Collisionless laser-energy conversion by anharmonic resonance.

After two decades of experiments with intense fs laser pulses the physical mechanism of collisionless absorption in overdense matter is still not understood. We show that anharmonic resonance in the self-generated plasma potential at a steep ion density profile may represent the leading physical absorption mechanism. Resonance provides for the phase shift of the free electron current which is compulsory for laser beam energy transfer to any medium and is capable of explaining the prompt generation of fast electrons with maximum energies exceeding many times their quiver energy, and the polarization dependence.