<p>Researchers have developed a novel method to track light fields directly within optical resonators. This enables precise measurements at the exact locations where future field-resolved studies of light-matter interactions will take place.</p><p>Scientists from the Department of Physical Chemistry at the Fritz-Haber Institute of the Max Planck Society and the Helmholtz-Zentrum Dresden-Rossendorf have developed a new experimental platform to measure the electric fields of light trapped between two mirrors with precision below a light cycle. These electro-optical Fabry-Pérot resonators allow for precise control and observation of light-matter interactions, particularly in the terahertz (THz) spectral region.</p><p>Through the development of a tunable hybrid resonator design and the measurement and modeling of its complex mode spectrum, physicists can now actively switch between nodes and maxima of light waves at relevant resonator locations. This study thus opens new paths for the exploration of quantum electrodynamics and the ultrafast control of material properties.</p>