The efficient absorption of wave energy is an important subject. This research direction is about how to design broadband perfect absorbers with ultra-thin thickness, which was challenging in practice. Our previous work utilizes the principle of coherent perfect absorption to acoustic absorption (Read more) and derives the conditions of ultra-thin metamaterials for broadband perfect absorption (Read more) and coherent perfect absorption (Read more). We further utilized the reciprocity principle and the traditional Brewster effect to realize the anomalous Brewster effect, where absorption and refraction can be tuned freely without destroying the reflection-less characteristic (Read more). Such a principle bestows Brewster absorbers with ultra-broad bandwidth, and can be extended to grazing incidence (Read more). We utilize metasurfaces to converge energy to a focal point such that energy can be harvested more efficiently (Read more).