Atoms and solids in strong laser fields.

High harmonics from ionizing gases irradiated by intense infrared light were discovered in 1989, but it took 22 years before similar phenomenon were discovered in solids. Electric field driven motion of the ionizing electron wave packet is the physical mechanism for high harmonics and, in gases, the re-collision between the wave packet and its parent ion is the most important process. The question arises ``is the mechanism similar for harmonics created by electrons and holes in transparent solids. To answer this question we measure the spectral phase of the harmonics by perturbing the wave packet motion with an interrogating pulse that adds a spectral or spatial marker. We apply a second harmonic beam parallel to the fundamental with the weak even harmonics that arise from the joint fields as the observable. We find that for ZnO and Si, the harmonics have a spectral phase characteristic of a generalized re-collision (the dipole moment requires two bands). In contrast, for SiO$_{\mathrm{2}}$ all harmonics all have the same spectral phase, indicating the single band origin of the harmonics. Solids are important for technology. Solids can be shaped, doped, coated or baised. We will show that high harmonics can be controlled by any of these methods while the harmonics reveal even subtle details about the solid.