
144 Light–Matter Interaction
In these cases the contributions, P
L
and P
NL
, belong to different materials.
1
Finally, there can be situations where the nonresonant (host) material as well as
a resonant (second) material produce a nonlinear polarization. Here P
NL
may be
decomposed into a nonresonant P
N,NL
and resonant part P
R,NL
:
P
NL
= P
N,NL
+ P
R,NL
. (3.1)
3.1. DENSITY MATRIX EQUATIONS
The semiclassical treatment has been most widely applied to discuss the inter-
action of short light pulses with matter. The electromagnetic field is used as
a classical quantity, whereas the matter is described in the frame of quantum
mechanics. This both allows a simple interpretation ...