Perhaps you should try some spelling variations:
acoustic metamaterial » acoustic metamaterials, elastic metamaterials
22optical materials » optical materials, optical material, acoustical materials
acoustic materials » acoustic metamaterials, plastic materials, elastic 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 metamaterial) OR (acoustic materials))))%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%((22optical materials) or (((acoustic metamaterial) OR (acoustic materials))))%22".
• Adding a wildcard symbol may retrieve word variants: %((22optical materials) or (((acoustic metamaterial) OR (acoustic materials))))%22*.