Ultrafast infrared nano-spectroscopic imaging of heterogeneous photoinduced dynamical processes

Low-energy responses and interactions at mid-infrared frequencies often play central roles in photoinduced dynamical phenomena, ranging from phase transitions in quantum materials to polaron formations in photovoltaic materials. However, the spatial heterogeneities and temporal evolution associated with such low-energy processes in the photoinduced transient state has remained elusive. Here, we develop and apply ultrafast infrared scattering scanning near-field optical microscopy (IR s-SNOM) to characterize photoinduced dynamics with far-from-equilibrium excitation and ultrahigh spatio-temporal-spectral resolution. For unambiguous determination of nano-scale phase and spectral responses, we develop optical pump - infrared probe nano-imaging with heterodyne detection. We demonstrate the performance of ultrafast IR s-SNOM to probe transient quantum states of the photoinduced heterogeneous metallic phase in vanadium oxide microrods, as well as spatially varying polaron-molecule coupling in lead halide perovskite films. The approach of ultrafast heterodyne IR s-SNOM with strong pump excitation is generally applicable to a wide range of strong transient field phenomena, providing for fundamental insight into heterogeneous electronic, structural, and dynamical properties of materials.