Another central area of research in our group is the use of light of predetermined field waveform to control and probe the dynamics of electrons in solids. Here in contrast to the research topic 2, the probing of the system does not utilize the absorption of an x-ray pulse. Instead, it records the properties of the nonlinear emitted radiation to deduce the physical dynamics. Solids exposed to light fields have been recently discovered to generate high harmonic radiation of the fundamental field. Our group has pioneering previous efforts as it has been the first to demonstrate that optically driven solids can emit high harmonic, extreme ultraviolet radiation reaching energies in excess of 45 eV. A few to single cycle pulse is focused on a thin transparent film of a dielectric or semiconductor to generate nonlinear emission. Typically, high harmonics are detected by a VUV-EUV spectrometer placed downstream the source.

The generation of high harmonics from bulk solids has opened up a new class of fundamental study in intense laser matter interactions. A key question which has still remained unresolved is the precise mechanism that leads to high harmonics in solids. Are the perturbative concepts of interband and intraband transitions valid to describe matter under strong fields? Does electronic dephasing act in favour of the one or the other mechanisms mentioned above? Can semiclassical pictures, that leave the band structure aside, be developed also in solids as successfully as they have been developed in atomic and molecular systems?

We have also shown that the emitted radiation embodies essential information both on the mechanism of the emission (intraband electron currents) and the fine structure of the bands. The first finding suggests that high harmonic radiation from bulk solid is a macroscopic probe of electronic currents whose nature is similar Both these findings influence dramatically the course of our current and future studies.