Tomoyoshi Motohiro

Archival data on the CD-R, which has been explosively popularized in the past five years, cannot be read with DVD-ROM drives or up-coming Blu-ray disc drives if these drives are equipped with a single laser of 650 nm or 405 nm in wavelength. To overcome this inconvenience inevitable in the near future, we developed a new class of recordable optical disc memories in collaboration with DENSO Corporation. The research activities reviewed here involve (1) proposal of a new recording mechanism, (2) observation of transient phenomena in the new recording mechanism, (3) materials design of thin-film multilayered systems, (4) optimal pre-groove design of optical disc substrates, whose details are described in the research reports in this special issue. The newly developed recordable optical discs were compatible with the conventional CD-R and could be also played with DVD-ROM drives, unlike conventional CD-R. Taking advantage of this first realization of DVD-compatibility, the new medium has already been used in software development for car navigation systems via built-in DVD-ROM drives. This "future compatibility", being an essential requisite for archival purposes, should see numerous applications of various types and shapes.

A new optical recording mechanism based on an interface reaction in thin film multi-layered systems was proposed. Its three basic concepts are (1) use of an interface exothermic redox reaction between a high reflective metal mirror layer and an inorganic dielectric transparent layer triggered by a recording laser irradiation and maintained by the exothermic heat of reaction, (2) reduction by thermal dissipation and enhancement of reactivity by simultaneous melting of the metal mirror layer at the recording laser irradiation, (3) disuse of poisonous materials. The performance of this new recording mechanism for write-once type optical disc memories was studied. Sufficient sensitivity was obtained with the interface reaction between the transparent GeS₂ layer and the highly reflective, low-melting In or Sn-57wt.%Bi eutectic alloy layer. The resultant systems showed monotonous wavelength dependences of the reflectance in contrast to specific optical absorption features of the organic dyes in conventional CD-R and DVD-R. This feature allows potential application to a write-once type disc memory compatible with CD (780 nm) and DVD (635 nm).

Akihiro Takeichi, Yasuhiko Takeda, Tatsuo Fukano,
Naohiko Kato, Tomoyoshi Motohiro

We constructed an apparatus for measuring rapid changes in reflectance of optical recording materials during a recording laser irradiation with a time resolution of less than 10 ns. This newly constructed apparatus enabled rapid screening of material candidates, using not optical disc samples but rather small flat test pieces. We applied the apparatus to the metal candidates in GeS₂/metal bilayer systems for recordable optical discs. Differences in the profiles of the reflectance changes during the laser irradiation were clearly elucidated depending on the metal layer materials. GeS₂/In and GeS₂/Sn-Bi (Sn-Bi: Sn-57wt.%Bi eutectic alloy) samples showed the largest and the most rapid decreases in the reflectance with low laser power absorbed in the samples. These results were consistent with those of the examination using optical disc samples.