Currently available linear accelerators are operated in pulsed mode to prevent a thermal overload of the entire system. Upcoming planned CW operations are possible using SRF photoinjectors with either warm or cold photocathodes. The use of warm cathodes in an otherwise cold cavity, which initially appears advantageous, unfortunately leads to a variety of difficulties. On the other side, a cold photocathode cannot be made from the efficient semiconducting materials such that metals with intrinsically lower quantum efficiencies have to be applied instead. Nanostructured surfaces can be used to increase the effective quantum efficiency by surface plasmon resonance enhancement. Optimized nanostructures enable the efficient application of a given laser with a specified wavelength and allow a promising operating regime for future photoinjectors.