Among photothermal converters, a nanostructured black silicon (b-Si) is a promising material because of its simple structure, inexpensive equipment, and high efficiency. We have pioneered the development of this material, which can absorb almost all light in the visible region. For example, it can be used for snow melting, and demonstrated that it can melt snow more efficiently than activated carbon and other materials. This is partly due to silicon's high thermal conductivity. The b-Si also has applications as an antimicrobial surface and acts as a "mechanical antibiotic" despite its hydrophilic nature. This property can aid in the purification action of contaminated water because it is biocidal upon mechanical contact with micrometer-and sub-micrometer-sized bacteria and does not affect the growth of larger cells.