A thin nitride layer formed at the interface of a Ge–Sb–Te recording layer and a ZnS–SiO2 protective layer successfully suppresses the phenomenon that reflectivity or signal amplitude becomes markedly small due to repeated overwrites. Based on secondary ion mass spectrometry (SIMS) observations, the 5-nm-thick interface layer was found to restrain sulfur atoms in the ZnS–SiO2 layer from diffusing into the Ge–Sb–Te layer and from changing the optical characteristics of the layer. Among several nitride materials, germanium nitride (Ge–N) sputtered film is found to have the most suitable properties as an interface layer: high barrier effect and good adhesiveness with Ge–Sb–Te and ZnS–SiO2 layers. The optical disk having the Ge–N interface layer achieves more than 5×105 cycles of overwrites with almost no changes in signal amplitude, reflectivity and jitter based on DVD-RAM specifications. The disk shows no degradation such as cracking, peeling, and corrosion after exposure to accelerated environmental conditions of 90°C and 80% RH for 200 h.

A 6-dimensional grand unified theory with the compact space having the topology of a real projective plane, i.e., a 2-sphere with opposite points identified, is considered. The space is locally flat except for two conical singularities where the curvature is concentrated. One supersymmetry is preserved in the effective 4d theory. The unified gauge symmetry, for example SU(5) , is broken only by the non-trivial global topology. In contrast to the Hosotani mechanism, no adjoint Wilson-line modulus associated with this breaking appears. Since, locally, SU(5) remains a good symmetry everywhere, no UV-sensitive threshold corrections arise and SU(5)-violating local operators are forbidden. Doublettriplet splitting can be addressed in the context of a 6d N = 2 super Yang-Mills theory with gauge group SU(6). If this symmetry is first broken to SU(5) at a fixed point and then further reduced to the standard model group in the above non-local way, the two light Higgs doublets of the MSSM are predicted by the group-theoretical and geometrical structure of the model.

We have confirmed the necessary conditions to achieve data recording at a channel bit rate of over 500 Mbps. We discuss the approaches and experiments adopted to realize such high-speed data recording by way of present available techniques. In order to clarify the necessary conditions for them, we have investigated the laser response speed required for recording, optimum write strategy and suitable media for high-speed mark formation. As a result of these studies, the over-500-Mbps data recording was realized by applying a new L-shaped write strategy with a laser pulse of 0.7 ns rising time to Te–O–Pd write-once media of the Blu-ray Disc format.

We have developed a semitransparent layer structure that is ideally suited to the laser-incident-side (front-side) recording layer of a rewritable dual-layer Blu-ray Disc (BD-RE). A TiO 2 top film and a memory film of GeTe-Sb 2 Te 3 characterize this layer structure. The TiO 2 film has a large refractive index, which works to increase the transmittance of the entire recording layer to more than 50%. It simultaneously increases the reflectivity contrast, defined by ðR cry À R amo Þ=ðR cry þ R amo Þ, where R cry and R amo (R cry > R amo ) are the reflectivities of the optical disc in the crystalline and amorphous parts, respectively. The GeTe-Sb 2 Te 3 memory film has optical constants that are appropriate for balancing the transmittance between the crystalline part (T cry ) and the amorphous part (T amo ) in the laser wavelength region of approximately 405 nm. Our experimental dual-layer disc shows high transmittances of T cry ¼ 51:6% and T amo ¼ 52:0% and a high reflectivity contrast of 0.71 in the laserincident-side layer. Dynamic examination showed that both recording layers have a good jitter value, satisfying the BD-RE specifications; more specifically, the laser-incident-side layer jitter value is 7.4% (spec.: 8.5%) and the back-side layer jitter value is 6.4% (spec.: 6.5%) through the front-side layer.

For multilayer rewritable optical discs, we developed a new adaptive write strategy named the S-22 (sequential 2T–2T) write pulse control method, which controls the laser pulse of the 2T-mark according to the combination of both the preceding and succeeding space lengths. By applying this write strategy, a rewritable triple-layer disc based on Blu-ray disc optical systems with a capacity of 100 Gbyte was realized. In particular, the advantage of this write strategy is significant for a transparent information layer. The bit shift of sequential 2T–2T patterns, which is a dominant error in 100 Gbyte rewritable discs, was well compensated. We experimentally confirmed that adopting the S-22 write pulse control method is effective to improve symbol error rate.