Nonlinear harmonic response in laser-plasma interactions

We examine some simple models describing aspects of the interaction of high power laser light with dense targets. Our particular interest is in the harmonic structure of the plasma response. We shall show that, as well as harmonics of the incident driving frequency, the response may also have subharmonic components. As occurs in many nonlinear systems period doubling (or, indeed, multiplication by some factor other than two) is seen. The normal approach to explaining the response of laser plasmas is in terms of interactions amongst linear modes. Our models, though much simplified representations of a real plasma, suggest that the response is not necessarily capable of explanation in these terms, but that frequencies other than at multiples of the driving frequency are likely to be an outcome of the nonlinear dynamics of high power laser-matter interactions.