Understanding and Engineering Contrast in Transient Interferometric Scattering Microscopy

Resolving weak transient optical responses in nanostructures is essential for understanding localized carrier dynamics. Transient interferometric scattering microscopy (T-iSCAT) has recently emerged as a highly sensitive ultrafast imaging modality for weak transient signals, yet its fundamental contrast formation mechanism and sensitivity enhancement have not been systematically demonstrated and validated experimentally. Here, we present a systematic comparison between T-iSCAT and transient absorption microscopy (TAM) under identical wide-field pump–probe conditions using Au and CdS nanomaterials. We demonstrate that T-iSCAT substantially enhances single-particle contrast through phase-sensitive interference while reporting comparable excited-state relaxation dynamics as TAM. Systematic substrate, morphology, and partial-reflector experiments further reveal that T-iSCAT contrast is governed by the local transient polarizability, reference-field amplitude and phase, scattering strength, and sample geometry. This work provides an experimental framework for interpreting and optimizing T-iSCAT contrast, establishing it as a complementary ultrafast imaging approach for weak, heterogeneous, and low-dimensional nanoscale systems.