Perhaps you should try some spelling variations:
22optical materials » optical materials, optical material, acoustical materials
critical materials » optical materials, chemical materials, artificial materials
acoustic material » acoustic materials, acoustic metamaterial, acoustic metamaterials

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 (((critical materials) OR (acoustic material))))%22.
• Adding a wildcard symbol may retrieve word variants: %((22optical materials) or (((critical materials) or (acoustic material))))%22*.