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