Foundations of Coherent Transients in Semiconductors

Short laser pulses are able to generate material excitations with a well-defined phase which is imposed by the optical excitation source. The generated coherent superposition state be described as an optical polarization which exists only in non-equilibrium situations. The coherence, i.e., the phase relations between the optical transitions that originate from the excitation, leads to several interesting effects in time-resolved linear and nonlinear optical spectroscopy. In this article, the basic principles that underlie these coherent transients are introduced and several examples are presented.