Perhaps you should try some spelling variations:
acoustic metamaterial » acoustic metamaterials, acoustic materials, elastic metamaterials
acoustic material » acoustic materials, acoustic metamaterials, plastic material
optical material » optimal maternal, optic materials, optics materials

Try widening your search to All Fields.

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: %((((((acoustic material) OR (acoustic metamaterial))) OR (optical materials))) OR (optical material))%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%((((((acoustic material) OR (acoustic metamaterial))) or (optical materials))) or (optical material))%22".
• Adding a wildcard symbol may retrieve word variants: %((((((acoustic material) OR (acoustic metamaterial))) or (optical materials))) or (optical material))%22*.