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