Terahertz Femtosecond Pulses

Abstract

As was shown in previous chapters, the large peak intensities associated with femtosecond laser pulses make them well suited to nonlinear wave-mixing processes, allowing the generation of new colors. Such processes include second-harmonic generation, sum-frequency generation, parametric oscillation and amplification, and continuum generation. However, there is yet another route for generating new frequencies, which relies on a quite different approach. It is actually based on a very old technique, first developed by Hertz in the last century: a transient polarization or current surge, occurring for example in a spark, will act as a source term in the Maxwell equations and radiate a pulsed electromagnetic wave. If the polarization transient does not exhibit any well-defined oscillatory feature but is, rather, a rapid change such as a step or a pulse, then the radiated wave has no well-defined frequency. Its spectrum will therefore be extremely broad and will peak at a frequency inversely proportional to the timescale of the transient. Because all these frequency components are emitted in phase, this technique is very well suited to the generation of broadband coherent radiation at the lower end of the electromagnetic spectrum.