Physical picture and important scales in laser- matter interaction :
Perturbed by an oscillating electric field a neutral atom acquires an oscillating dipole. In a homogenous medium, this is best described in terms of Polarization Which represents the dipole moment per unit volume.
First order perturbation gives
(1.4)
Next higher order terms in the perturbation series give terms that are:
(1.5)
(1.6)
Higher order terms are important because even though they are very small in magnitude they make qualitatively new phenomena possible. An analogy with spices in food is apt just as spices-even in very small proportions – make a qualitative difference to the taste of food, nonlinearities make qualitative changes to the behavior of optical waves. Smallness parameter for perturbation series is , where is the electric field of the electromagnetic field representing the laser beam and is the electric field that is seen by an electron in an atom.
a neutral atom acquires an oscillating dipole. In a homogenous medium, this is best described in terms of Polarization 
Which represents the dipole moment per unit volume.
, where 
is the electric field of the electromagnetic field representing the laser beam and 
is the electric field that is seen by an electron in an atom.