as a principal constituent has a refractive index of about 4 in order to obtain the antireflection condition the refractive index of the heat resistant protective layer should be less than about 4 and calculation reveals that the refractive index between 2 and 3 achieves optimum antireflection condition the antireflection condition is approximately expressed by the following equation wherein n is the refractive index of the protective layer l the wavelength of a laser and d the thickness of the protective layer a typical material satisfying the equation above is zinc sulfide zns n 2 3 as mentioned above however zns is excellent in the initial characteristics but it was found according to the inventors 39 experiments that the recording erasing cyclability was not satisfied therefore the inventors tried to improve this cyclability as a result they have found that a mixture of zinc sulfide and silicon dioxide has an excellent cyclability further inventors 39 experiments have revealed that materials exhibiting novel characteristics can fundamentally be obtained by quenching or vapor depositing two mutually solid insoluble components to produce a thin film constituted by uniformly finely divided and intimately mixed particles which look as if they are forcibly formed to be a solid solution these components of the materials are featured by being mutually solid insoluble as mentioned above the combination of these mutually solid insoluble components is one of a glassy component and a crystalline component the crystalline component is a chalcogenide such as a zinc chalcogenide such as zns znse or znte or a lead chalcogenide such as.