Laser-induced continuum structure

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Atomic and molecular continuum states may be structured by embedding a bound state into a previously unstructured continuum by a dressing interaction. Autoionizing resonances, in which a high lying discrete state is mixed by configuration interaction into a photoionization continuum, is the best known example in atomic physics. We discuss the process by which a dressing laser can embed a low-lying atomic state into a flat continuum to produce a tunable resonance of adjustable width. Such laser-induced continuum structures (LICS) behave in many ways like autoionizing states. We discuss how such states are formed and how they are probed in photoionization. Saturation properties, including the formation of stabilized dressed states and consequent population trapping, are reviewed. The effects of laser phase fluctuations on the interference phenomena responsible for LICS are included in both semiclassical Wiener-Levy and fully-quantum mechanical theories of multiplicative stochastic processes. We demonstrate how the laser fluctuations can lead to a dephasing of the LICS process and a destruction of population trapping, although these may be restored if the dressing and probing lasers have mutually correlated fluctuations. We also show that additional photoionization rates induced by strong dressing and probe laser fields can cause the population trapped in otherwise stable dressed states to decay away. Finally we discuss the complicating effects of competing transitions, laser pulse shapes and so on of relevance to the experimental observation of LICS.