Perhaps you should try some spelling variations:
22optical materials » optical materials, optical material, acoustical materials
acoustic material » acoustic materials, acoustic metamaterial, acoustic metamaterials
elastic materials » plastic materials, viscoelastic materials, ceramic materials

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: %((((22optical materials) OR (acoustic material))) OR (elastic materials))%22.
• Adding a wildcard symbol may retrieve word variants: %((((22optical materials) or (acoustic material))) or (elastic materials))%22*.