Observing how molecules bend, stretch, break, or transform requires sub-atomic spatial and few-femtoseconds temporal resolution. The emergence of High Harmonic Generation (HHG) not only enabled the development of attosecond table-top technology but also opened a new route towards attosecond spectroscopy which carries angstrom-scale spatial resolution, making high harmonic generation spectroscopy one of the most prominent tools to understand details about the structure and dynamics of composite, quantum, organic materials.
When light interacts with matter, it allows us to observe, probe and understand how matter behaves and works. If the intensity of the light that interacts with matter is low, three main things can happen: the matter absorbs the light, affecting the electrons of the atoms that compose the material, it transmits it or it reflects it. In any of these interactions with light, the electrons within the atoms that form the matter are only slightly perturbed by this light, undergoing quantum transitions from one energy level to another.
But what happens if the light becomes more and more intense? The conventional picture of light-matter interactions changes completely. When intense light interacts with matter, it reshapes the properties of the material, modifying its electronic and optical properties to a point where it can dramatically change the band structure of a crystal.
In a recent study published in Nature Photonics, researchers Álvaro Jimenez-Galan, Olga Smirnova and Misha Ivanov from Max-Born-Institut in Berlin, in collaboration with researchers from Weizmann Institute in Israel, Stanford University, and the Instituto de Ciencias de Materiales de Madrid (ICMM), demonstrate the link between nonlinear matter interactions in strongly driven crystals and the sub-cycle modifications in their effective band structures.
In their study, the team of scientists was able to use two-colour HHG spectroscopy to show that sub-femto-second variations of the electric field of the incident light-wave can generate a rapidly changing voltage that leads to sub-cycle modifications of the band structure of the material, which can drive changes in its macroscopic properties such as transmittance and conductance.
Their theoretical and numerical results carried out with HHG spectroscopy are capable of identifying the dynamical transitions between the conduction bands of the material as well as probing their structural dependence (in this case materials with large bandgaps, such as MgO). This opens a new window for the observation of new electronic phenomena in novel materials.
