ABSTRACT
This chapter considers the electric field of the laser pulse acting on the medium and exerting a force. A sufficiently intense laser pulse can modify the optical properties of the medium through its action on the electrons. When the field is weak, there is an instantaneous polarization that dissipates soon after the pulse. However, when the field is intense, it can cause significant displacement of the electrons and change the medium, at least during the time the laser is present. This implies that the laser can modify the medium strongly enough that the affected medium affects the pulse itself. These transient changes induced by the laser are known under the name "self-action" because the laser causes its own changes. Self-action phenomena can be very useful, especially for generating femtosecond laser pulses at different wavelengths and to convert longer pulses into shorter pulses.
