ABSTRACT
HHG is a nonlinear optical process that converts light of one
frequency, called the fundamental or driving, into light at integer
multiples of that fundamental frequency, called the harmonics. The
observation of harmonics can be considered as having two parts:
their generation at the single atom level and their propagation to
coherently build up a macroscopic signal over a number of emitters.
Below, we consider both low-order and high-order harmonic
generation [1].
