The interaction of light with atoms, molecules, semiconductors and dielectrics can lead to strong polarization of charges in these systems, generating harmonics of the incident light field. At sufficiently high intensities, e.g. 1013-1014 W/cm2, the number of emitted harmonics can be very large, ranging from tens to several hundreds and even more. The availability of such extremely broad coherent spectrum, spanning across several tens to hundreds of electronvolts, implies an extraordinary temporal resolution of the underlying charge dynamics, down to tens of attoseconds in typical experiments. High harmonic spectroscopy uses high harmonic emission to reconstruct attosecond electronic response in atoms, molecules, and transparent solids to intense light.
One recent example of our work in this topic is the application of high harmonic spectroscopy to detect attosecond laser-driven chiral hole dynamics in the two enantiomers of a chiral molecule, with a temporal resolution of about a tenth of a femtosecond. Another recent example is the use of high harmonic spectroscopy to follow a light-induced phase transition (insulator to metal) in a strongly correlated quantum material, a Mott insulator, with a temporal resolution of 1 femtosecond.