Terahertz cavity electrodynamics is an emerging research area, offering exciting new opportunities for research of light-matter interactions. In a new experimental work, researchers leverage electro-optic sampling to directly measure electromagnetic fields inside of a macroscopic planar cavity for the first time.

Unlike conventional cavities, which rely on external substrates that introduce interferences and dispersive effects, the cavities in this work are produced by depositing mirrors directly onto a free-standing electro-optic crystal. This allows direct measurement of the electric fields inside the cavity as opposed to relying on transmitted light. To develop a route towards measuring the local fields of strong coupling, they designed a tunable electro-optic cavity, which allows the extraction of the cavity field in combination with full cavity frequency tunability and the ability to introduce a sample. Surprisingly, this ‘hybrid’ cavity geometry displays avoided crossings of the cavity modes, a signature of strongly-coupled oscillators, even in the absence of any material excitations, typically observed in strong-coupling. These new findings spurred theoretical developments, explaining how these avoided crossings arise from the hybridization of standing waves in each of the constituent cavity layers.

Looking forward, they expect this novel strategy to provide critical new insights into cavity field-driven phenomena, particularly at the intersection of fundamental polariton research and laser-driven nonlinear excitations.