Transient

The initial phase of an alternating field has played a minor role in the force exerted by the field over free or bound charges. The main reason being that in most cases the transient behaviour has a slowly varying envelope.

However, in recent times either with passive or active schemes, very short optical pulses have been achieved. These pulses may contain only a few oscillations and the slowly varying envelope condition is no longer fulfilled.

Therefore we wish to reconsider the effect of a harmonic field firstly on free charges and secondly on bound charges.

Free charges

adiabatic approximation

Let us consider a slowly varying envelope harmonic field incident on a free charge.

The usual approximation is to consider the small velocities so that the magnetic contribution of the Lorentz force may be neglected.

This is reminiscent of the Lorentz - Drude model in the limit where the binding force is zero thus leading to free charges.

abrupt approximation On the other hand, we shall consider the opposite limit where the external field is abruptly turned on.

Titles for the effect we are describing

charge scattering due to transient harmonic fields

charge