For decades, metal photocathodes were used in the UV-regime by up-converting the fundamental harmonic of the excitation laser to reach the required energy for single-photon photoemission process. The main problems in this experimental approach are represented by low quantum efficiency at these frequencies, the high power required together with the efficiency for the third harmonic generation. Nowadays, the photocathode technology is paying great attention on a new generation of photoemitters, able to directly exploit the fundamental harmonic, showing the ability to simultaneously enhance and focus the electric field in a sub-wavelength focal spot, leading to non-linear optical processes as the multiphoton photoemission of the 3rd and 4th order.