  We explore the energetics of microwaves interacting with a double quantum dot
photodiode and show wave-particle aspects in photon-assisted tunneling. The
experiments show that the single photon energy sets the relevant absorption
energy in a weak-drive limit, which contrasts the strong drive limit where the
wave amplitude determines the relevant-energy scale and opens up microwave
induced bias triangles. The threshold condition between these two regimes is
set by the fine-structure constant of the system. The energetics are determined
here with the detuning conditions of the double dot system and
stopping-potential measurements that constitute a microwave version of the
photoelectric effect.
