Viridian green laser does not require high current, so the copper is good enough. There are metal magnesium is also very good, very good, but they are not. Of course, the next generation of the pursuit of high current LCLS II will not use metal cathode. But the metal cathode people naturally unwilling to low efficiency of evil. It has also developed a variety of surface coating materials or doping efficiency, in recent years the progress is quite big.
Then there are second kinds of viridian green laser. in telluride, antimonides III-V and several compounds. Efficiency is much higher than the metal, is basically 10 photons eject an electron. It is important that the antimonides and III-V compounds in green, and infrared light to generate power. Such as laser a frequency doubling laser energy is much higher, and the plastic is relatively easy.
The viridian green laser long wavelength, low photon energy, the electron temperature is low, quality is good. There is such a good thing, good for the other, the cathode also live. The problem is the biggest semiconductor cathode low life. Limited in the user device should be used.
The viridian green laser is the neutrino plated antimonides as photoelectric conversion materials, only their requirements as accelerator requirements. Telluride life can also only with ultraviolet light, the future LCLS II may use it. But in the discussion above 10mA high current device, have no choice, only antimonides.