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