The mechanism behind the second order non-linear injection current.

Question

In this paper the mechanism of second order non-linear injection current is defined as

Injection currents are produced by quantum interference between different polarization components or between one and two-photon absorption processes that lead to an asymmetric distribution of carriers in $k$ space.

Can any one explain this mechanism more clearly. More specifically, 

 1. What is meant by the quantum interference between polarization components and how this phenomena led to the electron hole separation?
 2. How the photon absorption led to asymmetric distribution of charge?