modification threshold and a higher recording sensitivity than those of silica glass, namely, the medium previously studied. Structural modification in Lumicera Ò occurs by light exposure for 10 s, suggesting that Lumicera Ò has potential for a recording speed of over 100 kbps. Data recorded in Lumicera Ò resists heating for 2 h at 1000 C and is expected to have a lifetime of over 300 million years. Moreover, the data recorded in Lumicera Ò was successfully read with a reading system based on a smart phone.