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