Correlation plays a central role in emergent phenomena, such as quantum ground states and collective excitations. Here, I will discuss what we can learn from time-domain sensing of correlation in two dimensional (2D) van der Waals (vdW) semiconductors, where excitonic transitions are intimately related to bandgap modulation, effectively dielectric constant, and Pauli repulsion. In the 2D vdW magnetic semiconductor, CrSBr, excitonic transition is found to strongly couple to magnetic order and this allows the easy detection of low energy (GHz-THz) magnons by visible-NIR light. In the 2D vdW magnetic semiconductor, NbOI2, we report the experimental discovery of a quasi-particle, the ferron, which may form the basis for new modes of information processing and control. In twisted bilayer MoTe2, we demonstrate exciton sensing as hitherto the most sensitive probe of electron correlation in moiré quantum matter, including signatures for a coveted quantum phase - the fractional topological insulator.