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The use of nonlinear optics for frequency conversion is now widespread in photonics, from complex optical parametric amplifiers to simple second harmonic generation in green laser pointers. Frequency conversion from visible to soft X-ray radiation is an extreme example of the process, but is possible given the very high intensities available from lab-scale laser sources. X-ray sources based on frequency conversion have many of the useful properties of the pump lasers – they can produce very short pulses (down to less than 100 attoseconds), they can have very high spatial coherence, and produce very well-collimated beams. However, the production process is not efficient, and the necessary pump lasers are both complex and expensive. This review is intended to provide basic information about the frequency conversion process – its physical basis (high-harmonic generation, or HHG), and its present and potential applications. The applications tend to fall into two categories: the science and engineering of attosecond pulses, and the production of soft X-rays for imaging and spectroscopy. The latter area will be covered within this chapter on “practical aspects” – excellent reviews of attosecond science are available elsewhere. Section 1 will review briefly the laser sources necessary for HHG. Section 2 will give an overview of the single atom processes that are responsible for the nonlinearity, and section 3 will look at the complex process of phase matching the nonlinear interaction, which is critical for high efficiency X-ray generation. Section 4 will consider experimental aspects of HHG, comparing the output characteristics of typical HHG sources with other sources in the soft X-ray spectral region, and section 5 will outline a few of the many applications for these sources.

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