This focus issue of Optics Express is concerned with coherent optical interactions in condensed matter. This field has shown tremendous growth over the past few years - a phenomenon at least partly due to the remarkable progress in ultrafast lasers and measurement techniques which has been a prerequisite to observing coherent transient phenomena on the short time scales required in condensed matter systems. There are many kinds of coherence which can be generated by short optical pulses in solids, including electronic interband and intraband coherence, and coherent phonon fields. The four papers in this issue investigate various aspects of these phenomena.

When an optical pulse is resonant with the interband exciton transition in a direct-gap semiconductor, an optical-frequency interband coherent polarization is generated; the resulting dynamics can be used as a powerful probe of the nature of the electronic states and scattering mechanisms in the material. Aoki et al. present investigations of excitonic quantum beats in GaN, a wide-band-gap semiconductor with considerable technological importance. Wang et al. discuss the effect of inhomogeneous broadening on the coherent dynamics of impulsively excited semiconductor-microcavity systems.

If the pulse excites electron-hole pairs in a semiconductor in the presence of an electric field, the resulting charge separation gives rise to a far-infrared (terahertz-frequency) polarization, which can be used to generate coherent THz radiation. Citrin shows how coherent control of exciton populations in biased quantum wells can be used to optimally generate novel THz waveforms.

Finally, an optical pulse can also generate coherence in the motion of the ions in a solid via impulsive stimulated Raman scattering. The coherence may be in the relative positions of the ions (coherent phonons) or in the variance of the ion positions (squeezed phonons); Garrett et al. show that both types of phonon coherence can be generated simultaneously via impulsive Raman scattering in SrTiO3. A series of movies illustrates schematically the nature of these phonon coherence.

I would like to thank the authors for their contributions to this focus issue, which we hope will serve to stimulate both further advances in the field and the format of electronic publishing.

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