Coherence in strong field harmonic generation

In this paper we discuss recent studies of harmonic generation in rare gases, including our current understanding of the special circumstances in the intense field regime which result in the observation of very strong high-order harmonics. The combination of a non-perturbative atomic response and enhanced phase matching in the medium is responsible for the high conversion efficiencies. The calculations point to those attributes of the microscopic atomic emission rates which yield the strong macroscopic, coherent harmonic output. We begin with a simple model of a one-dimensional electron in a strong oscillating field to provide some insight into the source of the single-atom emission spectrum. Next, we present a brief description of our calculations for realistic atoms. A review of recent experimental results is followed by a discussion of the method used for calculating the effects of phase matching on the emitted radiation. In the final section we discuss the agreement between the experimental and theoretical results, and describe the important differences between traditional, perturbative harmonic generation and that found in this new, non-perturbative, strong field regime.