 In Sun-free photovoltaics, the thermal emitter described above is finely tuned to emit specific wavelengths that the photovoltaic diode can utilize, while at the same time stopping the wavelengths that cannot be used by the diode.This is achieved by etching features of nanoscale like ridges and holes on a photonic crystal so that the light passing through the crystal can be modified.A slab of tungsten is used as a thermal emitter with billions of nanoscale pits on the surface, with each pit acting like a resonator.